As virtually all fly fishermen and fisherwomen know, caddisflies are insects that build small cases of various materials, like pebbles, sand, leaves or twigs, found in various freshwater habitats.
Perhaps the greatest reason for anglers’ interest is that caddisfly larvae, pupae and adults are a favorite food for trout, and thus make great bait.
Caddisflies belong to the order Trichoptera. The name comes from the Greek words trichos (referring to “hairs”) and pteron ( meaning “wing”). Not surprisingly, Trichopteran adults have special hairs and scales on their wings. The common name caddisfly probably comes from Middle Ages England when cloth salesmen, called “cadismen,” attached elongated strips of cloth to their garments as an advertisement of their wares. Many pond and lake species of caddisflies attach elongated stems, grasses and twigs to their special cases. These fuzzy-looking cases then serve as protection or camouflage for the larva.
Caddisflies are closely related to moths and with their wings folded tentlike over their abdomen, they are often mistaken for small moths. Besides this look-a-like resemblance to their close relatives, caddisflies can also produce silk from special silk glands in the larval head. Larvae use this silk to bind various materials into protective cases that surround their somewhat unprotected abdomen. As the larva grows in size, the case is enlarged to fit its bigger waist size.
Some cases serve as effective camouflage, protecting them from visual predators like birds or fish. In other situations, the case seems to give them buoyancy and keeps them entangled in floating or standing masses of plant stems. In faster-moving water, the case may act as an anchor or ballast, keeping the case builder from being washed downstream.
These cases can take on a large number of shapes and sizes and are as varied as the materials caddisflies find in the habitat. The most common caddisfly in the Kenai River is brachycentrus, and its distinctive case looks like an elongated box with squared-off corners. They cut tiny, uniform sections of grasses and leaf fragments and assemble them like a miniature, hollow log cabin. There are other groups in the Kenai River that make distinctive cases of uniform sand grains in an elongated tubular shape reminiscent of a hot dog.
Not all caddisfly larvae make cases. Some use their ability to spin silk to create ultrafine nets between gravel or twigs that filter out fine particles drifting in the water column. The plant and animal materials that are captured by these tiny homemade nets are then consumed by the larva. Sometimes they will consume the whole net and particles together and then re-spin a new capture net.
Once the larvae have grown to a sufficient size and receive the appropriate temperature and light cues from the environment, they form a pupal case. This case is attached to a rock or nearby structure. A few days later they crawl out of their pupal skin and emerge as an aerial insect.
In midsummer along the Kenai River we often see large numbers of small gray caddisflies (brachycentrus) flying upstream to mate and lay eggs somewhere along the river.
Depending on the species, and there is a great deal of variety here, most of the caddisfly life cycle is spent as an aquatic larva with only a week or so as an aerial adult. However, there are some caddisfly species from the Kenai River that overwinter as adults.
Fishermen and women have a great many fly patterns that are tied to mimic the adult caddisfly on the surface of the water, laying eggs in a jerky, erratic pattern. There are an equal number of flies tied to mimic the larval stage and even some designed to mimic the vulnerable emerging pupal stage. These fly patterns work all over Alaska, since there are caddisflies in almost every freshwater habitat.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the biology of the Kenai River watershed.