(Ornithodoros moubata ) to 75°C. (O. savignyi }. Species 

 having higher critical teraperatvires are more resistant 

 to desiccation at temperatiires within the biological 

 range. A broad -correlation is possible between these 

 powers of resistance and the natiiral choice of habitat, 

 Argasidae infest dry, dusty situations whereas Ixodidae 

 occupy a much wider variety of ecological niches. 



"*2. If the tick cuticle is rubbed with abrasive 

 dust, evaporation is enormously increased. Living 

 ticks partially restore their impermeability in moist 

 air by secreting wax from the pore canals on to the 

 surface of the damaged cuticle. 



'"3. Unfed ticks are able to take up water rapidly 

 through the wax layer when exposed to high humidities. 

 Water uptake, which is dependent on the secretory 

 activities of the epidermal cells, is completely in- 

 hibited by the abrasion of only part of the total cuticle 

 svtrf ace _ a fact which sxiggests that the cells are func- 

 tionally interconnected. Resistance to desiccation at 

 low humidities is achieved by a dual mechanism: active 

 secretion and the physical retention of water by the 

 wax layer, 



"%. In Argasidae the epicuticle consists of four 

 layers: the cuticulin, polyphenol, wax, and outer cement 

 layers. Only the three inner layers are present in Ixo- 

 didae, Since the wax layer is freely exposed in the 

 latter group, chloroform and detergents have a marked 

 action in increasing transpiration, particularly in 

 those species with low critical temperatures. In Ar- 

 gasidae the cement layer is very resistant to extrac- 

 tion but is broken down by boiling chloroform, 



"'5. The cuticulin, polyphenol, and wax layers 

 are all secreted by the epidermal cells. The water- 

 proofing layer, which is deposited on the completed 

 polyphenol layer, is secreted by the molting tick 

 relatively early in development and may be nearly 

 complete by the time limiting fluid is abundant. In 

 0. moubata the cement is poured out by the dermal 



- 155 - 



