Form and Function. 
Of all the ecological groups of animals none shows more 
splendid and ofttimes obvious adaptations of form to function 
than do those that live in the water. Surrounded by a re- 
markably uniform element, it is interesting to note how ani- 
mals of the most diverse classification have been molded to a 
common plan in response to the physical properties of the 
medium in which they dwell. 
Not only have widely different organisms responded in a 
similar manner to a common environment, but the very con- 
stant and uniform conditions of our fresh waters throughout 
the world have resulted in the greatest similarity of forms, 
widely dispersed over the earth. 
The insect life of the water affords an especially interesting 
study since it represents a secondary adaptation. It is believed 
that insects were all terrestrial in the beginning, but that some 
have invaded the waters and acquired those structural pecul- 
iarities that enable them to cope successfully with conditions 
in a medium some 775 times as heavy as air. 
An examination of the insect fauna of the water will indicate _ 
that it is a heterogeneous group, gathered together from the 
various orders of insects, and illustrating different stages of 
adaptation. 
As Bastin has stated “‘insects are essentially creatures of the 
earth and air,” but many species have adopted an aquatic life, 
some to spend only their immature stages in the water, others 
to linger there throughout most of their days, quitting it only 
to extend the foraging grounds through flight. 
To this latter group belong the water bugs. It is the purpose 
of this chapter to indicate some of the structural contrivances 
possessed by these insects, and the manner in which they are 
used. The chief modifications have taken place along the lines 
of locomotion, prehension, and respiration, and under these 
three heads the question will be discussed. 
In considering these various questions it is natural to treat 
the subject from the standpoint of degree of adaptation. This 
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