FACTORS INFLUENCING BODY TEMPERATURE 53 



perature of the body may differ from the surrounding 

 medium. For example, a dormant insect shows a tem- 

 perature which is practically the same as that of the air 

 about it, but an active insect lags behind as the air 

 temperature rises. When the air is very damp, body 

 temperature may be higher than that of air (Bachmet- 

 jew, 1901). Desert insects may show a body temperature 

 as much as 15 deg. C. below that of their surroundings. 

 Many animals in hot situations hide during the day or 

 during the dry, warm season and become active when the 

 direct rays of the sun are not present. In cool situa- 

 tions the opposite is often true. Some animals show 

 daily or seasonal rhythms in heat production and body 

 temperature. Even such a specialized homoiotherm as 

 a bank swallow may show considerable daily fluctuation 

 in body temperature (Stoner, 1926). Many homoio- 

 therms pass through long periods of rest, during which 

 they lose their powers of maintaining constant body tem- 

 perature. 



Insulation. — Poikilotherms during cold periods are 

 obliged to resort to situations where they will not freeze. 

 They commonly burrow into the soil or into mud where 

 the decay of organic materials prevents freezing; they 

 hide away in springs, caves, under leaves, and in logs 

 and hollow trees. They seek insulation or a warm situa- 

 tion in their environment. The skins of reptiles protect 

 them well from loss of water, but not so well from loss 

 of heat. Insulation for conserving heat is generally not 

 present on the exteriors of animals, except among 

 homoiothermic vertebrates. Hair, feathers, and fat are 

 the materials employed and they show more or less adap- 

 tation for particular situations and localities. Animals in 

 the tropics are generally thin and those in frigid regions 

 commonly have a thick layer of subcutaneous adipose 



