1916] Aquatic Hemiptera 365 



As an example, consider the hind wings of Ranatra. This 

 insect is obviously highly specialized, but these wings have 

 few if any cross- veins, and these few are mostly spurs arising 

 from the longitudinal veins. Equally obviously, such very 

 narrow wings as this insect has do not require stiffening cross- 

 veins, and in the course of time those that once were there 

 became obsolete. In Aphelocheirus, the insect's frequentation 

 of never-drying streams makes even the adventitious use of 

 wings unnecessary for any purpose, and so through ages of 

 disuse they have totally disappeared. Again, how by any 

 stretch of imagination can such a complex organ as the tarsal 

 plume of Rhagovelia be considered a primitive structure? In 

 the whole range of insects there is no similar highly specialized 

 swimming device known. No further comment on this phase 

 of the subject seems necessary. 



The true position in time of the highly interesting groups of 

 aquatic Hemiptera is to be determined by a careful consideration 

 of the volume of important work that has appeared in Europe 

 and elsewhere on the structure and habits of this series of 

 insect forms, a series which displays so many highly specialized 

 (and therefore not primitive) adaptations to a very special 

 environment. 



