Aquatic, stream-side entomology can be intimidating. There’s all that Latin to remember, and just when you start to feel like you know what you’re talking about, the taxonomy changes, and there’s more Latin to learn.

Luckily for fly anglers, trout don’t speak Latin and they could care less about taxonomy. A strong understanding of the behavioral characteristics, and the habitat requirements of different insects is far more valuable to the average fly fisher than the ability to differentiate between Baetis tricaudatis and Baetis bicaudatis or distinguish posterolateral spines from gills.

Few insects exemplify this more than Rhyacophila (Rhy-uh-co’-fil-uh) caddis. This unsung hero of the caddis family is vastly important on Yellowstone Country rivers like the Madison and Gallatin. Yet, how they live, and where they live in our waters is a mystery to many anglers.

The most unique characteristic of Rhyacophila caddis is also the most important for fly fishers to understand. These are free-living caddis.  The larvae don’t build a case, or spin a net. Rather, they roam freely on the bottom substrate of our rivers preying on other insects, detritus, and aquatic vegetation. This means that for the 30 or so weeks that it takes a larvae to mature and prepare to pupate, Rhyacophila is in the drift and available to trout making it one of the most abundant food sources in rivers where it is found.

Finding Rhyacophila caddis requires knowing a bit about their anatomy. Rhyacophila larvae have no gill structures, instead oxygen is absorbed directly through the skin. This demands water with high levels of dissolved oxygen. The cold, high gradient sections of the Madison, Gallatin, and many smaller headwater streams in Yellowstone Country have such water, and produce strong populations of Rhyacophila caddis.

The larval and egg-laying stages of Rhyacophila caddis are by far the most vulnerable to predation by trout, and the most important to imitate for fly anglers.

Larvae range in size from 14-16, and have a bright, almost neon-green and mottled-brown coloration. In absence of a shelter, Rhyacophila larvae will drop anchor lines of silk to secure themselves to rocks as they graze for food. This is a tenuous predicament in the rough and tumble waters of a river like the Madison, and a great number of larvae become dislodged, both accidentally and deliberately, into the drift.

Pupae are strong swimmers and quick emergers making them a tough target for feeding trout. Once they’ve left the water, the size 14-16 adults, with olive bodies and charcoal-speckled wings, spend little time on or near the water until females return to oviposit.

Female egg-layers dive to the bottom of the water column and lay their eggs with a string of silk on stream bed rocks. Once the eggs have been deposited, the females then drift haplessly in the current, slowly ascending back to the surface where they struggle to re-emerge from the water column and lay spent.

Two species of Rhyacophila caddis are found in Yellowstone country, R. bifila and R. coloradensis. Both species have virtually indistinguishable characteristics, and it is of no value to the fly angler to differentiate between the two.

While Rhyacophila larvae are present in the drift throughout the entire season, it is in the months of September and October, when all of our highly publicized summer hatches are gone and aquatic insects are at a premium, that these caddis are most important.

So, whether you memorize the Latin or not, be sure to remember these free-living, size 14-16, bright green caddis larvae the next time you are nymph fishing in fast water throughout Yellowstone Country, especially in September and October.