Caddis flies, order Trichoptera, are small flying insects that are probably more closely related to the butterflies and moths (order Lepidoptera) than to true flies (order Diptera). Their forewings are hairy and from this characteristic comes the name of the order: trikhos is Greek for hairy and pteron means wing. Caddis flies go through complete metamorphosis like the Diptera and the Lepidoptera but this is the only order whose larvae are entirely aquatic. These aquatic larvae are more important to humans than the adults because caddis fly larvae are a very important food source for fresh water fish, especially trout. If you are a fisherman, you have reason to be grateful to these insects and if you are a fly fisherman, you have probably made lures that resemble either the adult flies or their larvae.
There are over 5000 species of Caddis flies worldwide and they and their larvae can be found by almost any fresh water river, stream, lake or pond. The adults are usually small and drab with four moth-like wings and long, slender antennae. They are equipped with mouthparts designed to lap fluids but many do not feed. They are active at dusk and dawn and rest during the day. Their swarms are chiefly sexual in nature and mating takes place on the wing. Caddis flies reproduce once of year and then overwinter as larvae. They pupate in the spring and emerge as adults in early summer. The eggs are deposited in masses and covered in a sticky substance that attaches them to rocks or underwater vegetation.
The larvae are both omnivorous and voracious. They are caterpillar-like and have a hard cuticle over the head and thorax. They have small antennae, simple eyes, chewing mouthparts, and they breathe through external, filamentous, tracheal gills. They can use their six legs for walking, catching their prey and building a case to protect their soft abdomen. These cases are the most obvious signs that caddis fly larvae are in the water. They are made from sand grains, vegetable matter and even sticks and pebbles, all held together by a sticky glue. As the larva grows, it either adds to its case or moves out and makes a new one. Often caddis fly larvae can be seen dragging their protective cases along the bottom using their six strong legs. Case makers are usually herbivorous.
Some species construct silken nets under the water which face upstream and catch plankton and small crustaceans. While most species feed on plant material, those that are predatory make these webs and then use their forelegs to take and hold prey.
Caddis fly larvae are very important parts of aquatic food chains, consuming plant matter, plankton and crustaceans, and then being consumed themselves by fish, birds and frogs. The larvae and pupae may also be parasitised by wasps. Caddis fly larvae are also an indicator of the health of a river or stream. They are most common and diverse in healthy streams and become less numerous and diverse as pollution increases.