334 



Leech and Chandler: Coleoprera 



Fig. 13:30. Hydrophil idae. a, Hydrophilus Iriangularis, adult; b, Helophorus minutus, odult; c, 

 Tropisternus lateralis, adult; d, Enochrus sp., young larva containing bubbles of air if has 

 swallowed (a, Riley, 1881; b, Wesenberg-Lund, 1943; c, Blatchley, 1910; d, Williams, 1936). 



Annales de Biologie lacustre, 6:173-214, 23 text figs. 

 1929. Contribution a l'etude des Palpicornia. VII. Bull. 



et Ann. Soc. Ent. 3elg., 69:79-96, 1 pi. 

 RICHMOND, E. A. 



1920. Studies on the biology of aquatic Ilydrophilidae. 



Bull. Amer. Mus. Nat. Hist., 42:1-94, pis. 1-16 incl. 

 THORPE, W. H. 



1931. Miscellaneous records of insects inhabiting the 



saline waters of the California desert regions. Pan- 



Pac. Ent., 7:145-153. 



Family HYDROP HILID AE 



Water Scavenger Beetles 



The Ilydrophilidae and Dytiscidae comprise a majority 

 of the species of water beetles. Since they occur 

 together in the same ponds and streams, we tend to 

 overlook the fact that they belong to different sub- 

 orders of the Coleoptera, the Dytiscidae to the Ade- 

 phaga and the Ilydrophilidae to the Polyphaga. The 

 hydrophilids are divided into two groups, one aquatic, 

 the other terrestrial. The aquatic species in general 

 differ from the Dytiscidae in being flatter beneath, 

 more convex above, darker in color, and poorer swim- 

 mers. The predaceous dytiscids move their hind legs 

 in unison when swimming and obtain a powerful oarlike 

 stroke, but the hydrophilids move theirs alternately 

 after the manner of a walking insect. This is perhaps 

 correlated with food habits, for the largely vegetarian 

 hydrophilids do not have to be such fast swimmers 

 as the predaceous dytiscids. There is also a major 

 difference in their manners of obtaining fresh air at 

 the surface of the water. The Ilydrophilidae, which 

 have their largest and most used spiracles on the 

 thorax, break the surface film with their antennae, 

 adjacent to a front corner of the prothorax; the Dytis- 

 cidae, with major spiracles on the abdomen, come up 

 tail first, as do the larvae of both families. 



The Ilydrophilidae belong to a group of beetles 

 called the Clavicornia, because of the club-shaped 

 antennae. The name Palpicornia has also been used, 

 having reference to the palpi as tactile organs when 

 the beetles are under water. A vernacular name, 

 "silver beetles," describes the appearance of their 

 undersurface as they swim, for it is covered by a 

 silvery film of air, held by a pile of fine hairs. 



Adults of both, aquatic and terrestrial species are 

 largely herbivorous, but will eat dead animal tissue, 

 and a few are predaceous. The larvae of both groups 

 are carnivorous and cannibalistic, except perhaps 

 those of the genus Berosus, which reportedly feed 

 only on green algae. (Boving doubts this, because of 

 the mandibular structure.) 



The beetles and their larvae are an important source 

 of food for certain ducks and other aquatic birds, as 

 well as for shallow-water fish. The adults are eaten 

 by frogs and toads, and during flight periods are 

 presumably caught by birds and bats, and perhaps by 

 predaceous flies. 



Relationships. — The Hydrophilidae (in the broad 

 sense) do not show close relationship with any super- 

 family of the suborder Polyphaga except the Staphy- 

 linoidea, with which they seem to be connected by 

 means of the Spercheidae and Iiydraenidae. They 

 differ from the staphylinoid forms in having cantharoid 

 wing venation (the m-cu loop distinct; see fig. 13:26a 

 and b), short antennae with the true sixth segment 

 modified to form a cupule (fig. 13:25&), maxillary 

 palpi which are usually longer than the antennae 

 (fig. 13:36a), head usually with a Y-shaped suture, 

 elytra covering the abdomen, and (except Sphaeridi- 

 inae) aquatic habits. The genus Hydrochus here 

 retained in the Hydrophilidae, forms the family 

 Ilydrochidae of some classifications. 



Respiration. — Adults come to the surface head first, 

 turn slightly sideways, and break the surface film 



