Some sewage treatment plants are designed with trickle filters that harbor aquatic insects and other decomposers. The insect life in a pond, lake, or stream often has a significant impact on other organisms in the evironment. Insects are a major part of the diet for many fish and other small vertebrates. Aquatic insects may serve as biological control agents to clear weeds, like the water hyacinth, from commercial and recreational waterways. Marsh flies are parasites of snails and Coelopids feed on seaweeds.
Many insects live in the water as immatures and then move to terrestrial habitats as adults.
Some of these, like mosquitoes and black flies, spread human diseases in their adult stage.