1982 (Chapter 2) RUDOLPH Fisiology of Ticks Humidity Relationships and Water Balance of Ticks

Prévia do material em texto

C H A P T E R 2 
Humidity Relationships and Water Balance of 
Ticks* 
W. K N Ü L L E and D. R U D O L P H 
Institut fir Angewandte Zoologie, Freie Universität Berlin, Haderslebener Straße 9, 1000 Berlin 41, West 
Berlin, Federal Republic of Germany 
C O N T E N T S 
2.1. Introduction 43 
2.2. Loss of Water Vapour into the Atmosphere 45 
2.2.1. Critical Equilibrium Humidities 46 
2.2.2. Water Loss and Waterproofing of the Cuticle 48 
2.2.3. Respiratory Water Loss and Closing Device of the Spiracle 51 
2.3. Uptake of Water Vapour from the Atmosphere 53 
2.3.1. Adjustment of Water Steady State 56 
2.3.2. Uptake as an Active Process 57 
2.3.3. Site and Mechanism of Active Water Vapour Uptake 58 
2.4. Ecological and Behavioural Implications 62 
2.5. Conclusions 64 
References 66 
2 .1 . I N T R O D U C T I O N 
T h e life-cycle of t icks is c o m p o s e d of re la t ively sho r t p a r a s i t i c p h a s e s 
d u r i n g w h i c h e n g o r g e m e n t t akes p l ace a n d of e x t e n d e d n o n - p a r a s i t i c p h a s e s 
w i t h o u t access to food a n d l iqu id . T h i s i m p o s e s g r e a t d e m a n d s o n the i r 
w a t e r - b a l a n c e m e c h a n i s m s . W h e n a tick is suck ing b lood from its hos t it 
inges ts l a rge q u a n t i t i e s of w a t e r a n d ions w i th the b lood m e a l . I n o r d e r to 
c o n c e n t r a t e t he n u t r i e n t p o r t i o n of t he b lood a n d to m a i n t a i n o s m o t i c a n d 
i o n i c b a l a n c e , excess w a t e r a n d ions h a v e to be e l i m i n a t e d . I n ixodid ticks 
the sa l iva ry g l a n d s p l ay a s ignif icant role in r e t u r n i n g this excess of w a t e r 
a n d ce r t a in ions to t he hos t whi le t he tick is a t t a c h e d . I n r a p i d l y feeding 
a rga s id t icks t h e e l i m i n a t i o n of s u r p l u s w a t e r a n d s o m e ions is pe r fo rmed by 
a p a i r of coxal o r g a n s . T h e o s m o r e g u l a t o r y roles of t he sa l ivary g l a n d s 
in ixodid a n d the coxal o r g a n s in a rga s id t icks a n d the i r phys io logica l 
con t ro l a r e ex tens ive ly d i scussed by K a u f m a n a n d S a u e r in C h a p t e r 7 of this 
v o l u m e . 
* Supported by Deutsche Forschungsgemeinschaft, Schwerpunktprogramm "Biochemische 
und physiologische Mechanismen ökologischer Anpassung von Tieren". 
43 
44 W. Knülle and D. Rudolph 
W h e r e a s t h e e n g o r g i n g ixodid o r recen t ly e n g o r g e d a rga s id tick is faced 
w i t h t h e p r o b l e m of e l i m i n a t i n g s u r p l u s w a t e r , t h e non- feed ing tick h a s t he 
o p p o s i t e p r o b l e m — t h e c o n s e r v a t i o n of its b o d y w a t e r — d u r i n g the n o n -
p a r a s i t i c p h a s e . I t m u s t e n d u r e long pe r iods , often weeks or m o n t h s , w i t h o u t 
i m b i b i n g l iqu id , before it finds a n e w hos t ; in Amblyomma hebraeum, for 
e x a m p l e , as l i t t le as 2 % of t he t h r e e - y e a r life-cycle in E a s t e r n C a p e P r o v i n c e 
of S o u t h Africa is s p e n t on hos t a n i m a l s ( N o r v a l , 1977b) . D u r i n g non- feed ing 
pe r i ods , a t ick m u s t avo id d e h y d r a t i o n a n d m a i n t a i n its w a t e r b a l a n c e a g a i n s t 
t he d r y i n g p o w e r of a n a t m o s p h e r e in w h i c h the ac t iv i ty of w a t e r is n o r m a l l y 
m u c h be low t h a t of t he b o d y . E x h a u s t i o n of w a t e r reserves m e a n s d e a t h , 
whi le m a i n t e n a n c e of t he w a t e r b a l a n c e ensu re s su rv iva l . T h e c ruc ia l q u e s -
t ions a d d r e s s e d r e l a t e to t he n a t u r e of s t r u c t u r e s a n d of phys io log ica l as well 
as b e h a v i o u r a l m e c h a n i s m s w h i c h c o n t r i b u t e to t he m a i n t e n a n c e of b o d y 
w a t e r in t he unfed tick. W h a t a r e these devices , w h i c h r e g u l a t o r y p o w e r d o 
they possess in c o u n t e r a c t i n g phys i ca l g r a d i e n t s , a n d h o w d o they pe r fo rm 
u n d e r v a r i o u s e n v i r o n m e n t a l cond i t ions? 
T h e dev ices w h i c h t icks e m p l o y for w a t e r m a i n t e n a n c e a r e d ive r se . T h e 
w a x y l ip ids in t h e ep icu t ic le of t he gene ra l b o d y i n t e g u m e n t p r o v i d e a h igh 
d e g r e e of wa t e rp roo f ing , a n d m e c h a n i c a l sp i rac le c los ing devices p r o v i d e a n 
efficient m e a n s for r e s t r i c t ing t he loss of w a t e r t h r o u g h the o p e n i n g s of t he 
t r a c h e a l s y s t e m . O n the o t h e r h a n d , t icks d i sp l ay the r e m a r k a b l e abi l i ty of 
g a i n i n g w a t e r v a p o u r from u n s a t u r a t e d a t m o s p h e r e s . T h i s m e c h a n i s m p r o -
v ides a m e a n s for t h e r e p l a c e m e n t of w a t e r losses by a b s o r p t i o n of v a p o u r 
from t h e s u r r o u n d i n g a t m o s p h e r e . F ina l ly , b e h a v i o u r a l m e c h a n i s m s con-
t r i b u t e to t h e m a i n t e n a n c e of b o d y w a t e r , b o t h by r e d u c i n g tick e x p o s u r e 
to c o n d i t i o n s w h i c h m i g h t acce l e ra t e w a t e r loss be low cr i t ical levels a n d by 
i nc rea s ing tick e x p o s u r e to t he m i c r o h a b i t a t s w h e r e v a p o u r u p t a k e c a n occu r . 
All t hese dev ices h a v e the i r role in m a i n t a i n i n g w a t e r h o m e o s t a s i s . T h u s 
they a r e r e l a t ed to t h e d i s t r i b u t i o n , su rv iva l , o r i e n t a t i o n , a n d ac t iv i ty of t icks 
a n d to p a t t e r n s of d i sease t r a n s m i s s i o n by t icks . 
W e c a n on ly expec t to u n d e r s t a n d t h e h u m i d i t y r e l a t i onsh ips of t icks if 
w e c o m p r e h e n d t h e funct ion of t he s t r u c t u r a l a n d phys io log ica l m e c h a n i s m s 
w h i c h u n d e r l i e these r e l a t i onsh ip s a n d the i r p e r f o r m a n c e u n d e r v a r i o u s 
e n v i r o n m e n t a l c o n d i t i o n s . W e sha l l , therefore , d i scuss t he s t r u c t u r e s , p r o -
cesses, a n d m e c h a n i s m s r e l evan t to t he m a i n t e n a n c e of b o d y w a t e r in unfed 
ticks from a n e n v i r o n m e n t a l p o i n t of v iew. W e c a n largely exc lude c h a n g e s 
in t he w a t e r s t a t u s b r o u g h t a b o u t by t he e l im ina t i on of excre to ry p r o d u c t s 
a n d faecal m a t e r i a l s , as well as by t he p r o d u c t i o n of m e t a b o l i c w a t e r , f rom 
these c o n s i d e r a t i o n s . T h i s is poss ib le for a n u m b e r of r e a sons . F i rs t ly , v o i d i n g 
of faecal m a t e r i a l s a n d n i t r o g e n o u s exc re to ry e n d - p r o d u c t s , g u a n i n e a n d a 
s econd p u r i n e ( H a m d y , 1977), in all i n s t a r s of ixodid t icks takes p l ace w i t h i n 
a c o m p a r a t i v e l y s h o r t pe r iod of a t m o s t 4 weeks fol lowing the m o u l t . D u r i n g 
t h e po ten t i a l l y long non- feed ing p e r i o d p r io r to a t t a c h m e n t to a n e w hos t , 
Humidity Relationships and Water Balance of Ticks 45 
t he poor ly so lub le exc re to ry p r o d u c t s p rogress ive ly a c c u m u l a t e in t he rec ta l 
sac a n d t h e M a l p i g h i a n t u b u l e s . T h e a c c u m u l a t i o n of exc re to ry p r o d u c t s 
h a s a g r e a t s ignif icance for w a t e r c o n s e r v a t i o n d u r i n g the p r o l o n g e d n o n -
feeding p e r i o d s . Severa l r e l e v a n t s t ud i e s h a v e p r o d u c e d n o ev idence for a 
role of m e t a b o l i c w a t e r in t h e m a i n t e n a n c e of t he w a t e r s t e a d y s t a t e of unfed 
t icks (e.g. S a u e r & H a i r , 1971; M o n g i ,1978) . T h e r a t e s of r e s p i r a t o r y gas 
e x c h a n g e of unfed t icks a r e in fact r a t h e r low ( M a k l y g i n & Alekseev, 1960; 
Be lozerov , 1966) . 
T h e s e o b s e r v a t i o n s o n the s t o r a g e of exc re to ry p r o d u c t s , cessa t ion of 
de faeca t ion , a n d t h e negl ig ib le role of m e t a b o l i c w a t e r in the m a i n t e n a n c e 
of w a t e r h o m e o s t a s i s in t h e u n a t t a c h e d tick focus a t t e n t i o n o n the t w o 
o v e r r i d i n g d e t e r m i n a n t s for m a i n t a i n i n g w a t e r b a l a n c e , t he res t r i c t ion of 
w a t e r loss i n t o t h e a t m o s p h e r e , a n d the u p t a k e of w a t e r v a p o u r from the 
a t m o s p h e r e . 
2.2. LOSS O F W A T E R V A P O U R I N T O T H E A T M O S P H E R E 
T h e m o s t cr i t ica l p e r i o d s of t h e t ick life-cycle w i t h r e g a r d to w a t e r b a l a n c e 
a r e t hose of hos t - seek ing ac t iv i ty . I n these p h a s e s t icks h a v e to leave she l t e red 
p laces w i t h f a v o u r a b l e m i c r o c l i m a t i c c o n d i t i o n s (e.g. h igh re la t ive h u m i d i t y 
a n d m o d e r a t e t e m p e r a t u r e s ) a n d c l i m b u p t h e vege ta t ion or s tay o n the 
u n p r o t e c t e d soil sur face . T h e y a r e t h e n faced w i t h h u m i d i t i e s often well 
be low s a t u r a t i o n a n d w i t h h i g h e r t e m p e r a t u r e s . 
