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 below), and can grow up to 40 mm long, and have yellow heads with black stripes. They eat other insects, molluscs and plants, although some species are vegetarian.
Phryganea grandis, above and below , 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.
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.
Below 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.
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.