Caddisfly

Caddisflies make up the order Trichoptera, a diverse group of moth-like insects most closely related to true moths and butterflies, though their wings are covered in fine hairs rather than the flattened scales that give moths their powdery texture. There are thousands of described species worldwide, occupying nearly every kind of freshwater habitat.

The larvae are best known for building elaborate protective cases from silk combined with sand grains, small stones, twigs, or leaf fragments, a behavior unique among aquatic insects and one that varies distinctively by species, allowing biologists to identify many caddisflies from their case architecture alone. Some species instead spin fixed silk nets to filter food from the current rather than building a portable case.

Because caddisfly larvae are highly sensitive to water quality and require clean, well-oxygenated water to thrive, their presence or absence in a stream is widely used by ecologists as a reliable indicator of freshwater health.

• Adult resembles a small, dull-colored moth with wings held tent-like (roof-like) over the body at rest
• Wings covered in fine hairs rather than scales, giving them a somewhat fuzzy or dull texture instead of a moth's powdery sheen
• Long, thread-like antennae, often as long as or longer than the body
• No coiled proboscis like a butterfly or moth; mouthparts are reduced
• Larvae are soft-bodied and often live inside a portable case built from silk and debris (sand, plant bits, tiny stones), though some species are caseless and free-living or net-spinning
• Lookalikes: small moths (caddisflies lack scaled wings and a coiled proboscis)

Caddisflies are found worldwide, essentially everywhere there is clean freshwater, including streams, rivers, ponds, and lakes. Larvae occupy a wide range of microhabitats within these waters, from fast riffles to slow pools and still water, depending on species. Adults typically stay close to their natal water body, resting on streamside vegetation, rocks, or structures by day and becoming more active around dusk.

Adult caddisflies feed little, mainly on nectar or not at all in many species, and are most active at dusk and after dark, often drawn to lights near water. Larvae display an unusually wide range of feeding strategies depending on species: some are herbivorous grazers on algae and biofilm, some collect fine organic particles, some are predators of other small invertebrates, and net-spinning species filter drifting food particles directly from the current. As both prey and predator, caddisfly larvae form an important part of freshwater food webs and are a major food source for fish.

Females lay masses of eggs in or near water, often coated in a gelatinous substance. Larvae hatch and, in case-building species, immediately begin constructing a protective case suited to their species and habitat, growing and lengthening the case through several instars over several months to about a year. Mature larvae seal themselves inside their case or a fixed shelter to pupate underwater. The pupa eventually cuts its way free, swims or crawls to the surface, and emerges as a winged adult, which lives for only a short time to mate and lay eggs.