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Life in Freshwater

Caddis Flies (Trichoptera)

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Caseless Caddis Fly larva
Caseless Caddis Fly larva (Rhyacophila sp). Note the star-like gills protruding from the side of the abdominal segments

Caddis Flies are superficially like a moth but instead of scales on the wings there is a fine coating of hairs (the meaning of Trichoptera). They never develop a variety of colours. They lack the proboscis of the Lepidoptera and have very reduced mouthparts. So much so that they hardly consume food, maybe some nectar or water. They are a distant relative of the moths. It is the larval stage that is particularly interesting, which is aquatic. An exception is the larva of Enoicyla pusilla that lives in damp moss. They all have the ability to make silk threads. There are essentially 2 types of larval form which can be identified: those which produce a case around itself and the caseless form. The latter again can be split into two types: those which usually spin a web and those living freely as hunters. The cases are made from materials picked up in the habitat (leaf material, tiny sticks, sand and gravel) and are often the best form of identification as each species case tends to be specific.

Cased Caddis Fly larva
Cased Caddis Fly larva

Caddis Flies are widespread across Europe wherever water is available, either static or running. However, the adult is a strong flyer and can move some distance from ponds, streams and rivers, especially as it is attracted to light.

ECOLOGY: The Trichoptera is an important freshwater invertebrate. The larvae form a key part of the diet for fish such as salmon. Trout consume all stages including the adults. The larvae which make a case in which to live use small stones, sand, pieces of leaves and all glued together with a silk lining inside. The silk is spun with a secretion from labial glands (around the mouth) like moth larvae. When full-grown the larva pupates in the case which stops being mobile and may be glued down to a rock. The emerging adult bites its way out and floats to the surface of the water to fly off. Adults are mainly nocturnal.

The cased forms feed on vegetation whilst the caseless forms tend to be either carnivorous or detritivores. The predatory ones are those free living. The web-spinning, caseless larvae may create a silk tube in which to live whilst those in moving water spin the web between stones. This web may trap small invertebrates drifting with the current. This invertebrate drift, as it is known, is an abundant source of food as the current constantly dislodges animals living on the bottom of streams and rivers. Many of these caseless caddis larvae are associated with particular speeds of flow. However, much of what they catch will just be drifting detritus and other organic particulate matter on which the larva will feed (hence detritivore). Some particulate matter will be drifting plankton. Those larvae in especially fast flowing water, e.g. Hydropsyche, will have toughened upper parts to the thoracic segments (thickened external skeleton is refered to as being sclerotised). The larvae have their abdomens safely in the silk tube but the front section of the body protruding into the current. This sclerotisation may be an adaptation to protect them from small stones and other material hitting them.  

Adult caddis Fly being eaten
Caddis Fly adult being eaten by a scorpion fly

Caseless Caddis larva in silk web
Caseless Caddis larvae in silk web on a rock in fast flowing stream

Stone case of caddis fly larva
Stone case of caddis fly larva











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