1916] Aquatic Hemiptera 361 



The simple legs of the NepidcB are narrow, in Ranatra and 

 Amphischizops much elongate and in Nepa and its Old World 

 congeners, shorter and stouter and prismatic, with sparse hairs 

 in all. Naturally, they are poorly adapted to swimming, 

 and the bugs make but awkward and slow progress when in the 

 water. Ranatra, for example, alternates the second and third 

 pair of legs when swimming, a slow operation at best. In the 

 first pair, the coxa is very long, the femur is also long, and the 

 tibia closes over it and forms a scissors-like grasping claw. 



In the Corixidce also the hind legs only are specialized 

 for swimming. The anterior pair is broadened and flattened 

 of a more or less sickle-like shape and set with stout pegs and 

 hairs in series. These are held by some to be in the nature of a 

 musical organ. At least, some species make a chirping noise, 

 which it is claimed is produced by rubbing this strigil as it is 

 called, across the grooved front of the bug's face. The second 

 pair of legs is used in clinging to pebbles and other objects at 

 the bottom. 



In the three groups, CorixidcE, BelostomatidcE and Noto- 

 nectidce, the legs are nearly as useless for progression on land 

 as the wings are efficient for flying. These members have 

 been totally modified in their adaptation to subaquatic locomo- 

 tion and the very form of the insects has "suffered a sea- 

 change;" they are smooth, narrow and more or less tapering 

 back and front, especially toward the rear. In some, the smooth- 

 ness of the exterior is produced by the greater or less polish 

 of the hard parts — thorax, hemelytra — and in others by the 

 possession of a pile which inhibits the natural retardation 

 exercised by water on moving objects through friction, which 

 is overcome by the film of air imprisoned in this pile. This 

 device may be observed in Notonecta, especially in the darker 

 forms, in which the hemelytra are to be seen silvered with a thin 

 film of air when the bug is submerged. 



Protective mimicry is exhibited mainly in Ranatra, which 

 looks like a little brown twig caught in the vegetation. As 

 it is sluggish by nature, it remains in one place and waits for 

 its prey — Daphnias, young fish, free swimming aquatic larvae 

 of one kind or another — which it captures by means of its 

 raptorial scissors-like forelegs as the victims incautiously 

 swim past. The BelostomatidcE are more active and pounce 