T h e d i s t r i b u t i o n of different t ick species c lear ly ind ica t e s s ignif icant dif-
ferences in t he i r ab i l i ty to m a i n t a i n w a t e r h o m e o s t a s i s a g a i n s t t he e v a p o r a t i v e 
p o w e r of t h e a t m o s p h e r e . S o m e spec ies , e.g. Ixodes ricinus, c a n pers i s t on ly 
in m o i s t h a b i t a t s , a n d the i r d i s t r i b u t i o n is r es t r i c ted to p laces w i t h a d e n s e 
p l a n t cover . T h i s is in s t r ik ing c o n t r a s t to o t h e r s , e.g. Dermacentor marginatus, 
a tick p r i n c i p a l l y of s t e p p e a n d forest s t e p p e h a b i t a t s often w i t h a xe roph i l i c 
v e g e t a t i o n (Nosek , 1972; P o m e r a n t z e v , 1950; L ieb i sch & R a h m a n , 1976a, 
b ) . I n N o r t h A m e r i c a , D. variabilis is r e s t r i c t ed to h u m i d e n v i r o n m e n t s whi le 
D. andersoni l ives o n rocky s c r u b - c o v e r e d s lopes in w h i c h t h e g r o u n d m a y 
b e c o m e ve ry p a r c h e d d u r i n g the s u m m e r m o n t h s . F ina l ly , t h e r e a r e even 
s o m e tick spec ies like Hyalomma asiaticum i n h a b i t i n g de se r t a r e a s . I n t he 
K a r a - K u m D e s e r t t h e a d u l t s of t he spec ies , d u r i n g hos t - seek ing ac t iv i ty , a r e 
exposed to h u m i d i t i e s r a n g i n g from 15 to 5 0 % R H a t soil t e m p e r a t u r e s u p 
to 35—40°C, as m i c r o c l i m a t i c m e a s u r e m e n t s s h o w e d (Ba la shov , 1960) . 
2.2.1 . Critical Equil ibrium Humidi t i e s 
W h e n u n e n g o r g e d t icks a r e exposed to different s u b s a t u r a t e d h u m i d i t i e s , 
a t h r e s h o l d h u m i d i t y b e c o m e s a p p a r e n t be low w h i c h the t icks c o n t i n u o u s l y 
lose w a t e r . A b o v e th is p o i n t they a r e a b l e to m a i n t a i n the i r w a t e r level a t 
46 W. Knülle and D. Rudolph 
a s t e a d y s t a t e b y u p t a k e of a t m o s p h e r i c w a t e r v a p o u r for e x t e n d e d pe r iods 
u p to severa l m o n t h s . T h i s t h r e sho ld h u m i d i t y is k n o w n as t he cr i t ica l 
e q u i l i b r i u m h u m i d i t y ( K n ü l l e & W h a r t o n , 1964). 
D e s p i t e t he ve ry d i s p a r a t e h u m i d i t y cond i t i ons in t he e n v i r o n m e n t s of 
different tick species , t he cr i t ica l e q u i l i b r i u m h u m i d i t i e s a r e fairly un i fo rm, 
r a n g i n g f rom a p p r o x i m a t e l y 7 5 % to 9 4 % R H ( T a b l e 2 .1) . T h e a d u l t s of t w o 
ixodid t icks w h i c h live in de se r t h a b i t a t s , Hyalomma dromedarii a n d H. asiaticum, 
h a v e cr i t ica l e q u i l i b r i u m h u m i d i t i e s w h i c h a r e close to 8 0 % R H (Hafez et 
al., 1970a; B a l a s h o v 1960) . O n the o t h e r h a n d , two tick species f rom mo i s t 
t e m p e r a t e reg ions h a v e h i g h e r e q u i l i b r i u m h u m i d i t i e s , 9 4 % R H in Ixodes 
hexagonus (Lees , 1946b) a n d 9 2 - 9 3 % R H in Amblyomma maculatum a c c o r d i n g 
to H a i r et al. (1975) . Lees gives s o m e w h a t lower va lues , 8 8 - 9 0 % R H , for t h e 
l a t t e r species . 
T h e cr i t ica l e q u i l i b r i u m h u m i d i t y of a species , howeve r , is no t a n i n v a r i a b l e 
v a l u e . T h e h u m i d i t y t h r e s h o l d — a t w h i c h m a i n t e n a n c e of w a t e r s t e a d y s t a t e 
c a n still b e a c c o m p l i s h e d by c o m p e n s a t i o n of w a t e r l o s s e s — d e p e n d s u p o n 
the ene rgy reserves of t he t ick. Lees (1964) , for e x a m p l e , h a s d e m o n s t r a t e d 
in / . ricinus t h a t l o c o m o t o r act iv i t ies d u r i n g hos t seeking lead to a n acce l e ra t ed 
TABLE 2.1. CRITICAL E Q U I L I B R I U M H U M I D I T I E S OF SOME TICK 
SPECIES 
Critical 
equilibrium 
humidity 
Species Instar (% RH) Authority 
Amblyomma albolimbatum Nymphs 82.0-83.5 Bull and Smyth (1973) 
A. americanum Adults 80-82 Hair etal. (1975) 
A. cajennense Females 90-92 Lees (1946b) 
A. cajennense Larvae 80-85 Knülle (1966) 
A. hebraeum Adults 79.5-86.0 Osman (1978) 
A. lepidum Adults 84.0-90.9 Osman (1978) 
A. limbatum Nymphs 79.5-82.5 Bull and Smyth (1973) 
A. maculatum Females 88-90 Lees (1946b) 
A. maculatum Adults 92-93 Hair et al. (1975) 
A. variegatum Adults 80-85 Rudolph and Knülle (1974) 
Aponomma hydrosauri Nymphs 81.0-82.5 Bull and Smyth (1973) 
Ar gas arboreus Adults 85 Hefnawy etal. (1975) 
A. cooleyi Adults 78 Howell and George (1973) 
A. persicus Adults «280 Balashov and Filippova (1964) 
A. vulgaris Adults SS80 Balashov and Filippova (1964) 
Boophilus micro plus Larvae > 7 0 , sï95 Hitchcock (1955) 
Dermacentor andersoni Females 86-90 Lees (1946b) 
D. andersoni Larvae 80-85 Knülle (1966) 
D. reticulatus Females 86-88 Lees (1946b) 
D. variabilis Adults 87 McEnroe (1972) 
D. variabilis Adults 84-86 Hair et al. (1975) 
D. variabilis Larvae 80-85 Knülle (1966) 
Haemaphysalis 
Knülle (1966) 
leporis palustris Larvae ca. 85 Camin and Drenner (1978) 
Humidity Relationships and Water Balance of Ticks 47 
TABLE 2.1. (continued) 
Critical 
equilibrium 
humidity 
Species Instar (% RH) Authority 
Hyalomma anatolicum ι 
excavatum Adults 80-85 Rudolph (unpublished) 
H. asiaticum Adults 80 Balashov (1960) 
H. dromedarii Larvae 75 Hafez etal. (1970a) 
H. dromedarii Nymphs > 7 5 , Hafez etal. (1970a) 
H. dromedarii Adults > 7 5 , ^ 8 4 Hafez etal. (1970a) 
H. schulzei Adults 80-85 Rudolph (unpublished) 
Ixodes canisuga Females 92-96 Lees (1946b) 
I. hexagonus Females 94 Lees (1946b) 
I. ricinus Adults 86-96 Lees (1946b) 
Ornithodoros capensis Adults *S80 Balashov and Filippova (1964) 
0. concanensis Nymphs ca. 75 Cook (1973) 
0. moubata Nymphs 82-90 Lees (1946b) 
0. papillipes Adults ^ 8 0 Balashov and Filippova (1964) 
0. savignyi First nymph 75 Hafez et al. (1970a) 
0. savignyi Second nymph > 7 5 , =S84 Hafez etal. (1970a) 
0. savignyi Third nymph 84 Hafez etal. (1970a) 
0. savignyi Adults 84 Hafez etal. (1970a) 
0. tartakovskyi Adults =£=80 Balashov and Filippova (1964) 
0. tholozani* Males > 7 0 , *S90 Belozerov and Seravin (1960) 
0. verrucosus Adults S S 8 0 Balashov and Filippova (1964) 
Rhipicephalus 
appendiculatus Larvae ca. 70 Londt and Whitehead (1972) 
R. appendiculatus Adults 
Nymphs> 7 5 , *S85 Mongi (1978) 
R. bursa Adults 80-85 Rudolph (unpublished) 
R. pulchellus Adults 
Nymphs > 7 5 , ^ 8 5 Mongi (1978) 
R. sanguineus Pemales 84-90 Lees (1946b) 
a
 Reported as Alectorobius tholozani. 
d e p l e t i o n of t h e rese rve s u b s t a n c e s a n d , as a c o n s e q u e n c e , to a n inc rease of 
t h e cr i t ica l e q u i l i b r i u m h u m i d i t y . T i c k s w h i c h h a d b e e n ac t ive for a to ta l of 
12 d a y s o r less h a d a cr i t ica l e q u i l i b r i u m h u m i d i t y of a b o u t 9 2 % R H . T h o s e 
w h i c h h a d b e e n ac t ive for a to ta l of 2 0 - 3 0 d a y s usua l ly cou ld a t t a i n e q u i -
l i b r i u m on ly in h u m i d i t y - s a t u r a t e d a i r . T h e r e m a y a lso be differences b e t w e e n 
the e q u i l i b r i u m h u m i d i t i e s in different i n s t a r s of t h e s a m e species , a l t h o u g h 
these a r e u sua l l y sma l l . I n l a r v a e o f / / , dromedarii th is l imi t is close to 7 5 % 
R H , in n y m p h s a n d in a d u l t s it is close to 8 0 % R H (Hafez et al., 1970a) . 
I n g e n e r a l , t h e cr i t ica l e q u i l i b r i u m h u m i d i t i e s of different tick species s eem 
n o t to differ s ignif icant ly , a n d the i r va lue s a r e fairly h igh in t h e h u m i d i t y 
scale . T h e q u e s t i o n , therefore , is: W h i c h a t t r i b u t e s res t r ic t s o m e tick species 
to m o i s t h a b i t a t s a n d e n a b l e o t h e r s to pers i s t in r a t h e r d r y e n v i r o n m e n t s ? 
T h e r e c a n b e n o d o u b t t h a t d r a s t i c differences a m o n g different species in t he 
48 W. Knülle and D. Rudolph 
r a t e of w a t e r loss a t s u b - e q u i l i b r i u m h u m i d i t i e s a r e of c ruc ia l i m p o r t a n c e for 
a n u n d e r s t a n d i n g of these r e l a t i onsh ips . 
2.2.2. Water Loss and Waterproofing of the Cuticle 
S o m e species of ixodid t icks, w h e n exposed to h u m i d i t i e s be low the i r 
cr i t ica l e q u i l i b r i u m h u m i d i t y , lose w a t e r very r a p i d l y a n d d ie of d e h y d r a t i o n 
w i th in a few d a y s . O t h e r t icks lose w a t e r very slowly a n d surv ive for severa l 
weeks u n d e r t he s a m e cond i t i ons . A t 0 % R H a n d 25°C, unfed females o f / . 
ricinus lose u p to 5 0 % of the i r o r ig ina l we igh t p e r d a y a n d surv ive for on ly 
1 o r 2 d a y s (Lees , 1946b) . L ikewise , t he g r ea t suscept ib i l i ty of all i n s t a r s of 
D. variabilis to des i cca t ion p r e v e n t s t he ex tens ion of c o n t i n u o u s b r e e d i n g 
p o p u l a t i o n s in to d r i e r h a b i t a t s ( S o n e n s h i n e , 1979). I n c o n t r a s t , unfed females 
of D. andersoni lose on ly 1 % or 2 % of the i r o r ig ina l we igh t p e r d a y a n d 
surv ive for 27 d a y s a t 0 % R H a n d 25°C (Lees , 1946b) . Hyalomma asiaticum, 
a species l iv ing in t he dese r t , is even m o r e wa te rp roo fed ; a t 0 % R H a n d 
26°C it loses on ly 0 . 6 % of o r ig ina l we igh t p e r d a y a n d surv ives for m o r e 
t h a n 1 m o n t h u n d e r these cond i t i ons (Ba la shov , 1960). O t h e r species of 
ixod ids o c c u p y i n t e r m e d i a t e pos i t ions b e t w e e n these ex t r emes (Lees , 1946b) . 
