A Bug’s Life: Caddisfly

Imagine you are floating down a river wearing a thin coat of armor. You jostle against rocks as the current pushes you down a tight channel. Sometimes you’re able to grab onto a plant or branch hanging in the water. This gives you a sense of calm and safety as the water rushes around you. Clinging to a branch or rock, you finally have time to sneak a snack before carrying on your never-ending float. Now, imagine you’re the size of a penny. Imagine you’re a caddisfly.

Although this may seem like a fun and carefree life, there are risks for these little critters. They provide an excellent source of protein for fish, birds, and other small mammals looking for a snack. They are also highly susceptible to pollution– meaning they can only live in streams with very high water quality. As biologists, we look for caddisflies in the stream as an indicator of good water quality– the more, the merrier!

These underwater architects are more specialized than you think. There are over 16,000 caddisfly species known worldwide. Only about 280 of these call Oregon home. Depending on the species, these bugs can be herbivores, detritivores, or predators. Commonly, juveniles consume aquatic plants, algae, decaying organic matter, or other small(er) aquatic invertebrates. In contrast, adult caddisflies either feed on nectar or do not feed at all, as their mouthparts are not developed for solid food. 

These insects are one of the most interesting bugs to observe in our rivers and streams. They are known for their case-building abilities. Most caddisflies use pebbles, sticks, sand, and other natural materials to build a hollow case they can crawl into for protection. They can still move around by sticking their head and legs out the front of the case, crawling around riverbeds.

So, how do we identify these little guys? There are a few key characteristics we look for in identifying a caddisfly. The first (and easiest) is that most will have a protective case! Look for what seems like a tiny pile of sticks or pebbles stuck to the underside of a rock or clinging to some submerged vegetation. You might see something similar to the illustration below, as they’ll poke their heads out and crawl around the riverbed. Notice their three pairs of legs and the plates behind their head. The first and second segments are typically hard, while their abdomen is much softer.

Caddisfly larvae anatomy diagram, inside and outside of protective case.

Caddisfly larvae are the hermits of the bug world, crawling into their “shell” and hiding from the scary world around them. Similar to hermit crabs, they live in a borrowed home, collecting and constructing their case of treasures. In addition to protection, their cases also help keep them from floating away in strong currents by weighing them down. The pieces of casing are bound together by silk that is secreted from their mouths.

They spend about two years in the nymph stage underwater before undergoing a complete metamorphosis and emerging from the water. Adults only live for a few weeks as they attempt to fulfill their life cycle and reproduce near the water’s edge. They look fairly similar to moths, tending to stick around waterbodies where they emerged. Mostly active at night, they flock towards lights during their short lifespans on the surface.

Look for these little architects the next time you’re out on the river, and we hope you’ll love them as much as we do!