288 Insects and their Surroundings 



hereditary tendency for eyes to be developed as to 

 lead to the total loss of those organs. Accordingly 

 another factor has been invoked — that of " economy 

 of nutrition." There is a general tendency among 

 cave-animals to decrease in size, and their food- 

 supply is undoubtedly very limited. Hence to get 

 rid of any useless organ might be of considerable 

 benefit since the food-material necessary for its 

 support would be available for the nourishment of 

 useful parts. 



As a compensation for the loss of sight the senses 

 of smell and touch seem to be more highly developed 

 in cave-insects than in their relations of the upper 

 world. The feelers, which are the special seat of 

 these senses, are relatively longer in cave-species 

 than in above-ground members of the same genus. 

 This is markedly seen in the Bristletail, Campodea (fig. 

 6$^, while the length of the feelers in a species of 

 springtail {Entomohrya cavernarimi) found in several 

 caves in Kentucky increases as the depth of the cave 

 increases (167)- 



Springtails form the majority of the insect-popula- 

 tion of caves. The underground conditions carry to 

 an extreme the darkness and concealment of the usual 

 dwelling-places of these insects — beneath stones and 

 in damp earth. The cave-springtails pass through 

 their bodies the red earth which carpets the floor of 

 the underground galleries, and get food therefrom in 

 shape of vegetable refuse — fragments of plants carried 

 in by the subterranean streams or of fungi which 

 grow on the cave walls. Nourished on such meagre 

 fare the Springtails themselves form the food of the 

 Beetles of prey which share their gloomy dwelling- 

 place. 



Fresh-water Insects. — We have seen that Insects 

 are essentially creatures of the air and the land ; yet 



