Water 5-7 



changes in cuticular wax, whereas the tracheal loss increases gradually and 

 very slowly with temperature. In other insects there is less correlation between 

 water loss and saturation deficiency.^-J- i" The abihty of insects to survive 

 starvation depends on both temperature and desiccation; these two factors 

 probably act on different mechanisms and can be separated. At the same rela- 

 tive humidity the absolute vapor pressure of the atmosphere increases with 

 higher temperature, hence absolute humidity is more important than relative 

 humidity.^-' ^^^ 



Lost water is replaced by drinking in land insects and most insects take 

 water in the food. However, some insects, such as clothes moths, wax moth 

 larvae, and meal worms, get most of their required water metabolically. The 

 complete oxidation of 100 grams of carbohydrate (glucose) yields 60 grams 

 of water. Larvae of the flour moth (Ephestia) and of the beetles Triholium 

 and Dermestes, and also Tenehrio, eat more and yet grow more slowly at low 

 humidity. '^^ ^"^^ 



Another adaptive mechanism in osmotic regulation is the behavioral tend- 

 ency to select a region of optimal humidity. The optimal humidities for devel- 

 opment and for survival are hot necessarily the same, and they differ for various 

 species. ^^*' Insects taken from a moist environment appear more sensitive 

 to dry air than do insects from a dry environment; grasshoppers range widely 

 from hygrophilic to extremely xerophilic. Meal worm beetles, Tenehrio 

 molitor, choose the drier side of a humidity gradient and distinguish smaller 

 differences of humidity in the higher moisture range than in drier air. ^^ 

 Specific humidity receptors, probably pit and peg organs, are located on the 

 antennae. Other insects which avoid moist air are the mosquito Ctilex fatigans, 

 the cockroach Blatta orientalis, and the louse Pediculus hwnanus var. corporis. 

 Wireworms (Agriotes), on the other hand, avoid the lower humidity in a 

 gradient. They are able to distinguish differences as small as 0.5 per cent 

 relative humidity by hygroreceptors located on the head. ^^"^ The grain beetle 

 Ptiniis also collects in drier regions of a humidity gradient. •'•'* 1 he sign of 

 the response may depend on the state of hydration (Blatta, Ptinns), the insect 

 going to high humidity when desiccated or to a dry region when hydrated. *^" 



The onychophoran, Peripatopsis, lacks mechanisms for restraint of water 

 loss by the trachea and actually loses water twice as fast as an earthworm, 40 

 times as fast as a smooth-skinned caterpillar, and 80 times as fast as a cock- 

 roach. ^^'' Peripatzis loses water half as fast as an earthworm, but twice as 

 fast as a centipede, and 20 times as fast as a millepede. ^^- Restriction to a 

 damp environment may account for the lack of successful radiation of the 

 Onychophora as a group. 



Isopods occur in sea water, in fresh water, and on land. Their respiratory 

 organs present extensive moist surfaces. The nephridial canals of a fresh- 

 water species are larger than those of a marine species, and in both the canals 

 are larger than the canals in a land form. -"*' Terrestrial isopods lose water 

 more readily and are more restricted to a moist environment than myriapods. 

 A wood louse (Porcelio scaher) loses 4 per cent of its weight per hour in dry 

 air, compared with 0.05 per cent loss by a meal worm, 0.14 per cent loss by a 

 cockroach, or L8 per cent loss by an earthworm. ''-' The wood louse goes to 

 a region of higher humidity in a gradient. 



The most important means of prevention of desiccation in insects and 



