THE INSECT WORLD 



Temperature (°C). 25.6 29.4 30.3 33.0 



Speed of walking (cms/sec.) 2.62 3.49 3.57 4.17 



From the speed of walking, the soil temperature can be de- 

 duced with an accuracy of one degree. It is probably because 

 of this dependence on external temperature that insects are 

 so much more numerous in the tropics, and that it is there 

 that so much of their evolution has taken place. In many 

 groups, the temperate and still more the arctic species are 

 relatively recent invaders, often with special modifications 

 fitting them to the new conditions. This seems to be true, for 

 example, of our social wasps. 



FOOD AND WATER 



Insects are not only dependent on the external temperature 

 but also on some external source of water. Active life is 

 possible only if the body fluids are kept at a steady level. 

 Some features of the insects' external skeleton serve to retain 

 water as well as heat; such are the felted hairs which cover 

 some species. But this particular problem is more often 

 solved by modifications of physiology or behaviour. Most 

 species extract all water from their excreta before voiding 

 it, as will be seen later in the termites. Many species do not 

 actually drink water but obtain it from their food. Fraenkel 

 and Blewett showed in the flour moth that caterpillars reared 

 in very dry flour might produce a chrysalis containing more 

 free water than had been present in all the flour which had 

 been eaten. The caterpillars must therefore have combined 

 part of the flour with oxygen to form extra water. This 

 would explain their observation that caterpillars consume 

 more solid food on dry than on damper flour. 



The nests of social insects have among their functions that 

 of conserving water, and in artificial nests ants run about as if 



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