282 PRINCIPLES OF ANIMAL BIOLOGY 



them, however, cannot be dormant for any great length of time and still 

 live. Such species have to live in tropical regions. 



Some animals avoid the dangers of extreme temperatures by special- 

 ized habits. Certain wasps which dig in sand dunes cannot endure for 

 long the high temperature at the surface of the sand on sunny days. 

 They survive these temperatures by digging vigorously for a few seconds, 

 then flying about a few inches above the sand where the air is much cooler, 

 then returning to their digging for a brief period. 



The regulation of body temperature by birds and mammals has 

 already been described (page 120). This physiological feature enables 

 animals of these classes to range widely so far as temperature is con- 

 cerned. Among cold-blooded animals there is occasionally the ability 

 to regulate temperatures in groups. Honeybees can do this in colonies, 

 though each individual bee cannot. A certain amount of heat is liberated 

 in their metabolism; and if this is conserved in masses of bees, the tem- 

 perature may be considerably raised. Temperatures in their hives are 

 much higher than those outside in winter. 



One curious relation to temperature is the acceleration of metabolism 

 by fluctuating as compared with constant temperatures. Grasshopper 

 eggs develop much more rapidly at their optimum temperature if that 

 temperature has been interrupted by a cold period. The acceleration is 

 greater if the interruption by low temperature comes early than if it 

 comes late in development. Eggs are laid by these insects in late summer 

 and fall, over a period of many weeks. It would be expected that those 

 laid early would be the first to hatch the following spring ; but all the eggs 

 hatch about the same time. Those laid late in the fall enter the winter 

 in an early embryonic stage but are accelerated enough more in the spring 

 to enable them to overtake their older companions and emerge at the 

 same time. This is an important reaction to temperature, for if any of 

 the young grasshoppers emerged much later in spring or summer they 

 would miss the most favorable period of the year for passing through 

 their immature nymphal stages. 



Some anilnals change their reactions to other stimuli with changes of 

 temperature. Thus one of the leaf-boring beetles studied by Chapman 

 goes toward the light at high temperature and takes to flight if mechani- 

 cally disturbed, but avoids light at low temperature and draws up its 

 legs and falls if disturbed. These beetles live on a plant which grows in 

 water, and during the warm part of the day they are out on the leaves. 

 If their reactions were reversed and they were disturbed at this time, they 

 would fall into the water, but instead they fly away. Disturbance during 

 the cool part of the day, when they are hiding at the center of the plant, 

 merely causes them to fall into the recesses at the bases of the leaves. 



The structure of an animal sometimes depends on the temperature. 



