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Entomology

Glossosoma Caddisflies

The most common caddis in the Sierra. Understand this bug and you'll rule!


In every tumbling coldwater stream in California lives an insect so important to trout that day after day throughout the first half of the season, the fish will often eat nothing else. I consider it to be the most important of all California caddisflies, yet not one angler in a thousand has ever heard of it. I’ve never seen an imitation of it in any flyshop nor have I ever heard of anyone purposefully imitating its behavior. I’m willing to bet it is the insect most frequently responsible for anglers snapping their rods in frustration.

What angler hasn’t noticed all those little gobs of pebbles glued to instream boulders? Looking for allthe world like barnacles left high and dry at low tide, these scabs of pebbles chronicle high water levels on every freestone river, creek and stream in the state. Each and every cluster of pebbles, and in some waters there must be hundreds of millions, used to house an immature Glossosoma caddisfly.The Glossosoma is a member of the Family Glossosomatidae, the most primitive of all the case making caddis. These guys invented the technique of building a home. Like most inventions, this one was a good idea but lacked some of the more refined qualities of subsequent incarnations. Most caddis larvae make tubular homes that they simply extend as they grow. The Glossosoma builds a dome that, like a pair of kid shoes, must be discarded and replaced as they mature.

Glossosoma larvae live in gravel cluster colonies.

At dusk or dawn, huge numbers of Glossosoma crawl out of their shelters and release themselves to the current. The numbers are staggering. In one survey, Glossosoma larvae attained drift rates of 350 insects per hour through a one square foot portion of river. In a stream he was sampling, Gary LaFontaine calculated that a trout with a three foot feeding area was seeing up 1,600 drifting larvae in one hour. LaFontaine concluded that during certain times of the year, Glossosoma create more selective trout feeding than any other organism. I absolutely agree.

The larvae might drift for a few feet or a few hundred yards before it lands on the streambed. It quickly begins gathering gravel and within two hours has built a new home (more properly termed a test). This home is dome shaped with a hole on the bottom at each end of the dome. Across the bottom of the dome is a belly band woven from silk.

The larvae crawls onto a rock searching for algae and plankton upon which to feed. The larvae might travel one direction for awhile then turn around inside its case, stick its head out the opposite hole and continue on in another direction. After a week or so, the case once again becomes uncomfortably snug and the larvae once more vacates its home and casts its destiny to the current.

Some segment of the Glossosoma population is doing this every dawn and dusk throughout the early California summer. The heaviest drifts seem to occur about an hour after sunset.

At some point the larvae decide it’s time to pupate. They aim the holes of their home so the current percolates through, providing a fresh flow of oxygen. Then, for the final time, they glue their case to the rock upon which it sits. This affixing of the case to the cobbles and boulders of the streambed is excellent insurance against getting inadvertently swept into the drift. It also spells certain death should some upstream dam operator decide to abruptly drop the water level. Billions of Glossosoma are lost every year due to river fluctuations.

Caddis pupae, aided by bubbles trapped inside their sheath, swim toward the surface.

After several weeks of pupation, the pupa chews itself free and emerges from the pebbled dome, sometimes in the morning but more commonly aboutan hour after sunset. These size 16 burnt orange colored pupae are very active swimmers. They often congregate in the soft water immediately downstream of riffle areas. The pupae hide among the cobbles during the day, but at dusk they emerge to swim about in an erratic jinking movement.

One or two evenings after emerging from their rocky homes, a pair sparkling bubbles develop just under the pupal skin at the shoulders. Possibly aided by the buoyancy of these bubbles, the pupae swim to the surface, drift a short distance (about one minute) then the adult pops out and immediately flies upstream.

About an hour after sunset, adult Glossosoma caddis return to the river to lay their eggs. Glossosoma are one of the many caddis species which crawl and swim underwater to lay their eggs on the streambed. Having lost their gills, the adults are obligatory free air breathers and must carry their oxygen supply with them. This they do by cloaking their entire bodies in a bubble of air.

