The larval forms of insects are of most value to an Aquatic ecosystem. Some adult insects may fall onto the water to be consumed by fish or birds but the larvae makes the greatest contribution to an equatic ecosystem.
The adults are often seen flitting about around ponds and streams but the larvae of these insects are an important food source for fish. The larval form of dragonflies and damselflies are called nymphs. The are generally found in slow moving or standing water and tolerant of water with low oxygen levels. The larvae are predators that feed on other insect larvae, each other and for some species even fish fry.
The caddisfly larvae are generally found in the clear flowing water of some stream. The are somewhat sensitive to low oxgen levels and pollution.
The larvae of the dobson fly is the Hellgrammite.
These insects in their adult form look like giant mosquitoes. Fortunately they are not, they actually feed on mosquitoes. As larvae they feed on manure, sewage and organic matter in ponds.