'"In considering the mechEinisras involved in the ex- 

 change of water throiogh the cuticle the assiimption wsls 

 made that, in addition to active secretion, the passage 

 of water, and particularly its retention, is also in- 

 fluenced by the presence of lipoid material in the cuticle. 

 Ticks show great diversity in their powers of resisting 

 desiccation, and this was thought to be accounted for by 

 the specific nature of the waterproofing lipoid. Never- 

 theless, no direct evidence of such a conpDnent was ad- 

 vanced in this paper (i.e.. Lees l%6iA). 



"'Ramsay (1935B^, and more recently Wigglesworth 

 (194-5) and Beament (19A5), have shown that the imper- 

 meability of insects is entirely due to a thin, discrete 

 layer of wax or oil in the outermost part of the epi— 

 cuticle. Any agents such as abrasive dusts, wax sol- 

 vents, or detergents, which interrupt the continuity of 

 this layer, at the same time greatly increase transpira- 

 tion. Water loss through the wax layer is also enor- 

 mously increased if the temperature is raised above a 

 certain critical value methods devised by Wiggles- 

 worth for demonstrating the properties of the waterproofing 

 layers in insects have been applied to a number of species 

 of ticks observations on the structure and depo- 

 sition of the epicuticle, and on the fvmctions of the 

 dermal glands (are provided). The outermost layer of 

 the tick cuticle visible in ordinary sections has hither- 

 to been referred to as the '"tectostracunf" (Ruser 1933) 



(but) the similarity of this layer with the insect 



epicuticle is so marked that the abandonment of this term 

 seems fully justified.'" 



The results and conclusions of this work, Lees summarized as 

 follows; 



'"1. Ticks owe their impermeability primarily to a 

 superficial layer of wax in the epicuticle. After expo- 

 sure to increasing temperatures, water loss increases 

 abruptly at a certain critical temperatiure . The critical 

 temperature varies widely in different species, in Ixodidae 

 ranging from 32°C . (Ixodes ricinus ) to /V5°C . / Hyalomma 

 marginatxim (= savignyi V /; and in Argasidae from b30C . 



- 15^- 