Lees (1946b) h a s s h o w n t h a t these differences in w a t e r loss r a t e s a t 
s u b e q u i l i b r i u m h u m i d i t i e s c a n essent ia l ly be a t t r i b u t e d to differences in t he 
p e r m e a b i l i t y of t h e i n t e g u m e n t to w a t e r . T h e c o m p o u n d s r e spons ib l e for t h e 
s t r ik ing differences in t he wa te rp roo f ing of t he cut ic le a m o n g t icks a r e t he 
e p i c u t i c u l a r l ip ids ; t hey convey , d e p e n d i n g on the i r specific n a t u r e , different 
deg rees of w a t e r i m p e r m e a b i l i t y to t he i n t e g u m e n t of t icks as they d o in o t h e r 
a r t h r o p o d s . A r e l a t i o n s h i p b e t w e e n w a t e r i m p e r m e a b i l i t y of the cut ic le a n d 
the n a t u r e of t he w a x y l ip ids b e c o m e s a p p a r e n t w h e n the so-cal led t r a n s i t i o n 
t e m p e r a t u r e s of t he e p i c u t i c u l a r l ip ids of different species a r e c o m p a r e d . T h i s 
t e m p e r a t u r e — i n the ma jo r i ty of t e r res t r i a l a r t h r o p o d s it is well a b o v e the 
le tha l t e m p e r a t u r e for t he species in q u e s t i o n a n d is therefore of n o d i rec t 
phys io log ica l o r ecological s ignif icance—signif ies t he t e m p e r a t u r e a t w h i c h 
w a t e r p e r m e a b i l i t y of t he cut ic le inc reases a b r u p t l y d u e to c h a n g e s in the 
l ip ids . T h e n a t u r e of these c h a n g e s is no t c lear b u t p e r h a p s is r e l a t ed to a 
c h a n g e in p h a s e from crys ta l to l iqu id c rys ta l (Dav i s , 1974b; see C h a p t e r 1 
for a m o r e de t a i l ed d i scuss ion of t r ans i t i on t e m p e r a t u r e s ) . 
F r o m the s tud ie s of Lees (1947) a n d B e a m e n t (1959) it is c lear t h a t those 
species , w h i c h a r e m o r e r e s i s t an t to des icca t ion , a lso h a v e h i g h e r t r a n s i t i o n 
t e m p e r a t u r e s ; t h a t is, a h i g h e r t r ans i t i on t e m p e r a t u r e of t h e e p i c u t i c u l a r 
l ipids is a s soc ia t ed w i t h a h i g h e r d e g r e e of c u t i c u l a r i m p e r m e a b i l i t y to w a t e r 
a t o r d i n a r y b io logica l t e m p e r a t u r e s . W h e n different tick species a r e a r r a n g e d 
a c c o r d i n g to the i r r a t e of w a t e r loss, as d e t e r m i n e d a t 0 % R H a n d 25°C, this 
s e q u e n c e c o r r e s p o n d s closely w i t h the o n e o b t a i n e d for t he t r ans i t i on t e m -
p e r a t u r e s of these species . W e find / . ricinus, w i th a t r ans i t i on t e m p e r a t u r e 
Humidity Relationships and Water Balance of Ticks 49 
of 32°C, a t t he lower e n d a n d D. andersoni, w i th 44°C, n e a r t he u p p e r e n d of 
a g r a d e d ser ies . T h e t r a n s i t i o n t e m p e r a t u r e of t he l a t t e r species is only 
s u r p a s s e d b y t h a t of t he even m o r e w a t e r p r o o f e d species of a r id e n v i r o n m e n t s , 
such as Hyalomma savignyi w i t h 45°C a n d H. asiaticum w i th 52°C (Lees , 1947; 
B a l a s h o v , 1960) . A r g a s i d t icks h a v e w a x e s genera l ly m o r e w a t e r p r o o f t h a n 
those of ixod id t icks; c o r r e s p o n d i n g l y the i r t r ans i t i on t e m p e r a t u r e s a r e h i g h e r 
(Lees , 1947; B e a m e n t , 1959; B a l a s h o v & F i l i ppova , 1964; Hafez etal., 1970b) . 
I t m u s t h o w e v e r b e e m p h a s i z e d t h a t t he va l id i ty of s h a r p t r ans i t i on t e m -
p e r a t u r e s of c u t i c u l a r w a x e s h a s b e e n q u e s t i o n e d recen t ly (Gi lby , 1980; 
M a c h i n , 1980); t h e ev idence p o i n t s m o r e to a s m o o t h inc rease in p e r m e a b i l i t y 
w i t h t e m p e r a t u r e (Gi lby , 1980) . U n f o r t u n a t e l y s tud ies o n the ch emi ca l n a t u r e 
a n d q u a n t i t y of c u t i c u l a r l ip ids in co r r e l a t ion w i th wa te rp roo f ing of the 
i n t e g u m e n t a r e l ack ing for t icks. O n l y t he c h e m i c a l cons t i t uen t s of t he egg 
w a x l ip ids of B. microplus h a v e b e e n a n a l y z e d ( M c C a m i s h et al., 1977). I n 
insec ts it a p p e a r s t h a t l o n g - c h a i n s a t u r a t e d c o m p o u n d s p rov ide for m a x i m u m 
i m p e r m e a b i l i t y to w a t e r a n d ev idence shows t h a t insects a n d a r a c h n i d s 
w h i c h i n h a b i t de s i cca t i ng e nv i r o n m e n t s h a v e a sur face-wax co mp o s i t i o n t h a t 
p rov ides m a x i m u m i m p e r m e a b i l i t y to w a t e r ( H a d l e y , 1981 ). A m o r e p ro found 
u n d e r s t a n d i n g of t h e differential suscep t ib i l i ty of tick species to des i cca t ing 
h u m i d i t i e s m u s t a w a i t s tud ie s co r r e l a t i ng different deg rees of cut ic le w a t e r -
proof ing w i t h sur face w a x th i ckness a n d c h e m i c a l compos i t i on of c u t i c u l a r 
l ip ids . 
I t is c l ea r f rom v a r i o u s e x p e r i m e n t s t h a t m o s t of t he pass ive r e s i s t ance to 
w a t e r loss res ides in t he ep icu t ic le of t icks (Lees , 1947; Hafez et al., 1970b; 
R u d o l p h , 1976) . T h e w a x y l ip ids of t h e ep icu t ic le c a n n o t be v iewed as a 
s t a t i c phys i ca l dev ice ; they a r e u n d e r phys io log ica l con t ro l . T h i s is ev iden t 
f rom the i r r e s t o r a t i o n after superf ic ia l a b r a s i o n (Lees , 1947; Hafez et al., 
1970b) a n d a lso f rom c o n t i n u o u s c h a n g e s w i t h depos i t i on a n d r e m o v a l of 
l ip ids d u r i n g e n g o r g e m e n t a n d m o u l t i n g (Dav i s , 1974a, b ; Hafez etal., 1970b; 
a lso see C h a p t e r 1). 
W a t e r loss u n d e r s u b e q u i l i b r i u m c o n d i t i o n s m a y v a r y g rea t ly in different 
i n s t a r s of t h e s a m e species . O t h e r t h ings b e i n g e q u a l , we w o u l d expec t t he 
l a r v a e a n d a lso t h e n y m p h s to r e a c h the i r l e tha l t h r e sho ld w a t e r c o n t e n t 
u n d e r d e h y d r a t i n g c o n d i t i o n s m u c h soone r t h a n the a d u l t s . T h i s s h o u l d 
follow as a c o n s e q u e n c e of t h e d ra s t i ca l l y i nc r ea sed surface- to v o l u m e ra t io , 
s ince t h e s m a l l e r i n s t a r s h a v e m o r e surface a r e a from w h i c h e v a p o r a t i o n can 
o c c u r p e r u n i t of w a t e r . Hafez et al. (1970a) s h o w e d t h a t w a t e r loss a t 
s u b e q u i l i b r i u m h u m i d i t i e s w a s d r a s t i c a n d a b r u p t in H. dromedarii l a rvae , 
less d r a s t i c in n y m p h s , a n d still less in a d u l t t icks. I t is no t k n o w n w h e t h e r 
this is a s i m p l e c o n s e q u e n c e of size o r d u e to differences in t he wa te rp roo f ing 
s y s t e m . T h e differences in w a t e r loss b e t w e e n in s t a r s a r e reflected in the i r 
su rv iva l ; a t 4 2 % R H a n d 28°C, unfed l a r v a e su rv ived for 4 o r 5 d a y s , n y m p h s 
for 16 d a y s , females for 8 3 , a n d m a l e s for 101 d a y s . 
50 W. Knülle and D. Rudolph 
T h e r e a r e a lso g r e a t differences in t he r a t e of w a t e r loss a m o n g eggs of 
different species ( R e c h a v & M a l t z a h n , 1977; H e a t h , 1979). W h e n these 
species a r e a r r a n g e d in a g r a d e d series a c c o r d i n g to the wate r - loss r a t e s of 
the i r eggs, t he s e q u e n c e c o r r e s p o n d s closely to the o r d e r of a s imi la r series 
for w a t e r loss f rom a d u l t t icks (Lees , 1947; Lees & B e a m e n t , 1948). As in 
the o t h e r i n s t a r s , t he r a t e of w a t e r loss from the egg is d e t e r m i n e d essent ia l ly 
by p r o p e r t i e s of t he wa t e rp roo f ing w a x e s of t he egg surface . Lees a n d B e a m e n t 
(1948) h a v e s h o w n t h a t t he w a x is a p p l i e d to t he ou t s ide of the egg by G e n e ' s 
o r g a n w h i c h is eve r t ed shor t ly before ov ipos i t ion . W h e n eggs a r e p r e v e n t e d 
from c o m i n g in to c o n t a c t w i th G é n é ' s o r g a n they lose w a t e r r ap id ly , shr ive l , 
a n d d ie in d r y a i r . S ince t he G e n e ' s o r g a n is only a specia l ized a r e a of t h e 
gene ra l e p i d e r m i s of t he female , it is no t s u r p r i s i n g t h a t the t r ans i t i on 
t e m p e r a t u r e s of t he egg w a x e s in I x o d i d a e a r e nea r ly iden t ica l w i t h those 
o b t a i n e d for t he cut ic le w a x e s of the females . T h i s , however , is no t t h e case 
in A r g a s i d a e ; t he t r ans i t i on t e m p e r a t u r e of egg waxes is a p p r o x i m a t e l y 20°C 
lower t h a n t h a t for w a x e s of t he female i n t e g u m e n t . I n fact, Lees a n d B e a m e n t 
(1948) found t h a t the w a x e s recovered from G é n é ' s o r g a n a n d from the 
cut ic le surface of females in two Ornithodoros species h a v e ent i re ly different 
phys i ca l p r o p e r t i e s ; t he egg w a x is soft a n d v iscous whi le t h a t of the female 
is h a r d a n d c rys ta l l ine . T h e s e a u t h o r s p r o p o s e t h a t t he p rope r t i e s of t he egg 
w a x a r e la rge ly d i c t a t e d by the need for it to s p r e a d over the surface of the 
egg. A w a x w i t h good s p r e a d i n g p o w e r s , howeve r , is unl ikely to h a v e a h igh 
t r ans i t i on t e m p e r a t u r e . I n a rga s id t icks, the s p r e a d i n g abi l i ty h a s b e e n 
ach ieved on ly a t the expense of s o m e inc rease in w a t e r p e r m e a b i l i t y . 
