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Order of caddisflies

The caddisflies, or order Trichoptera, are a group of insects with aquatic larvae and terrestrial adults. There are approximately 14,500 described species, most of which can be divided into the suborders Integripalpia and Annulipalpia on the basis of the adult mouthparts. Integripalpian larvae construct a portable casing to protect themselves as they move around looking for food, while annulipalpian larvae make themselves a fixed retreat in which they remain, waiting for food to come to them. The affinities of the small third suborder Spicipalpia are unclear, and molecular analysis suggests it may not be monophyletic. Also called sedge flies or rail flies, the adults are small moth like insects with two pairs of hairy membranous wings. They are closely related to the Lepidoptera (moths and butterflies) which have scales on their wings; the two orders together form the superorder Amphiesmenoptera. The aquatic larvae are found in a wide variety of habitats such as streams, rivers, lakes, ponds, spring seeps and temporary waters (vernal pools), and even the ocean. The larvae of many species use silk to make protective cases, which are often strengthened with gravel, sand, twigs, bitten off pieces of plants, or other debris. The larvae exhibit various feeding strategies, with different species being predators, leaf shredders, algal grazers, or collectors of particles from the water column and benthos. Most adults have short lives during which they do not feed. In fly fishing, artificial flies called dry flies are tied to imitate adults, while larvae and pupae are imitated with artificial flies called wet flies or nymphs. It is also possible to use them as bait, though this is not as common as artificial flies and is known as bait fishing. Common and widespread genera such as Helicopsyche and Hydropsyche are important in the sport, where caddisflies are known as "sedges". Caddisflies are useful as bioindicators, as they are sensitive to water pollution and are large enough to be assessed in the field. In art, the French artist Hubert Duprat has created works by providing caddis larvae with small grains of gold and precious stones for them to build into decorative cases.

The name of the order "Trichoptera" derives from the Greek: [[:wikt:θρίξ (thrix, "hair"), genitive trichos + [[:wikt:πτερόν (pteron, "wing"), and refers to the fact that the wings of these insects are bristly. The origin of the word "caddis" is unclear, but it dates back to at least as far as Izaak Walton's 1653 book The Compleat Angler, where "cod worms or caddis" were mentioned as being used as bait. The term cadyss was being used in the fifteenth century for silk or cotton cloth, and "cadice men" were itinerant vendors of such materials, but a connection between these words and the insects has not been established.

Fossil caddisflies have been found in rocks dating back to the Triassic. The largest numbers of fossilised remains are those of larval cases, which are made of durable materials that preserve well. Body fossils of caddisflies are extremely rare, the oldest being from the Early and Middle Triassic, some 230 million years ago, and wings are another source of fossils.

Nearly all adult caddisflies are terrestrial, but their larvae and pupae are aquatic. They share this characteristic with several distantly related groups, namely the dragonflies, mayflies, stoneflies, alderflies and lacewings. The ancestors of all these groups were terrestrial, with open tracheal systems, convergently evolving different types of gills for their aquatic larvae as they took to the water to avoid predation. About 14,500 species of caddisfly in 45 families have been recognised worldwide, but many more species remain to be described. Most can be divided into the suborders Integripalpia and Annulipalpia on the basis of the adult mouthparts. The characteristics of adults depend on the palps, wing venation and genitalia of both sexes. The latter two characters have undergone such extensive differentiation among the different superfamilies that the differences between the suborders is not clear cut. The larvae of Annulipalpians are campodeiform (free living, well sclerotized, long legged predators with dorso ventrally flattened bodies and protruding mouthparts). The larvae of Integripalpians are polypod (poorly sclerotized detritivores, with abdominal prolegs in addition to thoracic legs, living permanently in tight fitting cases). The cladogram of relationships within the order is based on a 2002 molecular phylogeny using ribosomal RNA, a nuclear elongation factor gene, and mitochondrial cytochrome oxidase. The Annulipalpia and Integripalpia are clades, but the relationships within the Spicipalpia are unclear.

Caddisflies are found worldwide, with the greater diversity being in warmer regions. They are associated with bodies of freshwater, the larvae being found in lakes, ponds, rivers, streams and other water bodies. The land caddis, Enoicyla pusilla (family: Limnephilidae), lives in the damp litter of the woodland floor. In the United Kingdom it is found in and around the county of Worcestershire in oakwoods. One such opportunistic species is Gumaga nigricula (family: Sericostomatidae) which has been observed scavenging fish carcasses and even bits of deer flesh. This particular family of caddisflies is typically classified among the shredders, suggesting caution when classifying macroinvertebrates into strict ecological functional groups, as some may shift their diets opportunistically. They are an important part of the food web, both larvae and adults being eaten by many fish. The newly hatched adult is particularly vulnerable as it struggles to the surface after emerging from the submerged pupa, and as it dries its wings. The fish find these new adults easy pickings, and fishing flies resembling them can be successful for anglers at the right time of year. The adult stage of a caddisfly may only survive for a few weeks; many species do not feed as adults and die soon after breeding, but some species are known to feed on nectar. They form an important part of the diet of fish such as the trout. The fish acquire them by two means, either plucking them off vegetation or the stream bed as the larvae move about, or during the daily behavioural drift; this drift happens during the night for many species of aquatic larvae, or around midday for some cased caddisfly species, and may result from population pressures or be a dispersal device. The larvae may drift in great numbers either close to the bottom, in mid water or just below the surface. The fish swallow them whole, case and all.

