Larva & pupa biology
Most caddisfly larvae live in cases they build out of sand, rock, twigs, leaf pieces, and any other kind of underwater debris. Some even generate their own cases out of silk. There is tremendous variation in case style and also in the way the larvae manage their cases: whether they replace it as they grow or renovate their old one, and whether they carry it around or fix it to an object. Trout love to eat these larvae, case and all.
Other common caddis larvae build nets instead of cases. These are not residences but hunting traps, like tiny spider webs, designed to capture plankton and smaller aquatic insects the larvae eat. One larva may build more than one net and roam freely around the rocks and logs tending to each and ingesting the catch. The net-spinning families, in order of abundance, are Hydropsychidae
One large and primitive family of caddisflies, Rhyacophilidae
, needs neither cases nor nets. Most of its species are predators who stalk through rocky riffles killing other insect larvae and nymphs.
All of these types are especially prone to behavioral drift
, making them an important food source year-round for the trout in most rivers.
When caddis larvae are full-grown, they seek hiding places to pupate, either in their cases or in special cocoons. They are considered to be pupae throughout the radical reformation from grub-like larva into intricate winged adult. Some of the larva's body mass is consumed as energy for the development of the pupa, so the pupae and adults both have bodies one to three hook sizes smaller than their mature larvae. When pupation is complete, the insect which begins the emergence sequence is called a pharate adult
. It is no longer technically a pupa in the language of entomologists, but because anglers universally recognize the term "pupa" I use that convenional misnomer throughout this site.
Sometimes individuals within the same fall-emerging species mature at different rates. In some species, mature larvae compensate for this by entering an inactive phase called diapause
prior to pupation. Cool fall weather triggers the end of this phase for every individual within a few short weeks, synchronizing emergences that would otherwise be spread over several months. This boosts the quality of autumn caddisfly hatches like the giant western genus Dicosmoecus