72 INSECT PHYSIOLOGY 



kept in air with a relative humidity of 90 or 95 per cent., it is able, by 

 some mechanism that is not fully explained, to absorb water from 

 the atmosphere through the cuticle; but this property is very un- 

 usual; most insects cannot absorb water vapour even when the air 

 is saturated. On the other hand, the loss of water (when this occurs 

 by evaporation as opposed to excretion (p. 54) ) is markedly affected 

 by the water vapour in the air. The rate of loss, like that from in- 

 animate bodies, has been found, within certain limits, to be more or 

 less proportional to the saturation deficiency of the air. It is not, of 

 course, to be expected that such a law should hold very exactly, 

 especially at varying temperatures, because evaporation occurs 

 chiefly from the tracheal system, the ventilation of which varies with 

 the intensity of metabolism in the insect and so with the external 

 temperature (p. 16). 



But apart from the importance of water as a constituent of the 

 tissues, it plays a significant part in the reaction of the insect to 

 extremes of temperature in its environment. When exposed to high 

 temperatures, an insect may be killed (depending upon the condi- 

 tions of experiment) by the temperature itself or by the desiccation 

 consequent upon evaporation. Provided the insect in question is 

 sufficiently large, it will be able to lower its temperature below the 

 fatal level (for short periods at any rate) by evaporating water from 

 its body, and thus to exercise a limited degree of temperature control. 

 Most insects die when their water content is reduced to about 50 

 per cent, of the total body weight. But certain insects can survive in 

 a state of suspended animation or 'cryptobiosis', after losing virtually 

 all their water. This happens in the eggs of the Collembolan Smin- 

 thurus. The best known example among insect larvae is the Chirono- 

 mid Polypedilum which will survive in a desiccated state, with its 

 water content reduced to below 8 per cent., in the dried mud of its 

 breeding places, and can be kept in this condition for several years. 

 They will then withstand temperatures ranging from — 190° C. 

 (liquid air) up to 102-104° C. When moistened they quickly take up 

 water and recover. 



Cold resistance 



It is usual for insects to reduce their water content before going into 



