Abdomen, segment 10, has 2 pairs of stout, terminal hooks
Gyrinidae (Whirligig Beetles) are a familiar sight on freshwater ponds, lakes, open flowing streams, bog pools and swamps. They may be found in large aggregations or schools in late summer and autumn, which can be made up of a single species or many different species. Overwinter, pond-inhabiting species often fly to large streams and lakes. Divided vision, chemical defenses, and quick swimming movements allow them to avoid predators from above or below. Predominately scavengers, feeding upon the live or dead insects trapped or floating on the water surface.
The famale lays her eggs on stems of emergent vegetation a few centimeters below the water surface. In 1-2 weeks, the eggs hatch and pass through three instars. They crawl about on submerged objects, using their apical abdominal hooks and feed on small aquatic organisms. They can swim in an undulating fasion by using the abdominal gills, possibly as an escape mechanism.
Merritt, R W., K W. Cummins, and M B. Berg. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa: Kendall/Hunt Company, 2008