Shiny black, brown to olive beetles, sometimes with yellowish marks. Body is streamlined, oval, with the narrower end at the head. Antennae are threadlike. These beetles hang head downward, with the tip of the abdomen protruding from the water surface. The hindlegs are fringed with hairs and flattened for swimming. When swimming, they kick both hind legs simultaneously (not alternately). The swimming method helps distinguish them from the similar water scavenger beetles, which are in a different family.
Like other beetles, they have membranous hindwings that are covered by forewings that are thick, heavy shields (elytra). When the wings are closed, the elytra create a line straight down the back.
Larvae, called “water tigers,” are elongated, flattened and can be 2 inches long. They commonly come to the surface to draw air into spiracles (like snorkels) located at the hind end of the body. There are 3 pairs of legs, and the jaws are strong pincers that are used to grasp prey.
Adult length: maximum to about 1½ inches (varies with species); larvae can be up to about 2 inches long.
Habitat and Conservation
These beetles prefer quiet water at the edges of ponds and streams, floating gently among weeds. Before diving, they trap air between their wings and body, prolonging their time under water. Because predaceous diving beetles are strong fliers, they can fly away to new water if the pond they are in dries up, or if electric lights lure them away from “home.” This is why water beetles are sometimes found in birdbaths and swimming pools.
Fierce predators, these beetles do not hesitate to attack prey larger then themselves, including small fish, tadpoles and frogs. Their sharp jaws inject enzymes that digest their prey so that the juices can be ingested by the beetle. The larvae hunt by holding still, waiting with jaws wide open, and then strike suddenly, clutching the prey tightly with their jaws. As with the adults, the pincers are hollow, enabling them to begin sucking the juices of their prey while grasping it.
Eggs are laid on submerged vegetation. The time it takes for the larvae to grow large enough to pupate varies by species. They crawl out of water and burrow into the mud to pupate. In about a week, the adult beetles emerge and return to water. Adult males of some species have a special enlarged patch on the forelegs that helps them grasp the females during mating.
It makes sense to treat these beetles with respect, since their bites are painful—even if not medically significant. People in some countries (such as Mexico and Japan) eat dytiscid beetles. Most people count them as “beneficial,” since they eat other aquatic insects.
These plump, chunky insects are relished by many kinds of animals, including fish, birds, mammals and others. Although they are fast swimmers, those creeping around on land during or after a night of flying are easy pickings for terrestrial insectivores.