Caddisfly Larvae (Trichoptera)
The caddisfly order is the largest order of entirely aquatic insects. Many caddisflies are easily identifiable by the portable case they make that surrounds their soft bodies. These cases can be made of organic material, such as vegetation or debris, or small stones or sand grains bound together by silk the caddisfly produces. Some caddisflies create a retreat that they attach to a substrate. One family of caddisfly is free-living, building neither a case nor a retreat. Whether they are in a case or not, all caddisfly larvae are identifiable by their ability to create silk (which they use to make cases), the hardened skin on their head and on at least one thorax segment, three pairs of segmented legs, and lack of wing pads or visible antennae.
Caddisflies undergo complete metamorphosis, meaning that their lifecycle is similar to that of a butterfly: they have a larval stage, a pupal stage during which metamorphosis occurs, and an adult stage that looks entirely different from the larval stage. Caddisflies build an underwater cocoon where they live as a pupa. They then emerge from the cocoon, float to the water's surface or the shore, and finish their transformation to an adult, terrestrial insect. Most families of caddisfly complete only one of these generations per year.
Because the caddisfly order is so large, all feeding types and habitat preferences are represented. However, most caddisflies are herbivores that graze on phytoplankton, shred larger plants and detritus, and catch drifting algae in their nets or leg-hairs. The type of habitat a caddisfly prefers is often easily identifiable by the type of case it makes. Caddisflies that build cases out of organic matter are found in slow parts of flowing waters and shallow areas of lakes and ponds where debris can accumulate, and where the neutrally buoyant case won't be swept away. Caddisflies that live on soft sediment often build wide, flattened cases that act like a snowshoe, keeping the insect from sinking. In areas with faster current caddisflies make cases out of sand and rocks that are heavy and not as easily swept away. Caddisflies that build fixed retreats attached to rocks are also found in strong currents.
Caddisflies are one of the three most commonly used indices of aquatic ecosystem health, along with mayflies and stoneflies. They breathe dissolved oxygen by diffusion across their soft tissues, and they have a limited ability to cope with low dissolved oxygen by wiggling their bodies within their cases. However, they lack the ability to breathe atmospheric oxygen that some other more tolerant insects have. While most caddisflies are considered sensitive to environmental stress, some caddisflies are less sensitive. Some actually thrive on slightly polluted conditions with elevated nutrients, because it causes more periphyton, a favorite food, to grow.
Picture Key
Top: Caddisfly Larva (Brachycentrus americanus)
Bottom: Adult Caddisfly (Nerophilus californicus)
Source: Both photos are from The North American Benthological Society
Bibliography
Voshell, Jr., J. Reese; illustrated by Amy Bartlett Wright. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg (VA): The McDonald & Woodward Publishing Company. 442 p.
