48 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



in their bladder for times of drought. It has been noted above that 

 frogs excrete the water taken up by their skin through their kidneys, 

 and collect it in their bladder. This water contains only very small 

 amounts of urea, and is almost pure. Thus in the Australian deserts, 

 when the frogs bury themselves after the close of the short rainy sea- 

 son, their whole body is swollen up by the distended bladder, and 

 they can survive twelve and even eighteen dry months in this con- 

 dition.* 



Xeric animals are represented on the one hand by the myriapods, 

 insects, and arachnids and on the other by the reptiles, birds, and 

 mammals. These are not excluded from existence in humid regions. 

 Some, however, probably in consequence of specific adaptation to arid 

 conditions, as in steppe animals like the camel, are unfavorably af- 

 fected by higher humidities and amounts of rainfall. Since humid 

 regions and situations are relatively few, the xerocoles in general have 

 a wider distribution than the hygrocoles. 



The capacity to withstand dry air without danger appears in all 

 kinds of gradations. Many myriapods such as Lithobius are inade- 

 quately protected against evaporation. Many relatively soft-skinned 

 insects avoid direct sunlight and are active only in moist air, in the 

 twilight, at night, or after dewfall or rain. These include may flies 

 and stone flies; young mole crickets, which may easily be killed by the 

 sun's rays; mosquitoes, which swarm only in moist air and otherwise 

 remain in hiding in places sheltered from the sun and wind; most 

 termites, which carry on their building activities and foraging expedi- 

 tions only at night or in humid weather, as before rainstorms. 33 



Most insects are children of the sunlight. They are protected 

 against loss of water by their dense body covering and have few or no 

 skin glands. Nor does the excretion of waste require much water, as 

 a part of the excretory products is stored in the fat-body. Finally, 

 the tracheal breathing relieves the blood of an important function, by 

 carrying the oxygen directly to the points of consumption, so that it 

 is probable that smaller amounts of blood can care for the distribution 

 of nourishment and for the removal of excreta. How small the need 

 for free water may be in insects is best shown in the forms which can 

 live on very dry food, such as the larvae of meal worms {Tenebrio, 

 see p. 12) ; boring beetles (Anobium) , which eat the dry wood of old 

 beams and old furniture; or the skin beetles (Attagenus) and clothes 



* There is a dearth of exact information about the desert toads and spade- 

 foot toads of North America with respect to the physiological aspects of their 

 aestivation. 



