360 THE VARIATION OF ANIMALS IN NATURE 



a considerable extent brought into the scope of the organised 

 system of compensations. 



As regards internal organisation, the simplest type of 

 compensation is seen in the regulation of temperature. 

 Temperature control in vertebrates, and to a less extent in 

 the nests of social insects, is one of the most obvious examples. 

 Thus the temperature of the brood-cells of a beehive is much 

 higher than the surrounding air, and is usually maintained at 

 32°-35° C, according to von Buttel-Reepen (1915, p. 119). 

 The bees can also cool the hive by fanning with their wings. 

 In ants, Wheeler (191 3, chapter xii) records that the tempera- 

 ture of the nest may be io° C. higher than that of the air 

 outside. The workers, by moving the brood to different levels 

 in the nest, can expose them to the appropriate conditions. 

 Such species as Formica sanguinea, further, have separate winter 

 and summer nests. 



The control of temperature by choice of habitat is also 

 achieved in various desert animals. Chapman, Mickel and 

 others (1926) have shown that the temperature of the surface 

 of the soil on the Minnesota sand-dunes at midday is high 

 enough to kill most insects. The species which live there 

 escape destruction by appropriate behaviour. Some are 

 nocturnal and bury themselves deeply during the day. The 

 sand-wasps, however, show the most interesting modification 

 of behaviour, since they are active during some of the hottest 

 hours and have to make a burrow for their nest through the hot 

 surface. They take advantage of a peculiarity of the habitat, 

 namely, that a little below the surface and a little way above 

 it the temperatures are much lower. Thus, while burrowing, 

 they work very rapidly for a short period and then fly up into 

 the air for a rest. Later the burrow itself forms a refuge from 

 the surface conditions. By their plastic behaviour the wasps 

 avoid the destruction which might have been their lot for 

 seeking out so unfavourable an environment. 



Again, external skeletons (Arthropoda, Mollusca), tubes 

 and cases (Vermes, Crustacea, many larval Insecta), covered 

 runways (Isoptera, Formicidae, small mammals), clothes 

 and houses (man) are another method of resisting or con- 

 trolling the environment. Almost every feature of man's 

 environment, except a relatively small number of para- 

 sites, is under effective control, and the chief problem is 



