74 A^inals Entomological Society of America [Vol.1, 



The water striders which hve entirely on the surface of the water 

 have a rather special adaptation, but with the Naucoridae we 

 reach a group in which the aquatic life is well established. These 

 insects have their form well fitted for swimming, but their respi- 

 ration is dependent upon the atmosphere, and they must make 

 frequent trips to the surface. Somewhat more specialized are the 

 Belostomid^ in which the ability to remain under water is very 

 perfectly developed, but without special gill structures for aqua- 

 tic respiration. The Nepidse go further in that they have 

 long tubular structures which permit them to reach the surface of 

 the water without bringing the body up to the surface. The 

 water boatman and Corixa, though not more specialized in these 

 features, show in some other respects a more perfect adaptation 

 and are to be considered as among the most extreme of the 

 ac|uatic groups. It seems to me apparent from such a series as 

 this that the arrangement of groups should be made to correspond 

 as closely as possible with the successive stages in specialization 

 for the particular habitat which is present. That these insects 

 are descended from strictly terrestrial forms seems certain, and 

 that their various adaptations represent different degrees of per- 

 fection in adjustment to the new habitat will, I believe, fit in 

 most perfectly with the comparative study of their structure. 

 It is shown, perhaps, most decidedly in the condition of the an- 

 tennae and in the modifications of the legs in adaptation to swim- 

 ming. 



Most of the aquatic beetles appear to have but recent adapta- 

 tion to this habit, as all are obliged to secure air from above 

 water, and in most species a considerable ability to live outside 

 the water is present. It is also quite certain that the aquatic 

 habit has in this group been acquired independently in the 

 different families. 



With the evidence that may be accumulated in all the different 

 orders of insects it seems certain that in every group of insects 

 having ac|uatic habits we may confidently trace a derivation from 

 terrestrial ancestors, and hence in arranging phylogenies we may 

 consider that those less perfectly adapted are the more primitive 

 and those most perfectly fitted for aquatic life are the most 

 specialized. This will involve some radical changes from accepted 

 and time honored systems of classification, but will, I believe, 

 in every instance result in more rational and satisfactory group- 

 insfs. 



