CADDISFLIES: Trichoptera

PHYSICAL CHARACTERISTICS

Adult caddisflies are slender, mothlike insects that are usually drab in color, although some species are very brightly marked. They measure 0.048 to 1.76 inches (1.2 to 40 millimeters) in length. Both compound and simple eyes are present. Compound eyes have multiple lenses; simple eyes have only one. The chewing mouthparts are made up of long, fingerlike appendages on either side of very small jaws. The antennae (an-TEH-nee), or sense organs, are long, threadlike, and held together out in front of the body. Both the head and the thorax, or midsection, have wartlike bumps. All four wings are similar to one another in size and appearance. They are folded like a roof over the body when at rest. Most of the wing veins are straight and have very few branches, or cross-veins. The females of some species have very small wings and cannot fly. The legs are long, slender, and have scattered spines. The ten-segmented abdomen is tipped, with reproductive organs that vary in shape.

The larvae (LAR-vee), or young, resemble caterpillars. Their heads are distinct and hard with strong jaws. Each of the three segments of the thorax has a pair of well-developed legs. A thick plate covers one or more of the thoracic segments. The long, soft ten-segmented abdomen is slender or plump and sometimes has gills along the sides. The last abdominal segment has a pair of leglike structures.

GEOGRAPHIC RANGE

Caddisflies are found on all continents except Antarctica. Most species have relatively small ranges, and many are found only in one or a few countries. There are more than 11,000 species worldwide, with about 1,400 species known in the United States and Canada.

HABITAT

Most caddisfly larvae and pupae (PYU-pee), the life stage between larva and adult, are found in freshwater, but there are a few species that live on land or in the sea. The larvae of freshwater species usually live in cold clean flowing waters, but some species prefer warmer slower waters. They are very particular about water temperature and speed, dissolved minerals and pollutants, as well as the amount of sunlight. Several species can live together in a stream or river because each occupies habitats within the water that do not overlap. Predatory species wander about freely in the environment hunting for food animals, but many plant-feeding species live in protective cases built from pebbles, sand, or bits of vegetation from the bottom and held together with silk. A marine species from Australia and New Zealand spends part of its larval life eating tissues inside the body cavity of a living sea star (starfish). Later it leaves its host to build a case from seaweed.

The adults are usually active at night. They spend their days hiding in moist, cool habitats and are often found on vegetation growing along river banks.

DIET

Adult caddisflies eat only plant fluids such as nectar or sap. Depending on the species, the larvae eat bits of plant materials on the bottom or in the water, living plants, living and dead animals, or most or all of the above. Species that do not build cases usually feed on tiny bits of plants or prey on other insects. Case builders shred leaves, graze on living plants, or scrape algae (AL-jee) from rocks, wood, and other surfaces. Some species use their jaws to pierce threadlike algae and suck its fluids, one cell at a time.

BEHAVIOR AND REPRODUCTION

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.

CADDISFLIES AND PEOPLE

Some South American native peoples use larval cases as earrings and as beads for necklaces. Beginning in the 1980s, the visual artist Hubert Duprat utilized caddisflies to create unique sculptural forms. He first removed larvae from their natural habitat, and then he provided the larvae with different colored pebbles, sand, or ground up seashells or glass materials. The caddisflies used these materials to build "jeweled" cases. Since then other companies have used this method to make earrings, necklaces and other types of jewelry.

Salmon and other fishes eat caddisfly larvae, pupae, and adults. Fly fishermen make all sorts of lures that mimic the various stages of caddisfly development and use them instead of bait to catch fish.

A few species of caddisflies are considered pests. Some chew on wood structures in the water, while others nibble on rice plants and aquatic ornamental plants sold in nurseries. The adults are often attracted by the thousands to lights, clogging air conditioners with their bodies. Others lay their eggs on the shiny road surface, mistaking it for water. Thousands of crushed eggs make the roads slippery and a hazard to drivers.

WHEN IS A FLY NOT A FLY?

The common names for true flies (Diptera), such as bee fly, crane fly, fruit fly, hover fly, house fly, and robber fly, always have two words. But insects that are not dipterans have common names that are written as one word, like butterfly (Lepidoptera), caddisfly (Trichoptera), dobsonfly (Megaloptera), dragonfly and damselfly (Odonata), mayfly (Ephemeroptera), sawfly (Hymenoptera), scorpionfly (Mecoptera), snakefly (Raphidioptera), and stonefly (Plecoptera).

CONSERVATION STATUS

Four species of caddisflies are listed as Extinct by the World Conservation Union (IUCN). These four species no longer exist. All caddisflies are vulnerable to changes in water quality. Since many species are found only in a small region, the slightest disturbance in their environment may have a devastating effect on the entire population. Because of their sensitivity to water pollution, the presence or absence of caddisfly larvae is used as an indicator of water quality.

NO COMMON NAME (Triaenodes bicolor): SPECIES ACCOUNT

Physical characteristics: The larvae of this species measure 0.34 to 0.52 inches (8.5 to 13.0 millimeters) in length. They are brownish yellow in color. The adults have slender brown bodies. The antennae are very long.

Geographic range: This species lives in Europe and western Russia.

Habitat: The larvae are found on plants growing in shallows close to the river bank, usually at depths of 7.87 to 59.05 inches (0.2 to 1.5 meters).

Diet: The larvae eat green plants.

Behavior and reproduction: The larvae build cases with long bits of plant material arranged in a spiral. The cases become narrow at the end and measure 0.6 inches (15 millimeters) in length. Mature larvae remodel their cases just before pupation.

Females lay their eggs in a spiral pattern on aquatic plants.

Triaenodes bicolor and people: This species is a pest in cultivated rice fields.

Conservation status: This species is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

LaFontaine, G. Caddisflies. New York: The Lyons Press, 1994.

Tavolacci, J., ed. Insects and Spiders of the World. Volume 2: Beetle-Carpet Beetle. New York: Marshall Cavendish, 2003.

Wiggins, G. Larvae of the North American Caddisfly Genera (Trichoptera). 2nd ed. Toronto and Buffalo, New York: University of Toronto Press, 1996.

Periodicals:

"Surprising Snapshot of a Long-lost World. Geographica." National Geographic 190, no. 4 (October 1996).

Web sites:

"Trichoptera. Caddisflies." Ecowatch. http://www.ento.csiro.au/Ecowatch/Trichoptera/Trichoptera.htm (accessed on October 27, 2004).

"Trichoptera. Caddisflies." Tree of Life Web Project. http://tolweb.org/tree?group=Trichoptera&contgroup=Endopterygota#top (accessed on October 27, 2004).

"The Trichoptera (Caddis Flies)." Earthlife. http://www.earthlife.net/insects/trichopt.html (accessed on October 27, 2004).

Videos:

Bug City. Aquatic Insects. Wynnewood, PA: Schlessinger Media, 1998.