Caddisflies: Trichoptera

All caddisfly larvae spin silk to make nets to capture food floating in the water or build protective shelters. Shelters may be silken bags or made with small pebbles, sand, or plant materials attached together with silk to form a case. The materials used and the shape of the case vary with each species. The larvae can be categorized into five groups based on their case-building behavior. Free-living forms construct shelters only for pupation. Saddle-case makers build cases resembling tortoise shells. Purse-case makers are free-living until they are ready to pupate. Then they build silken purselike or barrellike cases. Net-spinners build a fixed silken retreat on the rocky bottoms of swift streams with a weblike net to capture bits of plants and animals floating in the water. Tube-case builders are probably the most familiar. They use bits of leaves, twigs, or small gravel to construct portable cases. With only their head, thorax, and legs sticking out of the case, the larvae drag themselves across the stream bottom to search for food. The ability of caddisfly larvae to build their own shelters allows them to live in a variety of aquatic habitats.

Caddisflies are ready to mate as soon as they emerge from their cocoons. Some females attract males with chemical scents or pheromones (FEH-re-moans). In other species, males gather in swarms and engage in dances to attract females. Some species also make sounds, but these have different meanings among different species. For example, drumming sounds may drive some species to mate, but in others the same sounds are part of a defensive behavior that signals an attack. Many species also flap or spread their wings as a part of courtship, but males of other species may use these movements as a sign of attack toward other males.

Males transfer sperm or a sperm packet directly to the reproductive organs of the females. Mating usually takes place near the larval habitat, either on streamside vegetation or on the ground. They may stay together for just a few minutes or several hours. Both males and females may mate several times with other partners.

The life cycles of caddisflies includes four very distinct stages: egg, larva, pupa, and adult. The adult female can stay under the water up to thirty minutes as she glues her eggs to rocks and plants. In some species the female carries her eggs on the tip of the abdomen. She then flies upstream, dipping her abdomen into the water to deposit the eggs. Other species simply lay their eggs on plants hanging over the water. Case-builders usually pupate inside their shelters. Even species that do not build shelters use silk to make a cocoon before pupating inside. Some species cover their cocoons with small loosely stacked pebbles.

After emerging from its pupa the adult cuts its way out of the cocoon and swims to the surface. The adults are mostly active in spring and summer, but a few species emerge in the winter. They live for only a short time and spend most of their time looking for a mate. Many species are attracted to lights at night. Since larvae are usually washed downstream by the water current, many species will fly short distances upstream to lay their eggs.