"Water beetle" is a term used for aquatic beetles from several families. These species include diving water beetles, water scavenger beetles and whirligig beetles. There are about 2,000 species of water beetles around the world. Sometimes called water tigers for their aggressiveness, water beetles belong to the Coleoptera order, and diving, or true, water beetles belong to the family Dytiscidae.
According to Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO), dytiscids generally have hard, smooth, oval bodies with no spine. They have flattened hind legs used as paddles and long, thin antennae. They are black, brown or green and range from 1/10 inch to 1 1/2 inches long. The hardened wing covers, or elytra, of females have grooves and in males, they are smooth, forming a cavity above the body.
Females are more selective about choosing a mate than males. Males have a variety of mating behaviours and females have corresponding behaviours designed to resist mating such as swift and erratic swimming. Males may have sucker-shaped leg setae allowing them to grab females, preventing their escape during mating.
The male fertilises the eggs inside the female. She then deposits the eggs into aquatic plants’ stems by making cuts in the stem. Each egg batch may have as many as 600 eggs.
Larvae and Pupa
The larvae are aggressive water predators and have long sickle like jaws to suck fluids out of their prey. Larvae sometimes attack larger animals and feed on other insects, leeches and worms, crustaceans and mollusks or tadpoles and small fish. Some species have a siphon attached to their abdomen used to gather air. They move out of the water before the pupal stage, forming a cell in damp soil near the water.
When the adult beetle emerges, it returns to the water. Water beetles prefer slow moving water in ponds, lakes, stagnant rivers, dams and pools. Beetles require air: adults surface to gather air into a chamber under their wing covers to stay submerged longer. Often, water beetles hang from the water surface, head down, exposing the breathing tubes openings (spiracles) located at the end of the abdomen. The air bubble stored beneath the beetles’ abdomen not only provides air supply for extended submersion, but also protects the spiracles when the beetle is under water. Some beetles have a plastron (a thin permanent layer of air around the body) or a physical gill allowing them to “breathe” under water. Adult water beetles eat through a normal mouth opening and tear larger prey into smaller pieces. Diving beetles eat invertebrates living or falling into the water. They also eat small tadpoles and fish.