Illinois Natural History Survey - University of Illinois

Species Spotlight: Caddisflies

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"Stretched over the brooks, in the midst of frost-bound meadows, we may observe the submarine cottages of the caddis-worms, the larvae of the Plicipennes; their small cylindrical cases built around themselves, composed of flags, sticks, grass, withered leaves, shells, and pebbles, in form and color like the wrecks which strew the bottom, --now drifting along over the pebbly bottom, now whirling in tiny eddies and dashing down steep falls, or sweeping rapidly along with the current, or else swaying to and fro at the end of some grass-blade or root."

Henry David Thoreau

Thoreau is describing the larvae of caddisflies, an insect order familiar to fly-fishermen and aquatic entomologists alike. Caddisfly adults are mothlike and have two pairs of black, gray, or brown wings that are held rooflike over the body. Their wings are covered with hairs instead of scales and they have relatively long, slender antennae. These insects are closely related to the butterflies and moths (Lepidoptera), but they are adapted for an aquatic life in the immature stages.

Caddisflies belong to the insect order Trichoptera, which means hairy wing. It is a large insect order with over 1,200 species in North America; 184 species are found in Illinois. The common name "caddisfly" comes from a Middle English term meaning bits and pieces of worsted yarn and refers to the larvae and the way they make their unique homes. Other common names include shadflies, sandflies, traveling sedges, caddis worms, and periwinkles.

Caddisflies have complete metamorphosis (different adult and larval stages). Eggs are laid in the water on aquatic plants, in overhanging vegetation, or at the shoreline. The eggs hatch into larvae, which are elongate, and 2-40 mm in length. They have chewing mouthparts and small, simple eyes. Only the forward part of the larval body (head, thorax, and legs) is hardened. The remainder is soft and flexible. As soon as the larvae hatch they begin construction of their "homes." The larvae construct an assortment of portable cases in which to house and protect themselves. The most studied and fascinating aspect of caddisfly biology is the form and behaviors relating to the cases in which they live.

The larvae use stone, sand, pebbles, or twigs to construct their houses and silk to hold it together. These cases are so distinctive that larvae can be recognized and identified by their cases alone. The larvae anchor themselves in the case with hooks at the tip of the abdomen and as the larvae molts the cases are enlarged by adding material to the entrances. There are usually five larval instars.

The majority of the larvae eat algae, diatoms, and other aquatic vegetation. However, a few larvae are carnivorous and instead of making a "house," these larvae are free-living and spin nets which they anchor to vegetation and use to seine for insects and crustaceans. To complete development the larvae seal the case (house) with silk, spin a cocoon, and pupate inside. About a month later the pupa swims up to the water surface, finds a rock or log to settle on, and emerges as an adult. Once the adult emerges it quickly flies away from the water. The adults can live 1-2 months and are usually found in damp woods, wetlands, and riparian areas.

Caddisfly larvae are common bottom fauna in most freshwater environments and they occur in association with all substrate types. Their cases and retreats are seemingly well-adapted for camouflaging and protecting them from predators; however, both adults and larvae are part of the diet of many sport fish (trout can consume case and all). While caddisflies are not as important as mayflies to fly-fishermen, they are the models for the Grannom, White Miller, Adams, and Grizzly King, all flies used by fishermen in North American waters.

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Susan Post, Center for Economic Entomology

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