Caddisflies develop in a four-stage process called complete metamorphosis. The stages are: egg, larva, pupa, and adult. In this process, the immature insects look very different from the adults. They also live in very different environments.
The adult caddisflies are winged insects. They fly around lakes, ponds, and streams. After they mate, the female caddisflies deposit their eggs into the water or very close to it.
When the larvae hatch from the eggs, they usually go to the bottom of the pond or stream. The caddisfly larvae are aquatic. They live underwater and breathe with gills.
Some caddisfly larvae grow very large. These larvae can reach almost 10.5″ in length. They move freely under the water and prey on minnows, tadpoles, insects, and other small animals.
Some caddisfly larvae make nets that they use to catch prey. The trumpet-net caddisflies in the family Psychomyiidae make tubular nets that they place in streams. Larvae in the family Hydropsychidae make cup-shaped nets. They direct the net opening upstream and then the larvae wait in a separate retreat near the net.
When they are newly hatched, larvae in the family Brachycentridae live near the shore in the shallow water. As they mature, they move away from the shore into the deeper water in the middle of the stream.
Many caddisfly larvae hide in shelter cases. Most of the larvae make their own case. They spin a silken material or they secrete a sticky substance from their mouth. There are openings for the larva to put out its legs and its head. Many of these larvae are scavengers that feed on algae and organic debris at the bottom of the pond or stream.
Many larvae make cylindrical cases and cover them with grains of sand. However, some caddisfly larvae make intricate shelter cases. Larvae in the family Helicopsychidae make a case that is shaped like a snail shell.
Some caddisfly larvae use material that they find to make their shelter cases. Larvae in the family Calamoceratidae use hollow twigs that they find on the bottom of the stream. They carry the twigs with them as they forage on the bottom for food.
Larvae in the family Hydroptilidae wait to make their case until they are almost fully mature. They spin a purse-shaped sack that is open at the ands.
Many of the case-making larvae use the case as a place to change into adult caddisflies. They attach the case to a stone or a plant at the bottom of the pond or stream. They seal the openings to keep predators out. The larvae change into adults inside the sealed cases.
During the summer, the changing stage, called the pupa stage, takes about two weeks. Larvae that mature in the fall usually spend the winter sealed inside their cases.
After the pupa stage is finished, the caddisfly comes to the surface of the water. It sheds the pupal skin and emerges as an adult caddisfly.