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Trichoptera = Photos
Caddis fly larvae are all aquatic and feed principally on decaying vegetation in water, though some species are also predaceous. The eggs are laid in masses either on stones, plants or other objects in the water or on objects above the water surface. The larvae are eruciform and build silken cases or tubes about themselves which are overlaid with particles of sand, small pebbles or other objects (Clausen 1940/1962)
There are about 12,010 described species. Also called sedge-flies or rail-flies, they are small moth-like insects with two pairs of hairy membranous wings. They are closely related to Lepidoptera (moths and butterflies) which have scales on their wings, and the two orders together form the superorder Amphiesmenoptera. The aquatic larvae are found in a variety of habitats such as streams, ponds, rivers, lakes, spring seeps, and vernal pools. The larvae of many species construct cases of silk that are covered with gravel, sand, twigs or other detritis. The name is derived from Greek: trich, "hair" + ptera, "wings".
Although caddisflies may be found in water of varying qualities, species-rich caddisfly assemblages are generally thought to indicate clean water. Together with stoneflies and mayflies, caddisflies feature importantly in bioassessment surveys of streams and other water bodies. Caddisfly species can be found in all feeding guilds in stream habitats, with some species being predators, leaf shredders, algal grazers, and collectors of particles from the watercolumn and benthos
Caddisflies are underwater architects as many species use silk for building throughout their larval life. Caddisflies can be loosely divided into three behavioral groups based on this use of silk: retreat-making caddisflies, case-making caddisflies, and free-living caddisflies. Those that build retreats build a net or retreat from silk and other materials and use it to catch food items such as algae, aquatic invertebrates and zooplankton from the flowing stream. Case-making caddisflies make portable cases using silk along with substrate materials such as small fragments of rock, sand, small pieces of twig, aquatic plants, or sometimes silk alone. Many use the refuges or cases throughout their larval life, adding to, or enlarging them as they grow. These may look very much like bagworm cases, which are constructed by various moth species that are not aquatic. Free-living caddisflies do not build retreats or carry portable cases until they are ready to pupate.
Many species of caddisfly larvae enter an inactive state called the pupa stage for weeks or months after they mature but prior to emergence as adults. Adult emergence is then triggered by cooling water temperatures in the fall, effectively synchronizing the adult activity to make mate-finding easier. In the Northwestern US, caddisfly larvae within their gravel cases are called periwinkles.
Caddisfly pupation occurs much like pupation of Lepidoptera. That is, caddisflies pupate in a cocoon spun from silk. Caddisflies which build the portable cases attach their case to some underwater object, seal the front and back apertures against predation though still allowing water flow, and pupate within it. Once fully developed, most pupal caddisflies cut through their cases with a special pair of mandibles, swim up to the water surface, cast off skin and the now-obsolete gills and mandibles, and emerge as fully formed adults. In a minority of species, the pupae swim to shore (either below the water or across the surface) and crawl out to emerge. Many of them are able to fly immediately after breaking from their pupal skin.
The adult stage of caddisflies is usually very short-lived, usually only 1–2 weeks, but can sometimes last for 2 months. Most adults are non-feeding and are equipped mainly to mate. Once mated, the female caddisfly will often lay eggs (enclosed in a gelatinous mass) by attaching them above or below the water surface. Eggs hatch in as little as three weeks.
Species that occur in most temperate areas complete their life cycles in a single year. The general temperate-zone lifecycle pattern is one of larval feeding and growth in autumn, winter, and spring, with adult emergence between late spring and early fall, although the adult activity of a few species peaks in the winter. Larvae are active in very cold water and can frequently be observed feeding under ice. Many caddisfly adults emerge in synchrony in large numbers. Such emergence ensures that most caddisflies will find a member of the opposite sex quickly. Mass emergences of this kind are called 'hatches' by salmon and trout anglers, and salmonid fish species will often switch to whatever species is emerging at a particular time. Fishermen take advantage of this behavior by trying to match their artificial flies to the appropriate caddisfly species.
Hoell, H.V., Doyen, J.T. & Purcell, A.H. 1998. Introduction to Insect Biology and Diversity, 2nd ed.. Oxford University Press. p. 320.