I n conc lus ion , m a n y ticks a r e ev iden t ly res t r ic ted to p e r m a n e n t l y moi s t 
h a b i t a t s b e c a u s e the i r r a t h e r w a t e r - p e r m e a b l e i n t e g u m e n t resu l t s in r a p i d 
w a t e r loss, d e h y d r a t i o n , a n d d e a t h a t s u b - e q u i l i b r i u m h u m i d i t i e s . T i cks 
o c c u r r i n g in m o r e a r i d e n v i r o n m e n t s can subs i s t for a long pe r iod a t low 
h u m i d i t i e s b e c a u s e of the i r m o r e wa te rp roo fed i n t e g u m e n t s . L a r v a e a n d 
n y m p h s of ixodid t icks a r e genera l ly m o r e suscep t ib le to des icca t ion t h a n 
a d u l t s . T h e y usua l ly feed on sma l l e r hos t a n i m a l s a n d the i r hos t - seek ing 
ac t iv i ty is n o r m a l l y res t r i c ted to she l t e red p laces w i th m o d e r a t e m i c r o c l i m a t i c 
cond i t i ons . C o m m o n hos t s for the l a r v a e a n d n y m p h s of H. dromedarii a r e 
E g y p t i a n gerbi l s ( H o o g s t r a a l , 1956) a n d these i n s t a r s r e q u i r e b u r r o w s for 
she l te r . T h e re la t ive h u m i d i t i e s in t he b u r r o w s r a n g e from 5 0 % to 1 0 0 % 
R H . T h e a d u l t s o f / / , dromedarii, howeve r , pa ra s i t i ze came l s a n d s ea rch for 
the i r hos t s in t he o p e n w h e r e they a r e often seen r u s h i n g ac ross t he dese r t 
t o w a r d a m o v i n g a n i m a l o r m a n (Hafez et al., 1970a) . T h e g r ea t differences 
in t he r a t e s of w a t e r loss a m o n g eggs of different species m a y be a m o n g the 
m o r e c ruc ia l factors r e s t r i c t ing t h e m to ce r t a in h a b i t a t s ( S o n e n s h i n e & 
T i g n e r , 1969; L o n d t & W h i t e h e a d , 1972; N o r v a l , 1977a, b) a n d , in c o m b i -
n a t i o n w i th t e m p e r a t u r e to l e rances , for t he res t r ic t ion of ce r t a in species to 
d i s t inc t c l ima t i c reg ions ( H e a t h , 1979). Eggs , however , a r e no t conf ron ted 
Humidity Relationships and Water Balance of Ticks 51 
w i t h e n v i r o n m e n t a l h a r d s h i p s a s soc ia t ed w i t h hos t finding. T h e y a r e exposed 
to t he m i c r o c l i m a t i c c o n d i t i o n s p r e v a i l i n g a t t he site of the i r depos i t ion ; 
f u r t h e r m o r e , t hey a r e la id in closely p a c k e d b a t c h e s often of severa l t h o u s a n d s 
a d h e r i n g t ight ly toge the r , w h i c h r e d u c e s t he surface a r e a exposed to t he 
a m b i e n t a i r . 
2.2.3. Respiratory Water Loss and Clos ing Device of the Spiracle 
In a d d i t i o n to t r a n s p i r a t o r y w a t e r loss f rom the gene ra l b o d y i n t e g u m e n t , 
w a t e r loss in t he cou r se of r e s p i r a t o r y gas e x c h a n g e via the t r a c h e a l sys t em 
m u s t a lso b e c o n s i d e r e d . I n a n u m b e r of tick species from b o t h famil ies , h igh 
C O 2 c o n c e n t r a t i o n s in t he a m b i e n t a i r c a u s e d r a s t i c inc reases of w a t e r loss 
( M e l l a n b y , 1935; B r o w n i n g , 1954a; H e f n a w y , 1970) . I t h a s been inferred 
t h a t these losses o c c u r r e d v ia t h e t r a c h e a l s y s t e m a n d t h a t C 0 2 ac t s on a 
c los ing dev ice of t he sp i rac les . T h e ex i s tence of a C C V s e n s i t i v e con t ro l 
m e c h a n i s m in t h e sp i rac les of ixod id a n d a r g a s i d t icks is, in fact, a p p a r e n t 
from H e f n a w y ' s (1970) e x p e r i m e n t s . 
R e m a r k a b l e w a t e r loss m a y o c c u r via t he t r a c h e a l sys t em as h a s been 
d e m o n s t r a t e d in t h e a d u l t s of Amblyomma variegatum. F e m a l e s s h o w a seven-
teenfold i nc rea se of w a t e r loss a t h igh C O 2 c o n c e n t r a t i o n s , in 0 % as well 
as in 9 3 % R H . T h i s loss c a n be a b o l i s h e d by b lock ing the sp i rac les w i th 
paraff in w a x ( R u d o l p h , 1976) . S ince l a r v a e of ixodid ticks h a v e n o t r a c h e a l 
sy s t em, w e s h o u l d expec t t h a t the i r w a t e r loss is unaffected by C O 2 , a n d th is 
s u p p o s i t i o n h a s b e e n e x p e r i m e n t a l l y verified for l a rvae of A. variegatum 
( R u d o l p h & K n ü l l e , 1979) . 
I n t icks, excess ive w a t e r loss f rom t h e t r a c h e a l sys t em in to the s u r r o u n d i n g 
a t m o s p h e r e is d o u b t l e s s p r e v e n t e d by a n effective c losing device in t he 
Time 
FlG. 2.1. Microbalance recordings of water loss of an individual female Amblyomma variegatum 
at 20°C and 93% RH: a, resting tick; b, tick showing locomotor activity. 
52 W. Knülle and D. Rudolph 
sp i rac le . I n a c t i v e female A. variegatum t icks a t 20°C o p e n the i r sp i rac les on ly 
1-2 t i m e s / h a n d t h e n on ly for 4 - 7 m i n as i n d i c a t e d by s h o r t p e r i o d s of 
a b r u p t l y i nc r ea sed w e i g h t loss (Fig. 2 . 1 , a). D u r i n g l o c o m o t o r ac t iv i ty , 
howeve r , t h e f r equency of sp i rac le o p e n i n g s inc reases m a r k e d l y , to ca. 15 
t i m e s / h a t 20°C (Fig. 2 . 1 , b). T h i s resu l t s in a d r a s t i c inc rease of w a t e r loss 
( R u d o l p h & K n ü l l e , 1979) a n d ind ica t e s h o w l o c o m o t o r ac t iv i t ies , e.g. 
hos t - seek ing b e h a v i o u r , c a n be expec t ed to s t ress the w a t e r reserves of t icks 
t h r o u g h inc rea sed w a t e r loss. 
C o m p r e h e n s i o n of s p i r a c u l a r r egu l a t i on h a s b e e n h a m p e r e d by a n insuf-
ficient u n d e r s t a n d i n g of t h e func t iona l m o r p h o l o g y of t he tick sp i rac le a n d 
its m o d e of ac t ion . I n p a r t i c u l a r , c o n s i d e r a b l e con t rove r sy h a s c e n t r e d on 
FlG. 2.2. Diagrammatic representation of the adult Amblyomma variegatum spiracle as seen from 
surface view and in transverse section. A, atrium; O, ostium; SO, subostial space; V, atrial 
valve. The arrow indicates the path of the air from the atmosphere to the main tracheal trunks. 
Air enters the spiracle via the aeropyles of the spiracular plate and passes via the interpedicellar 
space into the subostial space which can be separated from the atrium by the atrial valve. 
Goblets in the thick basal cuticle of the spiracular plate are not shown. 
t he ident i f ica t ion of t h e func t iona l o p e n i n g a n d the c losing dev ice of t he tick 
r e s p i r a t o r y s y s t e m . A s p i r a c u l a r p l a t e w i th a slit-like c rescen t ic s t r u c t u r e , 
t he so-ca l led o s t i u m , is ex te rna l ly visible (Fig. 2 .2) . T h e s p i r a c u l a r p l a t e is 
c o m p o s e d of a r a t h e r th ick ba sa l cut ic le a n d a th in surface cut ic le . I n n e r a n d 
o u t e r cu t ic le l ayers a r e c o n n e c t e d by n u m e r o u s ver t ica l , rod- l ike c u t i c u l a r 
p i l l a r s , t h e ped ice l s , w h i c h c r ea t e a n air-filled l a b y r i n t h , t he so-cal led in te r -
ped i ce l l a r s p a c e . T h e t h i n surface cut ic le is pe r fo ra t ed by m i n u t e ho les , t he 
a e r o p y l e s , a s h a s b e e n c lear ly d e m o n s t r a t e d by s t e reoscan e lec t ron m i c r o -
scopy ( H i n t o n , 1967; Sixl et al, 1971; Wool l ey , 1972; R o s h d y & H e f n a w y , 
1973; R o s h d y , 1974; R u d o l p h & K n ü l l e , 1979). T h e o s t i u m , w h i c h h a s 
r e p e a t e d l y b e e n mis ident i f ied a s t he func t iona l ex t e rna l o p e n i n g of t he sp i rac le 
Humidity Relationships and Water Balance of Ticks 53 
(Fa lke , 1931; B r o w n i n g , 1954b; R o s h d y & H e f n a w y , 1973; R o s h d y , 1974) , 
is a n ecdys ia l s ca r w h i c h t igh t ly closes u p t he fo rmer ecdys ia l o p e n i n g in the 
s p i r a c u l a r p l a t e t h r o u g h w h i c h t h e s h e d d e d n y m p h a l t r a c h e a l sy s t em is 
w i t h d r a w n ( H i n t o n , 1967; R u d o l p h & K n ü l l e , 1979). 
G a s e x c h a n g e b e t w e e n the tick a n d the e n v i r o n m e n t takes p l ace via t he 
ae ropy les of t h e s p i r a c u l a r p l a t e as H i n t o n (1967) h a s sugges ted . T h e a e r o -
pyles l ead i n t o t h e so-cal led i n t e rped i ce l l a r space , w h i c h is c o n t i n u o u s w i t h 
t he subos t i a l s p a c e , t he a t r i u m a n d the m a i n t r a c h e a l t r u n k s ( R u d o l p h & 
K n ü l l e , 1979) . T h e c los ing dev ice of t h e tick sp i rac le is loca ted in t he a t r i u m 
as a p r o m i n e n t a t r i a l va lve , fo rmed by t h e v e n t r a l wal l of t he a t r i a l c h a m b e r . 
O p e n i n g a n d c los ing ac t ions of t h e va lve c a n be o b s e r v e d in situ in d i s sec ted 
p r e p a r a t i o n s of A. variegatum he ld in R i n g e r so lu t ion a n d the C O 2 sens i t iv i ty 
of t he va lve is r ead i ly d e m o n s t r a b l e ( R u d o l p h & K n ü l l e , 1979). 
F u n c t i o n a l l y , t h e c o m p l e x s t r u c t u r e of t he t ick sp i rac le is no t fully u n d e r -
s tood . T h e s p i r a c u l a r p l a t e , w i t h its m i n u t e ae ropy les a n d the u n d e r l y i n g 
in t e rped ice l l a r s p a c e , m a y ac t as a diffusion b a r r i e r w h i c h r e t a r d s t he e scape 
of w a t e r to t he e n v i r o n m e n t . I t m a y h a v e a n o t h e r u n k n o w n funct ion as well , 
invo lv ing t h e n u m e r o u s c o n s p i c u o u s o r g a n s w h i c h t r ave r se t he th ick ba sa l 
cut ic le of t h e s p i r a c u l a r p l a t e a n d o p e n in to t he in t e rped ice l l a r s p a c e (Fa lke , 
1931; A r t h u r , 1956; R o s h d y & H e f n a w y , 1973; R o s h d y , 1974; Sixl & Sixl-
Voig t , 1974). I t is c lear , h o w e v e r , t h a t t h e a t r i a l va lve of t he tick sp i rac le 
r e p r e s e n t s a n efficient c los ing dev ice w h i c h is sub jec t to phys io logica l con t ro l 
a n d w h i c h p r e v e n t s excessive w a t e r losses from the t r a c h e a l l u m e n of t he 
tick i n to t he a m b i e n t a i r . 
