Caddisflies

A question from a student who had visited the park was “Why are there so many caddisflies in the park’s streams?” There are three parts to the answer. First, caddisflies are very common in most aquatic ecosystems, especially streams. Second, there are almost a thousand species of caddisflies known from North America. Each species has slightly different requirements. Because Rocky Mountain National Park has such a wide altitudinal gradient from about 7,600 feet to over 14,000 feet, there are many different types of streams and habitats in those streams for caddisflies to find a home. Finally, caddisflies are very abundant in clean water, and land management practices in our park provide some of the most pristine streams in the state.

Caddisflies are members of the insect order Trichoptera. Their larvae, which look something like caterpillars with legs, are the aquatic (water dwelling) forms. Adults are terrestrial (land dwelling, or more accurately, are flying insects). While few insects have aquatic life stages, those that do often make up more than 90% of the invertebrates in the park’s streams. The best scientific information available at present suggests all caddisfly larvae are aquatic. Most larvae will eat almost anything (omnivorous), although a few eat only plants (herbivorous) or animals (carnivorous). Caddisfly larvae make a living in a variety of different ways. Some are free-living, while others spin silk nets, and others spin silk and use it to bind together fragments from their environment to make a protective case. These cases can look like tubes made of sand grains, pine needles, sticks, leaves, or other materials. They can also look like purses, saddles, tiny spiral snail shells, piles of sand grains, or a variety of other structures. Most are nearly impossible to see underwater until they move. This camouflage probably evolved both to allow them to stalk their prey undetected, and to avoid their predators. Fish relish caddisfly larvae and fly fishers tie a variety of different flies to mimic caddisflies, both larvae and adults.