FORM AND FUNCTION 



281 



of the corresponding part, the one bearing the same 

 name in the biting mouth. That is, we say the parts 

 are homologous. 



What are the conditions which might arise to cause 

 this peculiar and interesting development ? Let us sup- 

 pose that, leaf- and vegetable-feeding insects became so 

 numerous as to devour 

 or greatly reduce the 

 whole food supply. Is it 

 not evident that many 

 insects would perish \ 

 But, if some of these 

 insects find it possible 

 to secure nourishment 

 from beneath the bark of 

 trees and dense outer 

 covering of plants, the 

 leaves of which have al- 

 ready been eaten, will 

 they not have a better 

 chance to exist than 



FIG. 214. Head of cicada, showing rnouth- 



tllOSe wllOSe dependence parts. Tip of mandible and maxilla en- 

 is wholly upon the foli- larged ' at the left 

 age? Insects which have learned to extract the juices, 

 finding this an easy and uncontested way of obtain- 

 ing sustenance, continue to seek nourishment in such 

 places. Succeeding generations having used the mouth 

 constantly in this way, this mouth has developed and 

 adapted itself to the work in hand, until at present there 

 exists a well-established mechanism for the imbibing 

 of fluids. 