2.3 . U P T A K E O F W A T E R V A P O U R F R O M T H E A T M O S P H E R E 
D e s p i t e t h e ve ry efficient w a t e r p r o o f i n g of t h e i n t e g u m e n t a n d the con t ro l 
of s p i r a c u l a r va lve func t ion , w a t e r losses w o u l d soone r o r l a te r d e pl e t e t h e 
w a t e r reserves in t h e t ick b o d y . T h e excep t iona l ly long surv iva l of unfed t icks 
in a n u m b e r of ixod id species , w h i c h u n d e r l a b o r a t o r y cond i t i ons m a y a m o u n t 
to severa l y e a r s (Ba l a shov , 1968) , cou ld no t b e e x p l a i n e d if these t icks h a d 
n o access to e n v i r o n m e n t a l w a t e r . 
L ike s o m e o t h e r a r t h r o p o d s , t icks possess t he r e m a r k a b l e abi l i ty to c o m -
p e n s a t e for w a t e r losses by ac t ive a b s o r p t i o n of w a t e r v a p o u r from a n 
a t m o s p h e r e w h i c h is well be low its p o i n t of s a t u r a t i o n . T h e lowest h u m i d i t y 
a t w h i c h c o m p e n s a t i o n is poss ib le is ca l led t he cr i t ical e q u i l i b r i u m h u m i d i t y . 
U p t a k e of w a t e r v a p o u r from s u b s a t u r a t e d a i r occu r s in a series of u n r e l a t e d 
mi t e s a n d a lso in insec ts b e l o n g i n g to d i s p a r a t e a n d phy logene t i ca l ly wel l -
s e p a r a t e d t a x o n o m i c g r o u p s . T h e s e a r t h r o p o d s often h a v e ex t r eme ly d iver -
gen t ecological r e q u i r e m e n t s ( rev iewed by E d n e y , 1977). T h e cr i t ical e q u i -
l i b r i u m h u m i d i t i e s w h i c h h a v e b e e n r e p o r t e d for v a r i o u s species r a n g e from 
4 3 % R H to n e a r s a t u r a t i o n . A b s o r p t i o n h a s b e e n d e m o n s t r a t e d a m o n g 
POT - c 
54 W. Knülle and D. Rudolph 
b lood - suck ing a r t h r o p o d s , as well as a m o n g those w h i c h feed on d r y v e g e t a b l e 
m a t t e r s u c h as seeds . I t is found a m o n g a r t h r o p o d s l iving in p e r m a n e n t l y 
h u m i d e n v i r o n m e n t s a n d o t h e r s i n h a b i t i n g h o t a n d xer ic dese r t s . O b v i o u s l y 
this p h e n o m e n o n h a s n o t evolved w i t h i n a ce r t a in phy logene t i c l ine n o r c a n 
it b e seen as a case of c o n v e r g e n t evo lu t ion by a r t h r o p o d s i n h a b i t i n g a ce r t a in 
t ype of h a b i t a t . A c q u i s i t i o n of the ab i l i ty to a b s o r b w a t e r from u n s a t u r a t e d 
a t m o s p h e r e s s e e m s to h a v e evolved i n d e p e n d e n t l y in different a r t h r o p o d 
g r o u p s . T h i s will b e e m p h a s i z e d l a t e r in t he d i scuss ion of t he sites of v a p o u r 
u p t a k e in t icks a n d o t h e r a r t h r o p o d s . 
U p t a k e of w a t e r v a p o u r from s u b s a t u r a t e d a t m o s p h e r e s occu r s in all 
m o b i l e i n s t a r s , l a r v a e , n y m p h s , a n d a d u l t s of t h r ee -hos t ixodids a n d a r g a s i d s 
b u t no t in the i r eggs ( H e a t h , 1979) . I n the one -hos t ca t t l e t icks of t he g e n u s 
Boophilus, e n g o r g e d l a r v a e a n d n y m p h s m o u l t o n t h e hos t a n d r e - a t t a c h 
w i t h i n a sho r t t i m e . T h u s n y m p h a l a n d a d u l t i n s t a r s a r e no t conf ron ted w i t h 
t he p r o b l e m of p r o l o n g e d su rv iva l on t he g r o u n d before finding a n e w hos t , 
a n d they h a v e n o n e e d to m a i n t a i n b o d y w a t e r for e x t e n d e d p e r i o d s . I n d e e d , 
unfed n y m p h s a n d a d u l t s of B. annulatus w h i c h w e r e r e m o v e d from the i r hos t 
i m m e d i a t e l y after m o u l t i n g a n d before r e - a t t a c h m e n t , s h o w n o ne t ga in of 
w a t e r v a p o u r , even f rom n e a r l y s a t u r a t e d a i r ( R u d o l p h & K n ü l l e , 1979) . I n 
c o n t r a s t t h e unfed l a r v a e of Boophilus species , w h i c h s p e n d the p r e - a t t a c h m e n t 
pe r iod off t h e hos t , s e e m n o t to differ f rom l a r v a e of m a n y o t h e r ixod id ticks 
in t e r m s of v a p o u r u p t a k e a n d longevi ty a t h igh h u m i d i t i e s (H i t chcock , 1955; 
W i l k i n s o n & W i l s o n , 1959). 
As a ru l e , e n g o r g e d t icks s h o w n o ne t g a i n of w a t e r w h e n exposed to h igh 
h u m i d i t i e s (Lees , 1946b; S a u e r & H a i r , 1971). E v e n w h e n pa r t i a l l y d e h y -
d r a t e d , ne t u p t a k e does n o t o c c u r a m o n g e n g o r g e d n y m p h s a n d females of 
/. ricinus (Lees , 1946b) . T i c k s w h i c h w e r e on ly pa r t i a l l y e n g o r g e d a l so failed 
to g a i n w a t e r . O n t h e o t h e r h a n d , S w e a t m a n (1967) r e p o r t s t h a t a few 
e n g o r g e d female Rhipicephalus sanguineus i nc r ea sed in we igh t d u r i n g the first 
d a y o r t w o of e x p o s u r e to 9 3 % R H a t b o t h 15° a n d 25°C. I t s eems t h a t m o s t 
e n g o r g e d t icks lose the i r ab i l i ty to t ake u p v a p o u r from the a t m o s p h e r e . T h e 
poss ib i l i ty exis ts , h o w e v e r , t h a t w a t e r a b s o r p t i o n m i g h t b e concea led by h i g h 
e v a p o r a t i v e w a t e r losses f rom the surfaces of e n g o r g e d t icks. 
U n d e r n a t u r a l c o n d i t i o n s , w e d o n o t k n o w if t he d r i n k i n g of l iqu id w a t e r 
p l ays a n y s ignif icant role in t he r e s to r a t i on of b o d y w a t e r . Lees (1946b) 
s t a t ed t h a t / . ricinus does n o t d r i n k w a t e r a n d neve r p r o b e s mo i s t ob jec t s . 
K n ü l l e (1966) offered d r o p s of w a t e r , co loured w i t h E v a n s b l u e , to pa r t i a l l y 
d e h y d r a t e d l a r v a e of D. variabilis a n d Amblyomma cajennense w i t h o u t o b s e r v i n g 
d r i n k i n g . O n t h e o t h e r h a n d , L o n d t a n d W h i t e h e a d (1972) o b s e r v e d t h a t 
l a r v a e of Rhipicephalus evertsi, R. appendiculatus, Boophilus decoloratus, a n d A. 
hebraeum i m b i b e w a t e r u n d e r l a b o r a t o r y cond i t i ons . W i l k i n s o n (1953) a n d 
W i l k i n s o n a n d W i l s o n (1959) a lso s h o w e d t h a t pa r t i a l l y d e h y d r a t e d l a r v a e 
of B. microplus i m b i b e w a t e r f rom d r o p l e t s . 
Humidity Relationships and Water Balance of Ticks 55 
2 10 20 30 40 50 60 
T i m e (days) 
FlG. 2.3. Changes in the body water content (expressed as percentage of the mean water content 
over a 9-week observation period) of four individual adult Amblyomma variegatum at 20°C and 
93% RH. Broken horizontal lines indicate individual mean water contents. 
T i m e (h) 
FlG. 2.4. A continuous recording of the weight of an individual female Amblyomma variegatum at 
20°C and 93% RH. Broken lines indicate periods during which the recorder ran out of scale. 
56 W. Knülle and D. Rudolph 
2.3.1 . Adjustment of Water Steady State 
T h e b o d y w a t e r of t icks, a t c o n s t a n t h u m i d i t i e s a n d u n d e r e q u i l i b r i u m 
c o n d i t i o n s , is n o t de l ica te ly b a l a n c e d . I n v e s t i g a t i o n s w i th female A. variegatum 
on a r e c o r d i n g b a l a n c e h a v e s h o w n longe r p e r i o d s ( 8 - 1 4 days ) of s l ight 
overa l l w a t e r loss a l t e r n a t i n g w i t h s h o r t e r pe r i ods ( 2 - 6 days ) of r a p i d u p t a k e . 
N o r e g u l a r p a t t e r n is r ecogn izab l e in e i the r the t i m e cour se o r the a m p l i t u d e 
of t he v a r i a t i o n s (Fig . 2 .3) . M o r e o v e r , t he pe r iod of w a t e r loss is i n t e r r u p t e d 
b y u p t a k e p h a s e s of ve ry s h o r t d u r a t i o n w h e n smal l ga ins occu r (Fig . 2.4) 
( R u d o l p h & K n ü l l e , 1978). 
U n d e r e q u i l i b r i u m cond i t i ons , t he a m o u n t of w a t e r in t he b o d y of t he t ick 
a p p e a r s to b e r e l a t ed to t he level of e n v i r o n m e n t a l h u m i d i t y ; t h e w a t e r 
c o n t e n t of t icks is h ig h e r a t h i g h e r a m b i e n t h u m i d i t i e s . T h e b o d y w a t e r level 
is m a i n t a i n e d as long as t he h u m i d i t y is u n c h a n g e d b u t ad jus t s to h i g h e r o r 
l ower p l a t e a u s w i t h c h a n g i n g a m b i e n t h u m i d i t i e s ( R u d o l p h & K n ü l l e , 1978) . 
I t is o b v i o u s t h a t v a p o u r u p t a k e takes p l ace w h e n e v e r a t ick is t r ans fe r red 
from lower to h i g h e r h u m i d i t i e s , even w h e n b o t h h u m i d i t i e s a r e a b o v e the 
cr i t ica l e q u i l i b r i u m . T h i s h a s a lso b e e n s h o w n for o t h e r ixodid a n d a rga s id 
t icks (Hafez et al., 1970a) . 
W e d o n o t k n o w the phys io log ica l b a s e for t h e a d j u s t m e n t of tick w a t e r 
c o n t e n t to e n v i r o n m e n t a l re la t ive h u m i d i t y . T h e r e m a y be n o phys io log ica l 
need for a d j u s t m e n t of b o d y w a t e r w i th in n a r r o w l imi ts . A c o n s i d e r a b l e 
t o l e r ance to c h a n g e s in h a e m o l y m p h o s m o l a r i t y a n d e lec t ro ly te c o n c e n t r a t i o n 
h a s b e e n r e c o r d e d for severa l ixod id tick species d u r i n g e n g o r g e m e n t . T h i s 
t o l e r ance h a s b e e n r e g a r d e d as a n in t r ins i c fea tu re of t icks (Lees , 1946a; 
T a t c h e l l , 1969; A r a m a n , 1972). F u r t h e r m o r e , unfed ticks c a n to l e ra t e losses 
of m o r e t h a n o n e - t h i r d of the i r to ta l b o d y w a t e r . T h e r e is l i t t le i n fo rma t ion 
on h o w this a m o u n t of w a t e r cou ld be r e m o v e d from b o d y fluids w i t h o u t 
h a r m f u l effects to t h e l iv ing t i ssues . T h e inves t iga t ions of C h i - Y e n et al. 
