Caddis-fly is the common name given to the insect order Trichoptera, of which between 4,000 and 5,000 species are known throughout the temperate zones of the world. All caddis-fly larvae are aquatic, living in freshwater and breathing by external gills on the sides of the abdominal segments, and all spin silk. The adult insects look much like moths and fly mainly at night.

Larvae

The most interesting feature of the caddis-flies is the life of the larvae, which varies in the different families and genera. They can be divided into two main types -- those which build portable cases and are almost all vegetarians, and those which live free and are at least partly carnivorous.

The case-builders use many materials in various ways to build their tubes. Members of the genus Phryganea cut pieces of leaves and stick them together with silk, while those of the genus Limnophilus use small stones and pieces of plant stems or empty snail shells. If removed from their cases and given beads or similar objects, some will use the artificial material to make new cases. Stenophylax and Heliopsyche use fine sand grains to make their cases. Cases made of stones or sands often have their weight reduced by a bubble of air trapped inside.

All caddis-fly cases are tubular and open at one end; the larva pushes its head and thorax out this end in order to feed and move about. The other end is closed with a silken mesh so that a current of water can flow through and aerate the gills. All caddis-fly larvae have a pair of hooked limbs at the back, used to hold onto the case. So tightly can a caddis-fly larva grip with those limbs attempts to pull it out of its case almost always result in injury to the larva.

Most larvae with non-portable cases live in silken tubes, in flowing water. Larvae of the species Plectronemia make a silk tunnel with the open end facing upstream widely flared to form a trumpet-shaped net. Any small animal or piece of plant material carried into this "web" is seized and eaten by the larva. Many other stream-dwelling caddis-fly larvae make nets of various shapes to gather food. When they are damaged, or choked with inedible material, the larvae clean and repair them.

Diet

Most tube-bearing caddis-fly larvae subsist primarily on the leaves and stems of live plants. The large case-bearing larvae of Phryganea catch and eat water insects as well as plant food. Most of the tube-dwelling or free-living larvae have a mixed diet.

The mouth parts of adult caddis-flies are vestigial, and they can take only liquid food. In the wild they probably feed from flowers with exposed nectaries.

Life History

Females lay their eggs in spring and summer. Some drop them on the surface as they are flying over, others crawl underwater and stick them to stones or plants in a jelly-like mass.

Some of the larvae do not make cases or tubes until they have moulted several times, others make tiny cases as soon as they hatch.

When the larva is fully grown, about a year after hatching, it pupates, inside the case if it belongs to a case-bearing species, otherwise in a silken coccoon. When the time comes for the adult insect to emerge, the pupa bites its way out of the case, being equipped for the purpose with strong mandibles, and swims to the surface of the water. There it splits open, releasing the adult caddis-fly, which can fly almost immediately on emergence.

The life history of a caddis-fly lasts approximately one year, of which the adult stage is but a very small fraction.

Questions or comments about this page?

Dr. Maurice Burton and Robert Burton. Funk & Wagnalls Wildlife Encyclopedia. New York:Funk & Wagnalls, Inc., 1970.

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Science.--Zoology.--Invertebrates.--Phylum Arthropoda..--Class Insecta (Insects).--Order Trichoptera (Caddis Flies).

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