More than 25,000 adult gyrinids that include 24 species were studied from all areas of Wisconsin. Dineutus discolor, Gyrinus aeneolus, G. analis, G. bifarius, and G. marginellus are lotic; D. assimilis, G. aquiris, G. confinis, and G. dichrous apparently breed in both lotie and lentic habitats; and the remaining species probably breed primarily in deep ponds or littoral areas of lakes and impoundments. Most collections were from streams in late summer and autumn because adults of almost all lentic species fly to streams to overwinter and these overwintering aggregations were easily collected. A key to adults of Wisconsin species was developed, and by using the gonocoxae and secondary sexual characters, females can be identified as accurately as males
