SECTION I 

 INTRODUCTION 



For practical purposes, one may consider any non-swimming aquatic 

 beetle found in the United States to be a dryopoid. Although a few, 

 such as Psephenus and LutroahuSj may be relatively conspicuous on rocks 

 projecting from riffles, the majority are very small, inconspicuous, 

 and slow-moving. Since the typical habitat of almost all dryopoids is 

 in riffles, rapids, or comparable lotic situations, the common name 

 "riffle beetle" is generally appropriate. Furthermore, since one 

 author or another has applied this name to the dryopoids alone, the 

 elmids alone, or the psephenids alone, while others have used it indis- 

 criminately for all, it might as well be used for the entire group of 

 aquatic dryopoids. 



The Superfamily Dryopoidea, in the Suborder Polyphaga, includes members 

 (the Limnichidae) that are very close to the Byrrhoidea, and others 

 that are closely allied to the Dascilloidea (most of the genera listed 

 under Psephenidae and Ptilodactylidae are treated as members of the 

 family Dascillidae by Amett (1963)). Crowson (1967) considers the 

 Superfamily Dryopoidea as being comprised of the families Psephenidae, 

 Eurypogonidae, Ptilodactylidae, Chelonariidae, Heteroceridae, 

 Limnichidae, Dryopidae, and Elmidae. The Eurypogonidae and 

 Heteroceridae are omitted from treatment here since none of our 

 representatives of these families are known to be aquatic, although 

 the heterocerids burrow in mud along the margins of streams, ponds, 

 and lakes. By far the most promising as indicators of water quality 

 are the elmids, but the psephenids, adults of Helichus (Dryopidae), 

 and larvae of Lutroahus (Limnichidae) should also be useful for this 

 purpose. 



Although somewhat detailed information concerning habitats of individual 

 genera and species is presented in the species list, a few general facts 

 concerning dryopoid life histories and ecology may be helpful. The 

 elmids of the tribe Elmini are the most completely aquatic of all 

 beetles. The eggs, so far as is known, are deposited on submerged 

 rocks or wood, usually on the under side. Here the larvae develop, 

 creeping about and feeding chiefly upon the algae which tend to encrust 

 such substrates or upon decaying waterlogged wood. Respiration is 

 accomplished by tufts of filamentous tracheal gills which are extruded 

 from a caudal chamber. The gills may be retracted and the chamber 

 closed by a trapdoor-like operculum. Mature larvae crawl out of the 

 water and pupate in small cavities beneath loose bark or rocks close 

 to the water's edge. Newly emerged adults of many species apparently 

 fly at night, and are attracted to lights. Upon returning to the water, 

 most individuals will never again emerge into the air, spending the 

 rest of their lives (several years in some species) in the same habitat 

 and utilizing the same food as the larvae. Their respiratory 