Caddisflies are best known for the portable cases created by their larvae. About thirty families of caddisfly, members of the suborder Integripalpia, adopt this stratagem. These larvae eat detritus, largely decaying vegetable material, and the dead leaf fragments on which they feed tend to accumulate in hollows, in slow moving sections of streams and behind stones and tree roots. The cases provide protection to the larvae as they make their way between these resources. The case is a tubular structure made of silk, secreted from salivary glands near the mouth of the larva, and is started soon after the egg hatches. Various reinforcements may be incorporated into its structure, the nature of the materials and design depending on the larva's genetic makeup; this means that caddisfly larvae can be recognised by their cases down to family, and even genus level. The materials used include grains of sand, larger fragments of rock, bark, sticks, leaves, seeds and mollusc shells. These are neatly arranged and stuck onto the outer surface of the silken tube. As the larva grows, more material is added at the front, and the larva can turn round in the tube and trim the rear end so that it does not drag along the substrate.

In contrast to larvae that have portable cases, members of the Annulipalpia have a completely different feeding strategy. They make fixed retreats in which they remain stationary, waiting for food to come to them. Members of the Psychomyiidae, Ecnomidae and Xiphocentronidae families construct simple tubes of sand and other particles held together by silk and anchored to the bottom, and feed on the accumulations of silt formed when suspended material is deposited. The tube can be lengthened when the growing larva needs to feed in new areas. More complex tubes, short and flattened, are built by Polycentropodidae larvae in hollows in rocks or other submerged objects, sometimes with strands of silk suspended across the nearby surface. These larvae are carnivorous, resembling spiders in their feeding habits and rushing out of their retreat to attack any unwary small prey crawling across the surface. In the family Philopotamidae, the nets are sac like, with intricate structure and tiny mesh. The larvae have specialised mouthparts to scrape off the microflora that get trapped in the net as water flows through.

The larvae of other species of caddisfly make nets rather than cases. These are silken webs stretching between aquatic vegetation and over stones. These net making larvae usually live in running water, different species occupying different habitats with varying water speeds. There is a constant drift of invertebrates washed downstream by the current, and these animals, and bits of debris, accumulate in the nets which serve both as food traps and as retreats.

Caddisfly larvae are aquatic, with six pairs of tracheal gills on the underside of the abdomen. The eggs are laid above water on emergent twigs or vegetation or on the water surface although females of some species enter water to choose sites. Although most species lay eggs, a few in the genus Triplectides are ovoviviparous. Some species lay eggs on land and although most are associated with freshwater, a few like Symphitoneuria are found in coastal saline water. Philanisus plebeius females lay their eggs into the coelomic cavity of intertidal starfish. The larvae are long and roughly cylindrical, very similar to those of lepidoptera but lacking prolegs.

Adult caddisflies are called sedges by anglers. Individual species emerge en masse at different times, and are used one after the other, often for only a few days each year, as models for artificial fishing flies for fly fishing in trout streams. Each type has its own angling name, so for example Mystacides is the dancer; Sericostoma the caperer; Leptocerus the silverhorn; Phryganea the murragh or great red sedge; Brachycentrus subnubilis the grannom; Lepidostoma the silver sedge; Oecetis the longhorn sedge; Cheumatopsyche the little sister sedge; Helicopsyche the speckled Peter, an important fishing fly in North America; and Hydropsyche the specked sedge, perhaps the most important caddisfly genus for anglers with over 50 species of net makers. Some species indicate undisturbed habitat, and some indicate degraded habitat. Although caddisflies may be found in waterbodies of varying qualities, species rich caddisfly assemblages are generally thought to indicate clean water bodies, such as lakes, ponds, and marshes. Together with stoneflies and mayflies, caddisflies feature importantly in bioassessment surveys of streams and other water bodies.

While caddisflies in the wild construct their cases out of twigs, sand, aquatic plants, and rocks, the French artist Hubert Duprat makes art by providing wild caddisflies with precious stones and other materials. He collected caddisfly larvae from the wild and put them in climate controlled tanks. He removes the larvae from their original cases and adds precious and semi precious items such as grains of gold into the tank. The larvae then build new cases out of precious items, creating a unique form of artwork. The resulting works are sold across the world.

In Japan the larvae of Stenopsyche marmorata are eaten as a delicacy called Zazamushi.