1. Ticks owe their impermeability primarily to a superficial layer of wax in the epicuticle. After exposure to increasing temperatures, water loss increases abruptly at a certain ‘critical temperature’. The critical temperature varies widely in different species, in Ixodidae ranging from 32 (Ixodes ricinus) to 45° C. (Hyalomma savignyi); and in Argasidae from 63 (Ornithodorus moubata) to 75° C. (O. savignyi). Species having higher critical temperatures are more resistant to desiccation at temperatures within the biological range. A broad correlation is possible between these powers of resistance and the natural 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 inhibited by the abrasion of only part of the total cuticle surface--a fact which suggests that the cells are functionally 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.
4. In Argasidae the epicuticle consists of four layers: the cuticulin, polyphenol, wax and outer cement layers. Only the three inner layers are present in Ixodidae. 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 Argasidae the cement layer is very resistant to extraction but is broken down by boiling chloroform.
5. The cuticulin, polyphenol and wax layers are all secreted by the epidermal cells. The waterproofing layer, which is deposited on the completed polyphenol layer, is secreted by the moulting tick relatively early in development and may be nearly complete by the time moulting fluid is abundant. In Ornithodorus moubata the cement is poured out by the dermal glands shortly after emergence. In Ixodidae the dermal glands undergo a complex cycle of growth and degeneration, but their products appear to add nothing of functional significance to the substance of the cuticle.
The epicuticle in an insect,
Rhodnius prolixus
(Hemiptera)
