Aquatic Hydrophilidae and Hydraenidae of Wisconsin (Coleoptera). I. Introduction, key to genera of adults, and distribution, habitat, life cycle, and identification of species of Helophorus fabricius, Hydrochus leach, and Berosus leach (Hydrophilidae), and Hydraenidae

The 3,920 Helophorus (Helophorinae) adults collected in Wisconsin included 16 species, two of which were new species. Helophorus orientalis and H. lacustris predominated. Three types of univoltine life cycles occurred: (1) adults overwinter in terrestrial habitats and larvae complete development in riparian habitats from early summer to early autumn, depending on the species; (2) the same as type 1, except eggs and a few adults overwinter; (3) larvae, pupae, and/or recently emerged adults overwinter and teneral adults occur in aquatic habitats in early spring. Nine species of Hydrochus (Hydrochinae) were found among 6,278 adults, with H. squamifer predominating. All had a similar univoltine life cycle in which adults overwinter in terrestrial habitats and enter aquatic sites in spring; larvae complete development in riparian habitats from late spring to late summer, depending on the species. Eight species and 5,167 adults of Berosus (Hydrophilinae: Berosini) occurred, with B. striatus predominating; their univoltine life cycles varied greatly among species. Some species overwinter as diapausing eggs (B. aculeatus, B. peregrinus, B. stylifer), others as adults in terrestrial habitats (B. pantherinus, B. striatus), one as aquatic larvae (B. hatchi), and another as adults and larvae (B. fraternus). Only 35 Hydraenidae adults were collected; included were Hydraena angulicollis, H. pennsylvanica, and Ochthebius lineatus. A generic key to aquatic Hydrophilidae adults and keys to species of Helephorus, Hydrochus, Berosus, and Hydraenidae in Wisconsin are provided. Following each species key is information about species that includes distribution and abundance in Wisconsin, range in North America, habitat cycle, life cycle, and notes on identification.