Trichoptera Families

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).Glenn B. Wiggins, ''Larvae of the North American Caddisfly General (Trichoptera)'', 2nd. ed. (Toronto: University Press, 1996), p. 3 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 are tied to imitate adults, while larvae and pupae are used as bait. 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.

Etymology

The name of the order "Trichoptera" derives from the Greek: (', "hair"), genitive ''trichos'' + (', "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.

Evolution and phylogeny

Fossil history

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. The evolution of the group to one with fully aquatic larvae seems to have taken place sometime during the Triassic. The finding of fossils resembling caddisfly larval cases in marine deposits in Brazil may push back the origins of the order to the Early Permian period.

Evolution

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 affinities of the third suborder, Spicipalpia, are unclear; the larvae are free-living with no cases, instead creating net-like traps from silk.

Phylogeny

The cladogram of external relationships, based on a 2008 DNA and protein analysis, shows the order as a clade, sister to the Lepidoptera, and more distantly related to the Diptera (true flies) and Mecoptera (scorpionflies).

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.

Distribution

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.

Ecology

Caddisfly larvae can be found in all feeding guilds in freshwater habitats. Most early stage larvae and some late stage ones are collector-gatherers, picking up fragments of organic matter from the benthos. Other species are collector-filterers, sieving organic particles from the water using silken nets, or hairs on their legs. Some species are scrapers, feeding on the film of algae and other periphyton that grows on underwater objects in sunlight. Others are shredder-herbivores, chewing fragments off living plant material while others are shredder-detritivores, gnawing at rotting wood or chewing dead leaves that have been pre-processed by bacteria and fungi; most of the nutrients of the latter group come from consumption of the bacteria and fungi. The predatory species either actively hunt their prey, typically other insects, tiny crustaceans and worms, or lie in wait for unwary invertebrates to come too close. A few species feed opportunistically on dead animals or fish, and some Leptoceridae larvae feed on freshwater sponges.

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 are typically classified as shredders, suggesting caution when classifying macroinvertebrates into strict ecological functional groups, as some may shift their diets opportunistically.

Like mayflies, stoneflies and dragonflies, but to a somewhat lesser extent, caddisflies are an indicator of good water quality; they die out of streams with polluted waters. 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. The winged insects are nocturnal and provide food for night-flying birds, bats, small mammals, amphibians and arthropods. The larval stage lasts much longer, often for one or more years, and has a bigger impact on the environment. 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.

Underwater structures

Cases

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.

Caddisfly cases are open at both ends, the larvae drawing oxygenated water through the posterior end, over their gills, and pumping it out of the wider, anterior end. The larvae move around inside the tubes and this helps maintain the water current; the lower the oxygen content of the water, the more active the larvae need to be. This mechanism enable caddisfly larvae to live in waters too low in oxygen content to support stonefly and mayfly larvae.

File:Caddisfly-larva.jpg, Larva with portable case of rock fragments
File:Caddisfly_Larva.jpg, Larva emerging from case made of plant material

File:Trichoptera larvenbau.jpeg, Larval case of Limnephilidae made of bitten-off plant pieces
File:L flavicornis 22088905 cropped.png, Case of '' Limnephilus flavicornis'' made of snail shells

Fixed retreats

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.

Silk domes

Larvae of members of the family Glossosomatidae in the suborder Spicipalpia create dome-shaped enclosures of silk which enables them to graze on the periphyton, the biological film that grows on stones and other objects, while carrying their enclosure around like turtles. In the family Philopotamidae