Trichoptera (caddis flies, caddis worms)

Trichoptera fast facts

  • 13,000 species worldwide, 1100 in Europe, over 230 in Northern Europe, and about 198 in Britain.
  • Both adults and larvae are important food source of certain fish and birds.
  • Trichos (Greek) = hair, pteron (Gk.) = wings.
  • Related to the Lepidoptera (butterflies and moths)
  • Adults. Usually found over or near freshwater.
  • 2 pairs hairy wings.
  • Usually the rear wing is broader than the fore wing.
  • At rest the wings are held roof-wise over the body.
  • Mouthparts greatly reduced.
  • Legs long with spurs at tip of tibia (see drawing below).
  • Length 7 - 25 mm.
  • Prominent, but small compound eyes and 3 ocelli.
  • Long antennae, bristle-like and many segmented.
  • Dull brown or grey.
  • Weak fliers, and fly mainly in late afternoon or night.
  • Larvae. Caterpillar-like with six legs.
  • Usually found inside an open-ended cylindrical case, or filter feeding using nets, or free-living predators.
  • Biting mouthparts.
  • Abdomen terminates in a pair of hook-like prolegs.
  • Aquatic in clean freshwater.
  • Eggs - Eggs laid underwater on plant stem in a gelatinous mass.

Caddis fly Eggs

The eggs usually hatch around two weeks after they have been laid. If the water has dried up in a seasonal pond or stream the eggs can lie dormant for a few months. The eggs are covered with mucilage which swells upon contact with water. To the naked eye the eggs look like miniature frog spawn, but the egg of Phryganea atrata right shows their microstructure.

Phtyganea artrata egg, caddis fly egg

Caddis fly case

Many caddis fly larvae make an open-ended cylindrical case to surround their bodies. The cases are made from various materials (see right, below right), and some species can be identified from their case alone.

As the larva grows the case is added to at the front end and pieces may be discarded from the rear end. This is why the front opening is wider. Although the case is large enough to hold the entire body usually the head and the three thoracic segments with the legs are outside the case.

The material used to make the case is held together by silken threads that come from a spinneret near the mouth. A current of water is continually passing through the case to come in contact with the gills on the abdomen.

If the larva feels threatened it can retreat into the case. It has two appendages at the rear of its abdomen which hold on to the silken lining of the case and can pull itself inside. Some species move around carrying their case, and others fix the case to the underside of a stone or other suitable material.

Larva living in fast-flowing water may add a larger stone or pebble to the case to stop themselves being swept away as can be seen below right where the case has one relatively large stone fixed to a case made of grains of sand.

caddis fly larva case

On the right are cases made of shells. The larvae choose both occupied and unoccupied shells. Below right is a case made of leaves, and above that one made of reed fragments. The larvae cut lengths of vegetation of the right size by measuring the vegetation against the length of the front of their body.

There are a few species which do not make a case and these usually have a tougher exoskeleton. Some of these construct nets between vegetation, the nets are used to trap food particles. Others are completely free-living.

Caddis fly Larva

The larvae are omnivorous. The movement of the larvae inside the case helps to draw a steady current of water past the gills enabling a constant supply of oxygen. The larvae have 6 legs, but also 2 terminal prolegs ending in hooks. These hold on to the case.

caddis fly larva case made of shells caddis fly larva case made of shells

They pupate inside the case while they are still underwater. Just before they pupate they close off both ends of the cases with a grid of silken threads. This gives them protection while still allowing a current of water to reach their gills.

Caddis fly pupa

The free-living larvae attach themselves to some suitable substrate with a silken net before pupation. So caddis fly pupae always have a case of some sort, even if it is just a silken net. Just before emerging as an adult the pupa breaks out of the case or web and crawls to the surface. Some crawl up plant stems or stones to emerge as adults, others break out of the pupal skin on the water surface. These use the old skin as a raft to rest on until their wings harden enough to fly.


caddis fly larva case made of reeds
caddis fly case made of leaves

Caddis fly Families featured

Phryganeidae Limnephilidae Phryganidae Molannidae
Leptoceridae Rhyacophilidae Hydropsychidae Polycentropodidae
Philopotamidae Odontoceridae Glossosomatidae  

Caddis fly Adults

Their flight is heavy and awkward, and they fly mainly at dusk. During the day they hide in waterside vegetation. Some adults feed on nectar or pollen, but many do not feed at all. A few species have wingless females.

The number spurs on each tibia are important characteristics used in identification (see Phryganea grandis right). Mating takes place while resting on vegetation, although in some instances mating may start on the wing. The life cycle usually takes one year.

Phryganeidae Family

The Phryganeidae family contains the larger caddis flies. There are about 450 species world wide, mostly in the northern hemisphere, and 10 British species. The adults range in length from 1.2 - 2.6 cm long, and can have a wingspan up to 60 mm. Usually they have 2 or more tibial spurs on their front legs and 4 on their mid- and hind legs. The larvae are found mainly in still water.

The larvae tend to build their cases out of plant materials, cutting the pieces into rectangles and arranging them in a spiral, and shown below. The larvae have a broad abdomen ending in small appendages with hooks (see Phryganea grandis right), and can grow up to 40 mm long. They eat other insects, molluscs and plants, although some species are vegetarian.

Phryganea grandis

Phryganea grandis, on the right, also known as the great red sedge, or murragh by fly fishermen, is the largest British caddis fly with a wingspan of 50 - 65 mm, and a body length as long as 20 mm. The adult has a grey or olive coloured body. It is found throughout the British Isles. The adults fly from May - July, in the evening. The female lays her eggs in summer in jelly ropes attached to water plants.

Phryganea grandis, adult caddis fly

Phryganea grandis larva

Phyrganeidae larva in case

The larvae are omnivorous, and live in still or slow moving water in weedy lowland lakes, ponds and canals, especially in limestone areas. The case is constructed of a spiral of pieces of leaf sections. When ready to pupate it fixes its case to something solid and covers, or partly covers both ends and pupates inside.

On the left is a typical Phyrganeidae larva in its case. The larvae are large, and have yellow heads with black stripes.

On the right is the case of a Phryganidae caddis fly larva that has been parasitised by Agriotypus sp., an Ichneumonid wasp. The adult wasps swarm over streams in spring and mate. The female wasp then crawls down a plant stem, into the water and down to the stream bed where she searches under stones for a Phryganidae caddis fly larva.

She lays an egg inside the case. When the wasp hatches from the egg it starts to eat the body of the caddis fly larva. However, it does no eat the nervous system, as it does not want to kill the caddis larva. Only once the caddis larva has fixed its case to a stone to prepare for pupation will the wasp kill it by eating all the caddis larva. Then the wasp makes its own silk cocoon inside the caddis case and emerges as an adult the following spring. A parasitised case is easy to recognise as it has a long thread hanging from it.

 

Agrotypus sp. Ichneuminid wasp parasitising a caddis fly larva

Glossosomatidae family

The small caddis fly on the right is in the Glossosomatidae family. The larvae feed mainly on algae, but can be carnivores on occasion. They live in fast-flowing water. Their cases are attached to boulders, are made of tiny stones and are flattish rather than rounded. Sometime huge numbers of cases can be found.

caddis fly in the Glossosomatidae family