56 



Comparative Animal Physiology 



stage of development, diet, humidity, season of year, and other factors, but the 

 order of magnitude is illustrated in Table 4, showing the freezing points. -^'^ 



The principal loss of water from insects is by evaporation, and the most 

 important mechanisms of water regulation are those limiting evaporative loss. 

 Insects with a soft cuticle, such as many larvae, undoubtedly lose considerable 

 water directly through the body surface. The chitin of most insects is nearly 

 water impermeable, except at high temperatures at which changes in the 

 cuticular wax occur. ^^^ The greatest water loss is through the spiracles. 

 When the spiracles are plugged by wax the water loss is reduced by about 

 two-thirds. Most of the remaining loss occurs at the intersegmental furrows 



TABLE 4 



(Bomhyx vtori; Gastrimargus-^^) . Whenever spiracles are kept open, as by 

 exposure to COo (in Rhodnius^^^^ or by increase in metabolism by activity 

 (in Blattd-'^^^, the water loss is increased many times. Those insects which 

 lack mechanisms for closing their spiracles lose more water than those which 

 have spiracular control. Most insects excrete their nitrogenous wastes and 

 either eliminate or retain feces in semisolid form, hence they lose little water 

 as solvent for wastes and in air have no problem of salt loss or retention. It is 

 probable -*^ that dilute urine is formed in the malpighian tubules and water 

 reabsorbed in the rectal glands. 



Since most water loss is by evaporation, the effects of relative humidity, air 

 movement, and temperature are important. For example, Mellanby ^"^ found 

 that adult bedbugs (Cimex lectularins) at 8° lose approximately 33 per cent 

 of their body weight by evaporation in 4 hours at per cent humidity and 

 about 12 per cent in the same time at 90 per cent humidity. Grasshopper 

 nymphs, potato beetles, and meal worms, but not lepidopteran caterpillars, 

 may actually take up water at humidity above 82 per cent.^''^' ^^*' This hygro- 

 scopic property of some insects deserves more study. If water loss is normally 

 low, death occurs at the same temperatures at all humidities; if water loss is 

 normally high, the insect CPedicidus, Lucilia, and others) can tolerate a higher 

 temperature at high humidity than at low humidity^"''' "* (Figs. 88 and 89). 

 The water content of meal moth larvae but not of pupae or adults varies with 

 the humidity.^'* In cockroaches, fleas, and butterflies, surface evaporation is 

 proportional to saturation deficiency of the air (difference in mm.Hg vapor 

 pressure between saturation and actual humidity) up to about 30°,^-^' -°^' 

 and survival time is inversely related to water loss. At higher temperatures 

 water loss in Blatta with spiracles plugged increases very sharply owing to 



