Aquatic Adaptations of Insects 273 



body are brought about by consolidation of the neck 

 vertebrae and shortening of the cranium. Smoothness 

 of contour, (and therefore diminished resistance to 

 passage through the water) is promoted by (i) the loss 

 of hair; (2) the loss of the external ears; (3) the 

 shortening and deflection of the basal joints of the legs; 

 (4) elongation of the rear portion of the body. Caudal 

 propulsion is attained in the whales by the huge 

 dorsally flattened tail; in the seals (whose ancestors 

 were perhaps tailless) by the backwardly directed hind 



legs. 



Compared with these marine mammals those of our 

 fresh waters show very moderate departures from 

 terrestrial form. The beaver has broadly webbed hind 

 feet for swimming. The muskrat has a laterally 

 flattened tail. The mink, the otter and the fisher, with 

 their elongate bodies and paddle-like legs, are best 

 fitted for life in the water, and spend much time in it. 

 But all fresh-water mammals make nests and rear their 

 young on land. 



Ill 



The insects that live in the water have adaptations for 

 swimming that parallel those of mammals, just noted; 

 but some other adaptations grow out of the different 

 nature of their respiratory system, and, more grow out 

 of the difference in their life cycle. The free-living 

 larval stage of insects offers opportunity for independ- 

 ent adaptation in that stage. Adult insects of but two 

 orders, Coleoptera and Hemiptera, are commonly 

 found in the water. These, as compared with their 

 terrestrial relatives, exhibit many of the same adapta- 

 tions already noted in mammals ; ( I ) approximation to 

 stream-line form, with (2) consolidation of the forward 

 parts of the body for greater rigidity; (3) lowering of 

 the eyes and smoothing of all contours; (4) loss of hair 