(1973) p r o v i d e s o m e i n d i c a t i o n t h a t t he h a e m o l y m p h ac ts as a w a t e r s to re 
in unfed t icks . T h e i r d a t a s h o w t h a t d e h y d r a t i o n a n d r e h y d r a t i o n a r e m e t 
by con t ro l l ed m o v e m e n t s of so lu tes a n d w a t e r b e t w e e n h a e m o l y m p h a n d 
n o n - h a e m o l y m p h c o m p a r t m e n t s . 
R e c o v e r y of a w a t e r deficit in t icks is faster a t h i g h e r h u m i d i t i e s a n d a lso 
faster a t h i g h e r t e m p e r a t u r e s . Pa r t i a l ly d e h y d r a t e d A. variegatum, for e x a m p l e , 
t ake u p w a t e r v a p o u r seven t imes faster a t 9 3 % R H t h a n a t 8 5 % R H . T h i s 
is a lso t h e case in o t h e r t ick species ; r ecovery of w a t e r deficits is a l w a y s faster 
a t h u m i d i t i e s n e a r t he s a t u r a t i o n po in t . U n d e r n a t u r a l cond i t i ons a fairly 
sho r t e x p o s u r e to h u m i d i t i e s close to s a t u r a t i o n will suffice for t he r e s t o r a t i o n 
of b o d y w a t e r deficits . N e t u p t a k e of w a t e r v a p o u r , however , does n o t t ake 
p l ace be low a lower t h r e s h o l d t e m p e r a t u r e w h i c h is a p p r o x i m a t e l y 9°C in 
/. ricinus (Lees , 1946b) a n d 5°C in Amblyomma americanum ( S a u e r & H a i r , 
1971) . 
Humidity Relationships and Water Balance of Ticks 57 
U p t a k e of w a t e r v a p o u r in t icks d e p e n d s o n the re la t ive h u m i d i t y of t he 
a t m o s p h e r e a n d , ev iden t ly , is i n d e p e n d e n t of t he a t m o s p h e r i c s a t u r a t i o n 
deficit. A t 9 0 % R H , a t 15°C a n d a t 25°C, respec t ive ly , t icks ach ieve s imi l a r 
e q u i l i b r i u m w a t e r c o n t e n t s a l t h o u g h t h e a t m o s p h e r i c s a t u r a t i o n deficit h a s 
a l m o s t d o u b l e d f rom 1.3 t o r r a t 15°C to 2.4 t o r r a t 25°C. T h i s impl i e s t h a t 
t h e ecological ly i m p o r t a n t t h r e s h o l d for n e t a b s o r p t i o n of a t m o s p h e r i c w a t e r 
v a p o u r is d e t e r m i n e d b y t h e re la t ive h u m i d i t y of the a m b i e n t a i r a n d is 
unaffected b y t e m p e r a t u r e w i t h i n a b r o a d r a n g e . T h i s d e p e n d e n c e m i g h t 
reflect t he n a t u r e of t h e u p t a k e m e c h a n i s m as will be d i scussed la te r . 
2.3.2. Uptake as an Act ive Process 
A tick m u s t o v e r c o m e a s t eep g r a d i e n t of w a t e r ac t iv i ty w h e n t r ans fe r r ing 
w a t e r f rom t h e a t m o s p h e r e , w h e r e it occu r s in low c o n c e n t r a t i o n , to its t issues 
a n d b o d y fluids w h i c h h a v e a c o n c e n t r a t i o n t h a t a l m o s t c o r r e s p o n d s to a 
1 0 0 % R H . T h e phys i ca l g r a d i e n t , w h i c h m u s t be reversed b y phys io log ica l 
m e a n s , b e c o m e s s t e e p e r w h e n the cr i t ica l e q u i l i b r i u m of t he tick species is 
lower . T h e m e c h a n i s m u n d e r l y i n g this r e m a r k a b l e p h e n o m e n o n is of c e n t r a l 
in te res t . 
I t is i m p o r t a n t to d e t e r m i n e w h e t h e r t he ac t ive u p t a k e m e c h a n i s m h a s a 
lower t h r e s h o l d of p e r f o r m a n c e . A t a first c o n s i d e r a t i o n it a p p e a r s t h a t a n y 
such l imi t s h o u l d co inc ide w i t h t he cr i t ica l e q u i l i b r i u m h u m i d i t y of e a c h tick 
species . T h i s is def ined a s t h e lowes t h u m i d i t y a t w h i c h a tick c a n res to re 
a w a t e r deficit b y a b s o r p t i o n of v a p o u r from t h e a t m o s p h e r e . H o w e v e r , t he 
so-cal led e q u i l i b r i u m b e t w e e n t h e w a t e r in t he o r g a n i s m a n d the e n v i r o m e n t a l 
h u m i d i t y is n o t s t a t i c . W a t e r is c o n t i n u o u s l y e s c a p i n g f rom the tick i n to t h e 
a t m o s p h e r e , b u t a t e q u i l i b r i u m th is w a t e r is r e p l a c e d b o t h by i n w a r d diffusion 
a n d b y ac t ive u p t a k e of v a p o u r . T h i s b a l a n c e , therefore , r e p r e s e n t s a s t e a d y 
s t a t e . 
Ac t ive v a p o u r u p t a k e m i g h t p r o c e e d a t h u m i d i t i e s be low the cr i t ica l 
e q u i l i b r i u m , b u t it w o u l d be concea l ed w h e n e v e r c o n c u r r e n t w a t e r losses 
exceeded t h e a m o u n t s a b s o r b e d , r e su l t i ng in ne t w a t e r loss. T h i s q u e s t i o n 
h a s b e e n reso lved w i t h t he use of t r i t i um- l abe l l ed w a t e r , a c o n s i d e r a t i o n of 
w a t e r e x c h a n g e k ine t ics , a n d t h e use of C O 2 as a tick a n a e s t h e s i a . T h e s e 
t e c h n i q u e s h a v e m a d e it poss ib le to s e p a r a t e t he ac t ive a n d pass ive m o v e -
m e n t s of w a t e r b e t w e e n l a r v a e of D. variabilis a n d the a t m o s p h e r e ( K n ü l l e 
& D e v i n e , 1972) . L a r v a e w e r e chosen b e c a u s e they h a v e n o sp i rac les ; a n y 
effect of C O 2 o n w a t e r loss d u e to t he o p e n i n g of t he sp i rac les c a n be exc luded 
from c o n s i d e r a t i o n . T h e d a t a f rom these e x p e r i m e n t s s u p p o r t t he hypo thes i s 
t h a t ac t ive a b s o r p t i o n of a t m o s p h e r i c w a t e r v a p o u r does not t ake p l ace over 
the en t i r e r a n g e of e n v i r o n m e n t a l h u m i d i t i e s ; i n w a r d t r a n s p o r t of v a p o u r 
ceased a t t h e re la t ive h u m i d i t y a t a n d be low w h i c h pa r t i a l l y d e h y d r a t e d 
58 W. Knülle and D. Rudolph 
l a r v a e c a n n o t a b s o r b lost w a t e r f rom the a i r . I n fact, th is m e a n s t h a t t h e 
cr i t ica l e q u i l i b r i u m h u m i d i t y i nd i ca t e s t he lower l imi t for ac t ive w a t e r u p t a k e . 
As a n ac t ive p roces s , u p t a k e of w a t e r v a p o u r is ene rgy d e m a n d i n g . T h e 
r e l at i o n s h i p b e t w e e n t h e cr i t ica l e q u i l i b r i u m h u m i d i t y a n d the ene rgy reserves 
h a s a l r e a d y b e e n m e n t i o n e d . D e p l e t i o n of t he rese rve s u b s t a n c e s in t he cou r se 
of l o c o m o t o r ac t iv i t ies m a y i m p a i r t he effectiveness of w a t e r v a p o u r u p t a k e 
as Lees (1964) h a s d e m o n s t r a t e d in / . ricinus. R e p e a t e d cycles of d e h y d r a t i o n 
a n d r e h y d r a t i o n l ikewise lead to a n acce l e r a t ed d e p l e t i o n of m e t a b o l i t e s , b y 
c o m p a r i s o n to i n d i v i d u a l s exposed c o n t i n u o u s l y to h u m i d i t i e s a b o v e t h e 
cr i t ica l e q u i l i b r i u m , as w a s s h o w n in Ornithodoras concanensis (Cook , 1973) . 
2.3.3. Site and Mechanism of Active Water Vapour Uptake 
I n t icks, as in o t h e r w a t e r - v a p o u r - a b s o r b i n g a r t h r o p o d s , t he l a rge surfaces 
of t he b o d y facing t h e s u r r o u n d i n g a i r — t h e gene ra l i n t e g u m e n t a n d t h e 
t r a c h e a l s y s t e m — h a v e b e e n sugges t ed as sites for u p t a k e of a t m o s p h e r i c 
w a t e r (Lees , 1946b; Be lozerov & Serav in , 1960; M c E n r o e , 1972) . B u t , su r -
pr i s ing ly , it h a s n o w b e e n d e m o n s t r a t e d t h a t t icks act ively a b s o r b w a t e r 
v a p o u r f rom t h e a i r v ia t h e m o u t h . Block ing the m o u t h w i t h paraff in w a x 
inh ib i t s w a t e r u p t a k e in ixod id t icks, a n d u n d e r these cond i t i ons t h e t icks 
lose w a t e r c o n t i n u o u s l y ( R u d o l p h & K n ü l l e , 1974; R u d o l p h , 1976; M c M u l l a n 
et al., 1976; R u d o l p h & K n ü l l e , 1978), t h u s it cou ld be inferred t h a t t h e 
m o u t h is invo lved in t he u p t a k e p rocess . T h e a c t u a l role of t he m o u t h reg ion 
w a s s h o w n by s i m u l t a n e o u s l y expos ing the r o s t r u m , w h i c h inc ludes t h e 
m o u t h , a n d t h e res t of t h e b o d y to s e p a r a t e a t m o s p h e r e s in different c o m -
p a r t m e n t s of a d o u b l e - c h a m b e r a r r a y (Fig. 2 .5; R u d o l p h , 1976). H u m i d i t y 
w a s ad ju s t ed to 9 3 % R H in b o t h c o m p a r t m e n t s b y d rop l e t s of s a t u r a t e d 
K N O 3 so lu t ion of k n o w n v o l u m e . T h e d r o p of K N O 3 so lu t ion a d j a c e n t to 
t he m o u t h p a r t s s h r u n k to a mo i s t c rys ta l of K N O 3 , whi le t h a t in t he t u b e 
c o n t a i n i n g t h e res t of t h e b o d y a p p e a r e d to be u n c h a n g e d . F r o m the a m o u n t 
of w a t e r w h i c h h a d e v a p o r a t e d from d r o p of K N O 3 so lu t ion , it w a s poss ib le 
Parafilm Parafilm Wax seal 
Μ KNO, „ | Ί — > I j I K N O . I 
w - — ^TXrsr^rsL—L 
Gauze 
11" "Jä^rsolütiorT 
FlG. 2.5. Double-chamber array for simultaneous exposure of the anterior and posterior portions 
of a tick to separated atmospheres. 
Humidity Relationships and Water Balance of Ticks 59 
to ca l cu l a t e t h e a m o u n t of w a t e r a b s o r b e d v ia t h e m o u t h . T h i s v a l u e a g r e e d 
well w i t h t h e n e t u p t a k e of u n r e s t r a i n e d con t ro l t icks ( R u d o l p h & K n ü l l e , 
1974, 1978) . 
P r e l i m i n a r y e x p e r i m e n t s w i t h a r g a s i d t icks s h o w e d t h a t b lock ing t h e m o u t h 
w i t h paraff in w a x i nh ib i t s u p t a k e of w a t e r v a p o u r from the a i r ( R u d o l p h , 
u n p u b l i s h e d o b s e r v a t i o n ) . I t is l ikely t h a t u p t a k e p r o c e e d s in a s imi l a r 
m a n n e r in b o t h famil ies of t icks . 
