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 above shows their microstructure.
Many caddis fly larvae make an open-ended cylindrical case to surround their bodies. The cases are made from various materials (see above and below), 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.
Above are cases made of shells. The larvae choose both occupied and unoccupied shells. Below 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.
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.
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.
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.
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.