WATER LOSS 229 



the insects. It is concluded, therefore, that meal-worms are able 

 to gain water from an atmosphere which is 90 per cent, saturated. 

 Other instances of a kindred nature are also quoted by Buxton in 

 his general review on humidity (1932). That the eggs of some 

 insects also absorb water is certain. Roonwal (1936), for example, 

 has recently shown that in the African migratory locust (Locusta 

 migratoria migratorioides) the water content of the egg increases 

 from about 52 per cent, of the net weight in the freshly laid egg to 

 82 per cent, in the fully developed egg. The water in this case is 

 most probably absorbed from the surrounding soil in which the 

 eggs are laid. 



Both climatic conditions and metabolism affect the rate at 

 which water is lost from the bodies of insects. This rate of water 

 loss, when measured in air of different humidities, bears a general 

 relation to the saturation deficiency of the air. According to 

 Mellanby (1935) practically all the water evaporated from the 

 body of an insect is lost through the tracheal system. This 

 conclusion is disputed by Ramsay (1935b), who showed that in 

 Periplaneta a considerable proportion of the water given off is 

 evaporated from the body-surface and at 30° C. a sudden increase 

 in the rate of evaporation occurs. This phenomenon, he states, 

 results from the presence of a thin film of fatty substance on the 

 body-surface which undergoes a change of state at that temperature 

 in that it shows a greatly increased permeability to water. Many 

 insects can reduce their body temperature below that of sur- 

 rounding air when the latter is tolerably dry and hot, and this 

 temperature regulation is brought about by evaporation. 



The existence of an insect in an atmosphere approaching 

 saturation depends on the excretion of water through the 

 Malpighian tubes and in the faeces. On the other hand, in an 

 insect living under extremely dry conditions, any loss of water 

 via the alimentary canal is practically nil. At the same time, by 

 excreting solid uric acid, which is insoluble, water is conserved 

 and any water present in the hind intestine appears to be resorbed 

 by means of the rectal papillae (Wigglesworth, 1932). The 

 regulation of the water content of the insect organism is, as 

 already mentioned (p. 222), also important in connection with its 



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