Length: 3 - 7 mm
Beetles in the genus Gyrinus have punctate striations on the elytra. Another key trait is that the scutellum is visible, not hidden as it is in genus Dineutus.
A 1921 article by H.C. Fall is pleasantly colloquial; he and his friends refer to aggregations of Gyrinus spp. as "schools" of beetles. It is interesting to see early twentieth century collectors' idea of collecting "a series":
Mr. Wallis has sent me two lots taken at Winnipeg and one at St. Norbert, Manitoba, the last named containing one hundred and eight specimens, all of which were maculiventris; while from Professor Bradley I have received a series of several thousand specimens taken at Silver Springs, Arkansas, which seems to contain nothing but analis.
Fall's article did go on to demonstrate that in large aggregations of Gyrinus spp., there are often four to seven different species present (Fall, 1921).
Photo data: 7 June 2011. Ramsey Canyon, Cochise County, Arizona.