Caddisflies, sedge-flies or rail flies (Order Trichoptera, from Greek trich, "hair", and ptera, "wings") are small moth-like insects having 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. Caddisflies have aquatic larvae and are found in a wide variety of habitats such as streams, rivers, lakes, ponds, spring seeps, and temporary waters (vernal pools). The larvae of many species make protective cases of silk decorated with gravel, sand, twigs or other debris.

Ecology

Although caddisflies may be found in waterbodies 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.

Underwater architects

Caddisflies are considered underwater architects because most species use silt for building throughout their larval life. Caddisflies can be loosely divided into three behavioral groups based on this use of silt: retreat-making caddisflies, case-making caddisflies, and free-living caddisflies. Those that build retreats build a net or retreat from silt 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 retreats 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.

Development

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 - see figure - or across the surface) and crawl out to emerge. Many of them are able to fly immediately after breaking from their pupal skin.