364 AN INTRODUCTION TO ENTOMOLOGY 



swimmers that are commonly seen floating at the surface of the water, 

 with the caudal part projecting sufficiently to admit of the air being 

 drawn into the air chambers. When in this position, their long, 

 oar-like, hind legs are stretched outward and forward ready for action; 

 when disturbed they dart away toward the bottom of the pond, 

 carrying a supply of air with them. 



Buenoa. — This genus, of which six species have been found in this 

 country, is composed of much more slender forms than is the preceding. 

 The habits of two of our species have been studied by Hungerford ('19). 

 These insects do not rest at the surface of the water as do some species 

 of Notonecta, but may be seen swimming slowly, or even poising in 

 midwater some distance beneath the surface. They abound in water 

 teeming with Entomostraca, upon which they largely feed. 



Plea.— The members of this genus are small insects, not exceeding 

 3 mm. in length. The shape of the body is quite different from that 

 of other back-swimmers, being highly arched behind. They are 

 found in tangles of aquatic vegetation, to the filaments of which they 

 cling when at rest. They feed on small Crustacea. Only one species, 

 Plea siriola, has been described from our fauna. 



Family NEPID^ 



The Water-Scorpions 



The members of this family can be distinguished from other 

 aquatic Hemiptera by the presence of a long respiratory tube at the 

 end of the abdomen. This tube consists of two long filaments, each 

 with a groove on its mesal side. By applying these 

 filaments together the grooves form a tube, which 

 conducts the air to two spiracles situated at the 

 caudal end of the abdomen. By means of this ap- 

 paratus these insects are able to rest on the bottom 

 of a shallow pond, or among rubbish or plants in 

 water, and by projecting this tube to the surface 

 obtain what air they need. 



With regard to the form of the body, two very 

 different types exist in this family. In one, repre- 

 sented by the genus Nepa, the body is a long oval, 

 Pi„ Ai-j—Nepa fl^^' ^"^ ^^^^ (^^g- 417); in the other, represented 

 apictdata. by the genus Ranatra, the body is almost linear and 



cylindrical (Fig. 418). An intermediate fonn, 

 Curicta, represented by two species, is found in Louisiana, Texas, and 

 Arizona. 



The water-scorpions are carnivorous; and with them the first pair 

 of legs is fitted for seizing prey. In these legs the coxae are very long, 

 especially in Ranatra; the femora are furnished with a groove into 

 which the tibias and tarsi fit like the blade of a pocket-knife into its 

 handle. 



