Caddisflies undergo complete metamorphosis. The caddisfly lifecycle includes four stages – egg, larva, pupa and adult.
Most of the life of a caddisfly is in the larva stage. They appear very grub like in appearance.
The larva undergoes five instars as they mature.
Development for most caddisfly larva takes about 1 year. A few have more than one generation per year and some take up to two years.
Prior to pupation the larva either spins a silken cocoon or seals off their case.
Different families and genera of caddis case makers have evolved specific case designs based on their habitat, camouflage and for protection.
They use their mouth parts and a jelly-like substance as a super-glue to construct their cases along with pebbles, twigs, sand, leave particles, wood particles, etc.
Fast water caddis larval cases are generally streamlined, solid and made of pebbles and other materials to stand up to the stream flow.
Slow water caddis larval cases are more randomly built and many use sticks, twigs, leave materials, etc.
Some carry their larval case around with them while others add to their case as they grow and still others abandon their case and build a new one as they go through their various instar stages.
Net spinners build silky spiderwebs in the riffles and runs along the bottom with their spiderweb-like nets in front of their makeshift homes. Most have some type of shelter facing the current waiting for food to be caught in their spiderweb net.
These caddis larvae have no case for protection. They use their anal hooks to attach to rocks along the bottom.
They roam the faster runs looking for food, many are predators feeding on other small aquatic insects and larvae, such as Rhyacophila (Green Rock Worm).
The caddisfly larva eventually seals their cases or spins a cocoon and go into pupation. After about 2-5 weeks the fully developed pupae breaks free of its cocoon or case and swims to the surface or crawls to the shoreline. This is referred to as the pharate adult. The adult caddisfly is encased inside a transparent, thin waterproof pupal skin (think of a person inside a diver's suit).
Note: Some caddisfly larvae go dormant and hide over the summer before going into pupation, such as the large stick caddis case larva that are so abundant and easy to see in the river in the Spring.
The pharate adult is somehow shrink-wrapped to protect the fully developed adult caddisfly as it makes it way out of the river or stream. Some authors have stated the causes an air bubble to be trapped inside the pupal skin that creates a silvery glow as the pharate adult makes its way to the surface.
This is the most vulnerable stage for caddisflies to trout and other predators.
Most caddisfly pupae species swim to the surface, but there are some pupae species that crawl to shallows to hatch along the shoreline.
For hatching aquatic insects, the surface film is a major barrier that the pharate adult must successfully break through to survive as a winged adult. Once the pharate adult breaks through the surface film the thorax section of the pupae breaks under pressure and the caddis fly adult quickly flies off.
Caddisfly adults quickly retreat to the vegetation along the banks. Often swarming over the stream or lake prior to mating. Mating takes place along the bank on the ground or in the bushes and trees.
After mating the females vary on how they deposit their eggs. Some dip their abdomen in the water to lay their eggs while others dive underwater to attach their egg mass to a substrate. Others lay eggs along the stream edge and rain water washes in the eggs into the stream.
The female caddisfly species that dive underwater to lay their eggs are the most vulnerable to trout.