Severa l o b s e r v a t i o n s p o i n t to a s ignif icant ro le of tick sa l ivary secre t ion 
in a t m o s p h e r i c w a t e r v a p o u r u p t a k e . W h e n a d u l t A. variegatum a r e severely 
d e h y d r a t e d t hey secre te a c lea r fluid w h i c h a c c u m u l a t e s b e t w e e n the m o u t h -
p a r t s a n d t h e p a l p s . I n low re la t ive h u m i d i t i e s th is qu ick ly d r i es to a w h i t e 
c rys ta l l ine sol id , b u t w h e n these t icks a r e t r ans fe r r ed to h u m i d i t i e s a b o v e 
t h e cr i t ica l e q u i l i b r i u m h u m i d i t y t h e c rys ta l l ine solid takes u p s u b s t a n t i a l 
a m o u n t s of w a t e r . W i t h i n a few seconds it d issolves a n d is s u b s e q u e n t l y 
i m b i b e d . T h e sec re ted s u b s t a n c e is h igh ly hyg roscop i c a t h u m i d i t i e s close 
to a n d a b o v e t h e cr i t ica l e q u i l i b r i u m h u m i d i t y of t h e tick, w h i c h is b e t w e e n 
8 0 % a n d 8 5 % R H . 
T h e r e is g o o d ev idence t h a t t h e e jected fluid o r ig ina te s in t h e sa l iva ry 
g l a n d s . I n t icks w i t h t h e che l i ce rae a n d t h e d o r s a l p a r t of t he c a p i t u l u m 
r e m o v e d , t h e b u c c a l cav i ty , t h e s a l i v a r i u m a n d t h e e n t e r i n g sa l iva ry d u c t s 
c a n b e o b s e r v e d . I n all t icks , r ega rd l e s s of w a t e r s t a t u s , a c lear sa l iva ry 
sec re t ion c a n b e seen in t h e s a l i v a r i u m a n d t h e sa l ivary d u c t s . F u t h e r m o r e , 
a p e r i o d i c p u l s a t i o n s in t h e r eg ion of t h e s a l i v a r i u m eject t he secre t ion fo rward 
u p o n t h e d o r s a l sur face of t h e h y p o s t o m e , f requen t ly u p to its d i s t a l po in t . 
S u b s e q u e n t l y , t h e sec re t ion is w i t h d r a w n in to t he p h a r y n g e a l orifice loca ted 
be low t h e s a l i v a r i u m . After e a c h eject ion u n d e r s u b e q u i l i b r i u m h u m i d i t i e s , 
s m a l l a m o u n t s of fluid c rys ta l l ize a n d a c c u m u l a t e o n t he d o r s a l surface of 
t h e h y p o s t o m e . T h e c rys ta l l ine s u b s t a n c e r e s e m b l e s t h a t seen in ex t r eme ly 
d e h y d r a t e d t icks as d e s c r i b e d a b o v e . P u m p m o v e m e n t s in t he reg ion of t he 
p h a r y n x c a n a l so b e seen in u n t r e a t e d t icks a n d fluid c a n be obse rved in t he 
b u c c a l c h a n n e l ( R u d o l p h , 1976) w h i c h p r o b a b l y i nd i ca t e s t h a t t h e h y d r a t e d 
sa l iva is s w a l l o w e d a n d p a s s e d i n t o t h e m i d g u t . 
S tud ie s o n t h e fine s t r u c t u r e of t ick sa l iva ry g l a n d s h a v e r evea led cells 
w i t h u l t r a s t r u c t u r a l fea tures c o m m o n to fluid-transporting ep i the l i a ( D z h a -
farov, 1965; K i r k l a n d , 1971; C o o n s & R o s h d y , 1973; M e r e d i t h & K a u f m a n , 
1973; D i e h l & M e g a w as q u o t e d in S a u e r , 1977; S a u e r , 1977; M e g a w & 
B e a d l e , 1979) . T h e m o s t c o n s p i c u o u s of these cells m a k e u p the a g r a n u l a r 
t ype I a lveol i . T h e s e cells exh ib i t ex tens ive infoldings of t he b a s a l p l a s m a 
m e m b r a n e w h i c h forms d e e p e x t r a c e l l u l a r c h a n n e l s , w h o s e o p e n e n d s face 
t he h a e m o l y m p h ; closely a s soc i a t ed w i t h these c h a n n e l s a r e l a rge n u m b e r s 
of m i t o c h o n d r i a . A s econd g r o u p of ce l l s—the so-cal led " w a t e r - c e l l s " — i s 
found in t h e g r a n u l a r a lveol i of e n g o r g i n g t icks (see C h a p t e r 7) . 
T h e r e a r e severa l r e a s o n s for a s s u m i n g t h a t a g r a n u l a r t ype I alveoli 
60 W. Knülle and D. Rudolph 
c o n t r i b u t e to t h e p r o d u c t i o n of t he o ra l secre t ion involved in t he u p t a k e of 
a t m o s p h e r i c w a t e r v a p o u r . I n a d u l t Hyalomma anatolicum excavatum t he sa l iva ryg l a n d alveoli of types I I a n d I I I c a n be comple t e ly de s t royed by h e a v y 
infect ions of Theileria annulata, l eav ing on ly the two m a i n sa l ivary d u c t s w i t h 
t he a g r a n u l a r t ype I alveoli i n t ac t . T h e s e severely infected ticks w e r e still 
ab l e to a b s o r b w a t e r v a p o u r , a l t h o u g h a t a r e d u c e d r a t e in c o m p a r i s o n to 
the un infec ted con t ro l s ( R u d o l p h , 1976). A l t h o u g h the i n v o l v e m e n t of t ype 
I alveoli in w a t e r a n d ion e l im ina t i on in feeding ixod ids h a s been r e p e a t e d l y 
s u s p e c t e d (Ba l a shov , 1968; K i r k l a n d , 1971), n o func t iona l i n v o l v e m e n t w a s 
revea led by u l t r a s t r u c t u r a l inves t iga t ions on the sa l ivary g l a n d s of e n g o r g i n g 
ticks ( B i n n i n g t o n , 1978; M e g a w & Bead le , 1979). C o o n s a n d R o s h d y (1979) 
even found a s ignif icant d e c r e a s e in t h e size of t ype I alveoli d u r i n g feeding 
in t h r ee species of ixodid t icks; the type I alveoli w e r e smal l e s t in t he final 
p h a s e of r a p i d e n g o r g e m e n t w h e n w a t e r a n d ion e l imina t ion w a s h ighes t . 
M o r e o v e r , t h e r e is good s u p p o r t for t he hypo thes i s t h a t t he w a t e r cells of t he 
alveoli I I a n d I I I , w h i c h d e v e l o p d u r i n g feeding, excre te t he bu lk of w a t e r 
d u r i n g e n g o r g e m e n t ( M e r e d i t h & K a u f m a n , 1973; B i n n i n g t o n , 1978; M e g a w 
& Bead l e , 1979; C o o n s & R o s h d y , 1979; see C h a p t e r 7) . 
I t a p p e a r s t h a t a fu r the r u n d e r s t a n d i n g of t h e co mp o s i t i o n a n d g e n e r a t i o n 
of t he s u s p e c t e d w a t e r a b s o r b i n g o ra l secre t ion of unfed ticks c a n be expec ted 
by focusing a t t e n t i o n on the t r a n s p o r t ep i the l i a of the a g r a n u l a r alveoli . 
T i c k s in b o t h famil ies a r e well e q u i p p e d w i th such alveoli . T h e y a r e confined 
to t h e a n t e r i o r r eg ion of t he sa l ivary g l a n d a n d a r e a t t a c h e d d i rec t ly to t he 
m a i n d u c t s . M e g a w a n d B e a d l e (1979) h a v e s h o w n t h a t t he gene ra l a p p e a r -
a n c e of t he p y r a m i d a l cells of these alveoli is r e m i n i s c e n t of the ep i the l ia l 
cells in t he a v i a n sal t g l a n d . T h e m o r p h o l o g i c a l cha rac te r i s t i c s a r e ind ica t ive 
of ep i the l i a invo lved in the secre t ion of h y p e r o s m o t i c fluids. W h i l e these 
alveoli a p p e a r to d e g e n e r a t e in feeding ixod ids , w h i c h a lso a p p e a r to lose 
the i r ab i l i ty to a b s o r b w a t e r v a p o u r from the a i r d u r i n g a n d after engo rge -
m e n t , n o r e d u c t i o n in t he size of t ype I alveoli w a s found in feeding Argas 
arboreus ( C o o n s & R o s h d y , 1979). I t is well k n o w n t h a t a d u l t a rga s id t icks 
t ake severa l b lood m e a l s a n d a r e a b l e to m a i n t a i n the i r w a t e r b a l a n c e for 
a c o n s i d e r a b l e t i m e b e t w e e n these m e a l s . 
I t w o u l d a p p e a r to be phys io logica l ly o v e r d e m a n d i n g to expec t t h a t w a t e r 
ac t iv i ty cou ld b e l owered to ca. 0 . 7 5 — c o r r e s p o n d i n g to the lowest cr i t ical 
e q u i l i b r i u m h u m i d i t y in t i ck s—wi th in t he t r a n s p o r t e p i t h e l i u m by ion con-
c e n t r a t i o n a l o n e , o r t h a t it cou ld o c c u r w i th in ex t r ace l lu l a r c h a n n e l c o m -
p a r t m e n t s . After e ject ion a n d e x p o s u r e to a m b i e n t cond i t i ons , o r g a n i c so lu tes 
in t h e sa l iva ry secre t ion m i g h t p r o v i d e the r e d u c e d w a t e r ac t iv i ty a n d the 
r e q u i r e d l o w e r i n g of w a t e r - v a p o u r p r e s s u r e . T h e q u a n t i t i e s of fresh sa l iva 
sec re ted by t h e fas t ing tick a r e min i scu le , a n d it h a s no t been poss ib le as yet 
to o b t a i n i n f o r m a t i o n on its w a t e r ac t iv i ty . 
S o m e c o n s t i t u e n t s of t he a c c u m u l a t e d solid r e s idue of sa l iva o n the r o s t r u m 
Humidity Relationships and Water Balance of Ticks 61 
of fas t ing Amblyomma variegatum i n d i v i d u a l s w h i c h w e r e exposed to d r y a i r for 
severa l weeks h a v e b e e n a n a l y s e d recen t ly (Dev ine , in p r e p a r a t i o n ) . T h e 
m o l a r p r o p o r t i o n s of s o d i u m , p o t a s s i u m , a n d ch lo r ine a r e 1 .1 :1 .7 :3 .8 . T h e s e 
ra t ios a r e q u i t e different from those in t h e sa l iva of feeding t icks, 1 .8 :0 .1 :1 .4 , 
w h i c h excre te excess w a t e r a n d ions inges ted w i t h t he b lood m e a l ( H s u & 
S a u e r , 1975) . I t a p p e a r s t h a t sa l iva ry eject ion by fast ing a n d d e h y d r a t i n g 
ticks is l ikewise bes t c o n s i d e r e d as a n e l im ina t i on of excess ions d u r i n g 
pe r iods w h e n o s m o t i c a n d ionic s t ress resu l t s from w a t e r losses. S u c h o s m o -
r e g u l a t o r y sa l iva t ions m i g h t d e t e r m i n e t he p a t t e r n of v a p o u r a b s o r p t i o n in 
d e h y d r a t i n g t icks . 
