Length: 8 - 15 mm
Two genera of Whirligig Beetles are found in northeastern North America; they are Gyrinus and Dineutus. Gyrinus species measure 8 mm or under, and have a visible scutellum. Dineutus, pictured here, measures 9 mm or more and the scutellum is not visible.
Dineutus contains 18 species in North and Central America, but is absent from South America (Gustafson and Miller, 2015).
A laboratory study by Romney and Rossman (1995) found that Whirligig Beetles in genus Dineutus tended to mass together when they were relatively well-fed, and that the beetles dispersed considerably when they were hungrier. Presumably massing together offers some advantage as defense from predators, but feeding is more successful when the beetles are not competing so directly for the same tiny arthropods.
The photos below show groups of beetles in genus Dineutus. The first and third photos below are from West Virginia's Elk River, Braxton County. The middle photo below is from the Sunflower River in Sharkey County, Mississippi.
A congener included on this site is Dineutus longimanus from the Caribbean.
Below: video of Dineutus sp. on the North Fork of the South Branch of the Potomac River.