An aquatic insect is any insect that depends on water for at least one part of its life cycle, typically during the egg, larva or nymph (miniature, wingless adult) stages. Unlike primarily land-dwelling insects, aquatic insects have adaptations for living in water, such as gills, streamlined bodies, air-trapping hairs or the ability to breathe through specialized body surfaces. Most inhabit freshwater ponds, lakes, streams, rivers and wetlands, although a small number live in coastal or marine environments.
Although land insects vastly outnumber aquatic insects in both overall species diversity and total global biomass (because freshwater habitats cover less than one percent of Earth's surface), aquatic insects have an effect far out of proportion to their abundance. They frequently dominate local animal communities, make up the vast majority of living matter in freshwater, and serve as a vital food source for birds, fish and other wildlife.
Aquatic insects do not belong to a single scientific order. Rather, they span various insect groups, and many are aquatic only during their immature stages. In their adult stages, they transition to the air to fly over water, pollinate plants and migrate between separate wetland ecosystems.
The largest and most ecologically prominent groups include caddisflies, which have more than 16,000 described species and rank among the most diverse fully aquatic groups. Many larvae construct protective cases from sand or leaves, while others spin underwater nets to filter food. They process plant debris, recycle nutrients and supply abundant prey for fish.
Estimates suggest that roughly 18,000 beetle species spend all or part of their lives in water. Operating as predators, scavengers and algae grazers across both the water's surface and the muddy bottom, they occupy a wider variety of freshwater niches than almost any other insect group.
Dragonflies and damselflies include roughly 6,000 to 7,000 described species. Their aquatic nymphs are active predators that consume mosquito larvae, other insects, tadpoles and sometimes small fish. Adults maintain this predatory role in the air, helping regulate pest populations around wetlands. They are also culturally important, attract nature tourism and serve as visible indicators of wetland health.
Mayflies encompass roughly 3,000 to 4,000 described species. Their nymphs graze on algae and organic particles while serving as key food sources for fish, amphibians and birds. When they emerge in massive swarms, they transfer nutrients from water to land, supporting terrestrial food webs. Because mayflies are among the most sensitive to pollution, their presence usually signals clean water.
Several thousand species of aquatic true bugs live on or beneath the water surface, with some using trapped air to breathe. A few rare species from other insect families also live in water, including aquatic moths, lacewings, springtails, and certain ants and wasps.
A widely cited global review estimates that about 33 percent of all aquatic insect species are threatened with extinction. Groups such as mayflies, stoneflies, caddisflies, dragonflies and damselflies face the highest documented risks. Specific species face individual pressures: for example, recent assessments found that roughly 16 percent of dragonfly/damselfly species and up to 20 percent of mayfly species are threatened.
The greatest pressures on these insects include wetland destruction, chemical pollution, pesticides, dams, invasive species, deforestation and climate change. Species restricted to a single spring or lake are especially vulnerable, as the loss of just one small habitat can wipe out their entire global population.