T h e finding t h a t t h e t h r e s h o l d for ac t ive v a p o u r u p t a k e in t icks is r e l a t ed 
to t he re la t ive h u m i d i t y of t h e a t m o s p h e r e a n d is no t m a r k e d l y inf luenced 
by t e m p e r a t u r e w i t h i n a b r o a d r a n g e m a y reflect t he n a t u r e of t he v a p o u r 
p r e s s u r e l ower ing p r inc ip l e . I f v a p o u r a b s o r p t i o n is a c c o m p l i s h e d by m e a n s 
of a c o n c e n t r a t e d a n d h y g r o s c o p i c so lu t ion d i s p l a y i n g p r o p e r t i e s c o m m o n 
for s u c h so lu t ions , w e w o u l d , i n d e e d , expec t a r a t h e r t e m p e r a t u r e i n d e p e n d e n t 
re la t ive h u m i d i t y t h r e s h o l d for v a p o u r a b s o r p t i o n . W h a t e v e r t he v a p o u r 
p r e s s u r e l ower ing p r i nc ip l e m i g h t be , it is b iological ly feasible to sugges t t h a t 
t he e n t r a i n e d w a t e r , fol lowing inges t ion of t he sa l iva , cou ld be d i l u t ed w i th in 
t he g u t u n d e r t h e inf luence of t h e i n t e r n a l e n v i r o n m e n t o r re leased by 
m e t a b o l i c d e g r a d a t i o n a n d s u b s e q u e n t l y a b s o r b e d in to t he h a e m o l y m p h by 
c o u p l e d so lu te t r a n s p o r t . 
T h e v a p o u r u p t a k e sy s t ems found a m o n g a r t h r o p o d s a r e of i n d e p e n d e n t 
e v o l u t i o n a r y or ig in . E v e n in those cases w h e r e t he s a m e gene ra l si te of 
c o n d e n s a t i o n is i m p l i c a t e d , t he s t r u c t u r a l a r r a n g e m e n t s , func t iona l c h a r a c -
ter is t ics , a n d phys io log ica l p r o p e r t i e s of t he a b s o r b i n g sys t em a r e p ro found ly 
different. T h i s is o b v i o u s w h e n o ra l u p t a k e in t icks is c o m p a r e d w i t h t h a t 
in o t h e r m i t e s a n d in t he c o c k r o a c h Arenivaga. T h e l a t t e r possesses a ve ry 
spec ia l ized s y s t e m invo lv ing t w o p r o t r u s i b l e b l adde r - l i ke h y p o p h a r y n g e a l 
s t r u c t u r e s o n e i t he r s ide of t h e m o u t h , covered w i t h a t h in l ayer of h y g r o s c o p i c 
fluid w h i c h is s u p p l i e d from g l a n d s loca ted on the ins ide of t he l a b r u m 
( O ' D o n n e l l , 1977a, b , 1978; M a c h i n , 1979). I n h o u s e d u s t mi t e s of t h e family 
Pyroglyphidae a n d in r e l a t ed s to red p r o d u c t m i t e s , up t a k e a p p e a r s to b e 
m e d i a t e d by a h y g r o s c o p i c sec re t ion of t h e s u p r a c o x a l g l a n d s . I t is a s s u m e d , 
t h a t th is sec re t ion is exposed to t he a m b i e n t a i r o n the p a i r e d s u p r a c o x a l 
p l a t e s . T h e v a p o u r e n r i c h e d fluid t h e n flows via t he p o d o c e p h a l i c c a n a l i n to 
t he p r e b u c c a l cav i ty a n d is finally p u m p e d by a m u s c u l a r p h a r y n x in to the 
m i d g u t ( W h a r t o n & F u r u m i z o , 1977; W h a r t o n , 1978) . L ikewise , rec ta l 
u p t a k e of v a p o u r in t h e t h y s a n u r a n Thermobia differs f rom t h a t in l a r v a e of 
t he bee t l e Tenebrio. I n t h e fo rmer , t he t r a n s p o r t e p i t h e l i u m of a s e p a r a t e d 
pos t e r io r r eg ion of t he r e c t u m , t he a n a l sacs , is involved ( N o b l e - N e s b i t t , 
1970a, b , 1975, 1977, 1978; N o i r o t & N o i r o t - T i m o t h é e , 1971). I n t h e la t te r , 
u p t a k e is m e d i a t e d t h r o u g h a c o m p l e x s t r u c t u r a l a r r a n g e m e n t , k n o w n as t h e 
62 W. Knülle and D. Rudolph 
c r y p t o n e p h r i c sys t em, w h i c h encloses t he e n d s of t he M a l p i g h i a n t u b u l e s in 
a n e n s h e a t h e d c o m p a r t m e n t s u r r o u n d i n g the pos t e r io r p a r t of t he r e c t u m 
( R a m s a y , 1964, 1971; G r i m s t o n e et al, 1968; N o b l e - N e s b i t t , 1970a, 1973, 
1977; D u n b a r & W i n s t o n , 1975; M a c h i n , 1975, 1976, 1978, 1979) . 
I n s o m e species , spec ia l ized s t r u c t u r e s a n d essent ia l c o m p o n e n t s of t he 
m e c h a n i s m m a y h a v e b e e n evolved for t he p r i m a r y p u r p o s e of c o m p e n s a t i o n 
for w a t e r loss by u p t a k e of a t m o s p h e r i c w a t e r , as a p p e a r s to be the case o n 
Arenivaga. I n o t h e r s , a s in Tenebrio a n d in t icks, t he s i t ua t ion seems equ ivoca l . 
U p t a k e in Tenebrio cou ld b e v iewed as a n a c c o m p a n i m e n t of t he m e c h a n i s m 
of faecal d e h y d r a t i o n . T h i s m i g h t be sugges ted by the fact t h a t fas t ing l a r v a e 
lack a d e q u a t e m e a n s of r e g u l a t i n g a t m o s p h e r i c v a p o u r u p t a k e d u r i n g p r o -
longed e x p o s u r e to h i g h h u m i d i t i e s ( M a c h i n , 1975) . I n fas t ing t icks, sa l iva t ion 
m a y b e a t t u n e d to t h e t icks o s m o r e g u l a t o r y needs a n d factors con t ro l l i ng t h e 
secre t ion of sa l iva cou ld a lso d e t e r m i n e t he p a t t e r n of v a p o u r a b s o r p t i o n . 
T h e d i s c o n t i n u o u s u p t a k e p h a s e s seen in A. variegatum a n d o t h e r t icks u n d e r 
e q u i l i b r i u m c o n d i t i o n s m a y reflect th is t u n i n g (Fig. 2.3) a n d m i g h t i n d i c a t e 
t h a t p r o d u c t i o n of v a p o u r - a b s o r b i n g secre t ions is i n t e r m i t t e n t . Amblyomma 
variegatum is a b l e to ad jus t i ts m e a n b o d y w a t e r c o n t e n t a t e q u i l i b r i u m wi th in 
l imi ts of ± 2 % . T h i s m a y c o r r e s p o n d w i t h t h r e s h o l d va lues w h i c h , w h e n 
s u r p a s s e d , t r igger o s m o r e g u l a t o r y r e sponses by the sa l ivary g l a n d s . 
2.4. E C O L O G I C A L AND B E H A V I O U R A L I M P L I C A T I O N S 
I n n a t u r e , b e h a v i o u r a l m e c h a n i s m s m a k e a m a j o r c o n t r i b u t i o n to t ick 
w a t e r b a l a n c e . T h e suscep t ib i l i ty of t he eggs to des icca t ion seems n o t to be 
c ruc ia l b e c a u s e t h e r ep le t e female c a n select f avourab le m i c r o h a b i t a t s for 
egg d e p o s i t i o n w h i c h bes t p r o t e c t eggs from low h u m i d i t i e s a n d h igh t e m -
p e r a t u r e s (Pa t r i ck & H a i r , 1979). F u r t h e r m o r e , s y n c h r o n i z a t i o n of egg 
depos i t i on w i t h t he b e g i n n i n g of t he r a i n y season a p p e a r s to be i m p o r t a n t , 
b o t h for egg su rv iva l a n d d e v e l o p m e n t of t he l a r v a e in s o m e tick species 
l iv ing in s emi -a r id reg ions ( O s m a n , 1978). T h e l a rvae of m a n y ixodid t icks 
r e m a i n in t h e i m m e d i a t e n e i g h b o u r h o o d of t he site of egg depos i t i on a n d 
t h u s a r e no t conf ron ted w i t h e x t r e m e c l ima t i c cond i t i ons . T h i s is, howeve r , 
no t t h e case in those i n s t a r s p re fe r r ing l a rge r hos t a n i m a l s , e.g. t he a d u l t s 
a n d n y m p h s , w h i c h in the cour se of hos t seeking a r e forced to s p e n d a 
c o n s i d e r a b l e p r o p o r t i o n of t he n o n - p a r a s i t i c p h a s e s exposed to h u m i d i t i e s 
well be low t h e cr i t ica l e q u i l i b r i u m h u m i d i t y . D u r i n g the hos t - seek ing p h a s e , 
m a n y ixod id t icks l iv ing in p l a n t covered h a b i t a t s m o v e to t he t ips of g rass 
s t e m s w h e r e they d i sp l ay q u e s t i n g b e h a v i o u r . After a pe r iod of unsuccessful 
q u e s t i n g they m a y d e s c e n d to the g r o u n d l i t ter . S u c h pe r iod ic ver t ica l 
m o v e m e n t s , from g r o u n d l i t ter to g rass t ips a n d b a c k to the soil cover h a v e 
b e e n extens ive ly s t u d i e d in a d u l t s of Ixodes ricinus a n d / . persulcatus (Lees & 
Humidity Relationships and Water Balance of Ticks 63 
M i l n e , 1951; K h e i s i n , 1953) a n d in l a r v a e of Haemaphysalis leporispalustris 
( C a m i n & D r e n n e r , 1978) . 
W h e n exposed on g r a s s t ips to d e h y d r a t i n g cond i t i ons be low its cr i t ica l 
e q u i l i b r i u m h u m i d i t y ( 5 0 % R H a t 25°C) , / . ricinus a d u l t s r a p i d l y lose w a t e r 
a t t h e r a t e of a b o u t 1 0 % of t he i r o r ig ina l we igh t p e r d a y (Lees , 1946b) . 
R e p e a t e d l y , after los ing a ce r t a in a m o u n t of w a t e r , t he tick r e t u r n s to t h e 
b a s e of t h e g r a s s . H e r e t h e h u m i d i t y c o n d i t i o n s a r e close to s a t u r a t i o n a n d 
the tick c a n r ep l en i sh its w a t e r deficit b y a b s o r p t i o n of v a p o u r from the 
a t m o s p h e r e (Lees , 1948) . 
A s c e n d i n g a n d d e s c e n d i n g m o v e m e n t s o n t he vege ta t ion s h o u l d no t b e 
seen exclus ively as a c o n s e q u e n c e of a d i r ec t ed h u m i d i t y r e sponse . T h e 
ver t ica l m o v e m e n t s in l a r v a e of H. le pons palustris, for e x a m p l e , a r e con t ro l l ed 
p r i m a r i l y b y a c o m b i n a t i o n of r e sponses to h u m i d i t y a n d l ight . U n f e d , 
w a t e r - s a t u r a t e d l a r v a e a r e p h o t o p o s i t i v e a n d r e s p o n d nega t ive ly to m o i s t u r e . 
T h i s c o m b i n a t i o n of r e s p o n s e s g u i d e s t h e tick o u t of t he moi s t m a t a n d u p 
the v e g e t a t i o n . If n o hos t is c o n t a c t e d , des i cca t ion occu r s after a few h o u r s , 
d e p e n d i n g on t h e a m b i e n t h u m i d i t y , a n d the tick b e c o m e s insens i t ive to l ight 
a n d r e s p o n d s pos i t ive ly to m o i s t u r e . T h e tick t h e n follows the h u m i d i t y 
g r a d i e n t d o w n t h e v e g e t a t i o n to t h e m a t , w h e r e it c a n res to re its w a t e r 
b a l a n c e b y a b s o r p t i o n of a t m o s p h e r i c w a t e r v a p o u r . U p o n s a t u r a t i o n , t he 
tick c a n r e t u r n to h i g h e r v e g e t a t i o n . O n l y w a t e r - s a t u r a t e d t icks a s c e n d the 
vege t a t i on ( C a m i n & D r e n n e r , 1978) . V e r t i c a l m o v e m e n t s of t icks m a y a lso 
b