360 Annals Entomological Society of America [Vol. IX, 



all. When not pressed, they paddle slowly, alternating the 

 middle and hind legs. In some of the exotic genera {SphcBro- 

 dema and Limnogeton), the hind legs are prismatic and sparsely 

 fringed, obviously representing in this respect a less specialized 

 type, and certainly a far less agile one. 



In the NotonectidcB, the hind pair only is used, and the legs 

 are moved simultaneously. In passing, it may be remarked that 

 this apparently obvious statement is repeated, because in some 

 of the earher observers, still quoted, it has been said that the 

 legs alternate laterally in swimming. While in this group the 

 legs are not so broad as in the Belostomatidce, the comparatively 

 thicker and longer hair fringe makes them very effective, 

 and the folding back of the hairs on the return stroke tends 

 to facilitate their use and by a species of feathering motion, 

 avoids the retardation of a heavy back stroke. Plea is an 

 exception in this family, however, in that its short little legs 

 are narrow and set only with sparse bristles. Naturally, while 

 the little bug swims about, it employs a short, clipping stroke, 

 very different from the free, long, powerful movement of the 

 others. It much prefers to walk, sometimes on the surface 

 film, clinging back down, at others among the water weeds it 

 frequents. The two front pairs of legs in this group are modified 

 into raptorial legs, a rather unusual condition, and they are 

 held closely appressed to the sternum when in repose, but when 

 at the surface and awaiting their prey, they are somewhat 

 outstretched and may be seen as four little eminences on the 

 surface by the observer. 



The Naucoridce have legs but little adapted for swimming. 

 They are more or less hairy or bristly, but not flattened like 

 the legs of the Belostomatids. Nevertheless, they contrive 

 to move quite rapidly while swimming, and some indeed, as 

 Aphelocheirus, are able to breast rapid streams. On land, 

 however, they walk quite freely for water bugs and especially 

 so if compared with the previously named families, which are 

 very awkward and feeble when moving on land. This difference 

 can be readily understood if it be borne in mind that they 

 live concealed in aquatic vegetation, among which they ordi- 

 narily creep. The raptorial front legs are very powerful in 

 this family, the femora being greatly thickened, and the tibi^ 

 strong and curved, terminating in a single sharp tarsal claw. 