An ovipositing caddisfly adult swims through the water column breathing from its sparkling bubble of air.

The bubble feeds oxygen to the caddis as it swims and crawls about the streambed. As the oxygen is consumed, the pressure differential shifts and oxygen from the water is drawn into the bubble thus replenishing the caddis’ supply.The bubble encrusted caddisflies look nothing less spectacular than sparkling, animated diamonds. In the relative dark of the evening stream bottom, the bubbles reflect any available light and seem to glow from within. To say these guys are highly visible is a gross understatement.

To summarize: Just about every evening of early summer starting about an hour after sunset, #16 pale colored larvae free themselves from their homes and drift, en masse downstream. Shortly thereafter, #16 burnt orange or purpilish colored pupae emerge from hiding and start swimming about. Many of these sport sparkling bubbles of air and ascend to the surface and emerge. Only a short time after that, #16 brown adult caddis from an earlier night’s hatch return to the river and travel about the streambed to lay their eggs.

The typical fisherman experiences this sequence initially by seeing a sudden burst of caddis adults winging their way upstream. There might be thousands of bugs in the air, on your arms, behind your glasses and in your hair. At the same time, trout are rising, slashing and flashing about at or just below the surface. The excited angler ties on a #16 elk hair caddis and flails the water for about twenty minutes until the rises stop and the caddis disappear. Rarely does he catch a fish. Frequently he cusses in despair and has been known to stalk off the river leaving a pile of broken fishing tackle in his wake. Sound familiar? You’re not alone.

Relax. Get on the water about an hour after sunset. Tie on a #16 orange, pink or cream colored larva imitation. Being a hack I often use a Bird’s Nest because they work pretty well and offer the opportunity to immediately switch gears. Add some split shot and high stick the imitation downstream in a drag free drift right along the river bed. You are imitating the helplessly drifting larvae. Trout are expecting it.

Bird's nest nymph treated with powdered floatant holds an air bubble exactly like the ovipositing caddis.

When the first trout rises, or you start to experience a burst of caddis activity. Tie on a #16 Birds Nest and a small split shot. Rub the Birds Nest in Powdered floatant (I am partial to Tite Lines’ Dry Fly Float) and lob it out and across the stream. The nymph will be buoyant from the floatant but the shot will help it break the surface. Actively retrieve the fly with a twitchy and erratic strip. You are imitating the actively swimming pupae. All those rising, slashing fish you’ve experienced in the past were NOT taking adults, they were chasing down the sparkly pupa just under the surface. That’s why they ignored the elk hair caddis.

As soon as the rises stop, and I don’t care how many caddis are still buzzing about, take that Bird’s Nest, make sure it is bone dry, treat itwith powdered floatant and fish it along the bottom of the riverbed with little or no drag. The rises stopped because the trout settled back down to the streambed to graze on the highly visible and vulnerable caddis adults.

Unless I have a specific reason for doing otherwise, the E/C (emergent/cripple) Caddis is my first choice of fly on any Sierra stream.

If you’re in the mood for some dry fly action (who isn’t) tie on a #16 crippled caddis pattern. Its shuck-trailing, bicolor body supported by the flared wing and parachute hackle makes the E/C caddis (Emergent Crippled caddis) a dead ringer for a crippled Glossosoma. Caddis are very efficient emergers but by my own estimation about five percent don’t survive the transition and the adult gets trapped in the pupal shuck.

Five percent of many thousand hatching caddisflies equates to a huge number of bugs that will remain easy pickings for hungry trout. Fish the E/C caddis immediately following the caddis hatch and use it as a searching pattern throughout the day. Fish are attuned to the seeing the cripple and will often suck them in even while other types of insects are available. The E/C caddis is my default fly. If nothing is on the water suggesting I do differently, the E/C gets fished. And the Glossosoma is one of the reasons why.