The Trichoptera, or Caddis Flies, are an order of small insects best known for their case-making aquatic larvae. There are about 12,000 known species. The adults look like small, hairy moths, usually drab brown or grey and quite cryptic. They are usually seen around streams and ponds or lakes. They have well developed compound eyes, reduced mouthparts which can be used to take water or nectar, long simple antennae, and two pairs of hairy coupled wings that are held roofwise over the body when at rest. The hind wings are shorter and broader than the forewings, and both pairs have simple venation with few cross veins. The legs are strong and have spines at the joints.
The aquatic larvae have hard heads and soft abdomens. They have chewing mouthparts and small antennae. Most make cases of sand grains or plant materials glued together to form a tube. The soft abdomen is protected by the case, which the larva drags around with its strong legs. A few species are free living or make a silk retreat. Free living larvae are usually predatory while the case makers are herbivores.
Most adults are active at dusk, night time or in the early mornings. During the day they shelter in vegetation or hide in cool, dark and damp habitats such as under bridges or under rock overhangs. They mate on the wing at dusk, then the females go into the water to lay their eggs or lay them on surface objects. The eggs hatch into larvae that go through several instars before pupating into the adult form. Case makers move about by crawling, while those who do not have cases can usually swim as well as crawl. As with many insects, most of the caddis fly's life is spent as a larva. The adults usually do not feed and are quite short-lived, their main purpose being reproduction.
Caddis fly larvae are a very important group in many stream food chains. They feed on aquatic vegetation, and then are preyed upon by fish as well as other insects. Trout fishermen are particularly indebted to the humble caddis fly larvae, which form a large part of the diet of trout. Adult Trichoptera are food for nocturnal predators, such as small bats, and may also be eaten by frogs when the females are egg laying. A few species are also economically important as rice paddy pests whose larvae eat the young rice shoots. Because Caddis fly larvae are sensitive to pollution, they are also a good indicator of water quality.
Information sources: Britten etal 1979 Insects of Australia CSIRO