1916] Aquatic Hemiptera 355 



voir; the NotonectidcE the abdominal channel; and the NepidcB 

 the tube type. In the forms with pile and a dorsal reservoir, 

 practically the entire abdomen is pilose and the dorsum more 

 or less concave, forming a reservoir under the convex wings, 

 which is ordinarily filled with air. This air is renewed by 

 absorption from the atmosphere direct and also by reoxygenation 

 from the contained air in the water. In the Corixidce, the 

 abdomen also can be seen covered with a silvery coating of 

 air imprisoned in the pile. The insects of this last group, as a 

 general rule, remain at the bottom, holding fast to pebbles, 

 grains of sand, and other inequalities affording a hold, and from 

 time to time may be seen to pass the third pair of legs through 

 the air coating. Now and again they swiftly ascend, impinge 

 upon the surface-film and dart back to the bottom. The air 

 they carry makes it unnecessary for them to exert themselves 

 to go up, but their descent, while quick, is laborious, and it is 

 this extreme lightness that compels them to anchor themselves, 

 so to speak, in order to remain at the bottom. The NaucoridcB 

 haunt the aquatic vegetation among which they creep, and to 

 renovate their stored air, they come to the surface, where the 

 tip of the abdomen breaks through the surface film and the air 

 they carry is purified by diffusion or perhaps by being expelled 

 by body movements, fresh air being then drawn in to replace it. 

 In this family, or group, as in the Belostomatidce, the dorsal 

 part of the abdomen is hollowed out, but only slightly, and is 

 covered with a heavy pile, which affords storage for much air 

 and makes it unnecessary for the bug to come to the surface 

 with any degree of frequency to purify this stored air, which to 

 some degree must be done also by absorption of the oxygen 

 given off by the aquatic vegetation it haunts. In the Belosto- 

 matidce, however, there are two more or less retractile strap-like 

 appendages, the lengthened peritreme of the sixth abdominal 

 spiracles, which are moved from the connexivum and open 

 dorsally at a slit in the abdomen, which on the under^side is 

 covered in both sexes by the genital plate. These strap-like 

 appendages have been employed as specific characters, based on 

 their comparative lengths, but as they are, as already stated, 

 more or less retractile, individuals of the same species, L. 

 americanus for instance, will have them of different lengths. 

 They have also been considered to be a part of the genital appa- 

 ratus and perhaps to be ovipositors, but a simple dissection 



