requirements are met through the use of a plastron (Thorpe, 1950; 

 Thorpe and Crisp, 1949) . Various parts of the legs and body, especially 

 on the ventral side, are covered with a hydrofuge tomentum or pile which 

 maintains a film of air when the beetle is submerged. This film, which 

 is in contact with the air reservoir beneath the elytra, provides 

 adequate gaseous exchange in the well-aerated lotic situations occupied 

 by the beetles. Small bubbles of oxygen photosynthetically produced by 

 algae and other aquatic plants provide an additional source of oxygen 

 and can be incorporated into the plastron. Since the gaseous film is 

 essential to these beetles, it is not difficult to understand why they 

 cannot tolerate excessive pollution by such wetting agents as soaps and 

 detergents. 



Elmids of the tribe Larini are less thoroughly aquatic. The adults are 

 essentially riparian, usually occurring at or just above the water line 

 in rapids and creeping beneath the surface only for oviposition 

 (presumably) . They take flight readily, often after dropping onto the 

 water surface and being swept a short distance downstream. Otherwise, 

 the life history is like that of the Elmini. 



Psephenus and Eubrianax^ in the family Psephenidae, exhibit a pattern 

 very much like that of the Larini, except that the females may remain 

 submerged for days as they go about their task of oviposition beneath 

 rocks. Mature larvae (water pennies) crawl out and pupate beneath the 

 larval carapace. Details are unknown for the members of the Eubriinae, 

 but since the adults are found in shrubbery rather than at the water's 

 edge, it is quite possible that the adults never enter the water, perhaps 

 ovipositing on objects overhanging the stream as do such neuropterans as 

 the sisyrids and dobsonflies. Pupation, at least in Eotopria, is 

 comparable to that in Psephenus. 



In the family Limnichidae, Lutroohus has a life history that is also 

 very much like that of the Larini, although the adults of some species 

 may enter and remain under water for indefinite periods of time. 

 Females insert their eggs in such substrates as travertine. The other 

 genera of limnichids occurring within the United States are apparently 

 not aquatic even as larvae. Their life histories are unknown. The 

 adults are included in the key only because they may be taken near water 

 (in fact, Lirrmichus commonly falls onto the water from trash lodged in 

 the stream, and readily flies from the water surface as do LutrochuSj 

 Psephenus, Phanoaerus, and Lara) . 



Chelonarium (Chelonariidae) is aquatic only in the larval stage, as are 

 those members of the Ptilodactylidae listed here. Other ptilodactylids 

 are not aquatic at all. 



