There are at least 500 species in North America alone. The larvae are called water tigers and have sharp pincers to eat tadpoles, their siblings, and other critters. Adults eat bugs, fish, tadpoles, each other, and frogs. They bring air bubbles underwater with them to breathe. In some species, oxygen from the water can diffuse into the bubble and extend the time a beetle can stay underwater.
Adults spend a lot of time flying around and show up at night lights.
During the winter, they stay in the pond. In China, some people eat them. Through these, the beetles pump in toxins and enzymes that render the insides of the victim into liquid that the beetle can suck back for food. Drawings of a larval and an adult predaceous diving beetle can be seen at this water bug site . Whirligig beetles, Dineutus species, are some of the funniest insects to show up at my ponds.