While often associated with land, a diverse group of insects has adapted to life in water. These aquatic insects spend at least part of their life cycle submerged, often as immature stages like larvae or nymphs of familiar flying insects such as dragonflies and mayflies. They exhibit remarkable adaptations, allowing them to thrive in aquatic ecosystems.
Where Aquatic Insects Thrive
Aquatic insects inhabit diverse freshwater environments, from flowing rivers and streams to still lakes, ponds, wetlands, and even temporary puddles. While most are freshwater, a few specialized species, like sea skaters, live in marine habitats. Conditions vary, with some insects needing clean, fast-moving water, while others tolerate stagnant or polluted conditions.
Favorable habitats include shallow pond edges, lake margins, and oxygen-rich stream riffles. Within these, insects occupy microhabitats: on the water’s surface, in the open water column, or in bottom sediments (benthos) for shelter and food.
How Insects Survive Underwater
Insects have evolved various adaptations to obtain oxygen, move, and feed underwater, despite typically being air-breathing. Many aquatic insects, especially immature stages, use gills to extract dissolved oxygen. These gills can be external (leaf-like, filamentous) or internal, like the rectal gills of dragonfly nymphs that also aid in jet propulsion.
Other insects use breathing tubes (siphons) that extend to the surface, such as those used by mosquito larvae and water scorpions. Some carry a temporary air bubble, often under their wing covers, which acts as a physical gill. This bubble provides initial oxygen and continuously draws dissolved oxygen from the water through diffusion.
A more advanced adaptation for underwater respiration is the plastron, a thin, rigid film of air held against the insect’s body by a network of tiny, water-repellent hairs. This stable air layer acts as a permanent physical gill, enabling continuous oxygen uptake from the water. This allows insects like riffle beetles and water boatmen to remain submerged indefinitely. Beyond respiration, aquatic insects show specialized locomotion, using streamlined bodies and modified legs for swimming. Some, like water striders, glide on the surface tension. Their feeding strategies are diverse, including predators, detritivores, herbivores, and filter feeders.
Notable Aquatic Insect Examples
Dragonfly nymphs, the aquatic immature stage of dragonflies, are agile predators in ponds and streams. Their internal rectal gills aid breathing and jet propulsion.
Mayfly nymphs are typically found clinging to rocks in various freshwater habitats, from clear streams to lakes. These sensitive insects breathe using external gills along their abdomen and primarily feed on algae or detritus, though some species are carnivorous.
Caddisfly larvae construct protective cases from silk, pebbles, or plant material, carrying them along the streambed. These larvae have filamentous gills and break down organic matter.
Water striders glide on the surface of still or slow-moving waters, using long, hydrophobic legs to distribute weight. They prey on fallen insects.
Water boatmen are small, oval bugs that swim upside down with oar-like hind legs. They often use an air bubble or plastron for respiration.
Diving beetles are powerful swimmers and predators that can spend their entire lives in water. They carry an air bubble under their wing covers, acting as a temporary oxygen reserve and physical gill, allowing extended underwater hunting.
Importance of Aquatic Insects
Aquatic insects play important roles in freshwater ecosystems, contributing to their health and balance. They are a foundational component of aquatic food webs, serving as a primary food source for many organisms. Fish, amphibians, birds, and some terrestrial animals rely on them for sustenance, both as immature aquatic stages and adult flying forms.
Beyond being prey, these insects are instrumental in nutrient cycling and organic matter decomposition. Species like caddisflies and midges break down decaying plant material and debris, releasing nutrients like nitrogen and phosphorus back into the water. This process maintains ecosystem productivity.
Aquatic insects are also recognized as bioindicators of water quality. The presence or absence of sensitive species, such as mayflies, stoneflies, and caddisflies, can signal the environmental health of a freshwater body.