Turtle Case Caddis

The little Glossosoma caddisflies may be small and inconspicuous to the angler, but they are often abundant and available to trout.

Overcast skies and clear water, so where are the fish? That's what I was thinking as I stood knee deep in the tailout of the riffle. The evening before I caught a couple ten-inch rainbows on a caddis nymph fished deep. But the water seemed to hold more promise than just a couple small trout. The only signs of insect activity were the small caddis adults crawling over the rocks along the bank. I was hoping for a mayfly hatch, but the water flowed peacefully with no surface activity to break its smooth surface.

Out of the corner of my eye I saw the quick flash of a fish below me. I watched intently - nothing. Then I glanced down towards my feet and was greeted with a peculiar sight. A dozen tiny caddis pupae were crawling up my waders. Small dark gray adults flew off leaving pale empty pupal cases clinging to my legs like tiny burs stuck to your sock. A hatch was on!

I scanned the water. Still no surface activity, but I did notice a small caddis adult skitter across the surface. If fish were feeding they must be taking pupae near the bottom. I picked a pupa off my leg and pulled out a fly box with small emerger patterns. The natural was yellowish brown with black wingpads. The closest pattern I had was a size 18 dirty yellow soft hackle. I tied it on, adding a small split shot ten inches up the leader, and cast it up into the riffle. It floated seven or eight feet, then ZIP, the strike indicator was gone and a 14 inch trout broke the quiet surface dancing through the riffle with me trying to lead and it unwilling to follow. Ah, the joy of success. For the next hour fish hit that small soft hackle like no tomorrow. In fact it proved to be effective morning and afternoon for the rest of my two-day trip.

When I got home I took a few specimens I collected along the river and identified them as Glossosoma velona. Never heard of them? Their common name is turtle case or saddle case caddis. Still never heard of them? You're not alone. In his book Caddisflies, Gary LaFontaine says this about Glossosoma:
The inconspicuous species of Glossosoma have never been highly rated even among caddisflies, possibly because they are case makers as larvae and sporadic emergers as pupae. They are not particularly prominent as adults either. They are easy to dismiss overall as only a secondary food source for trout, one of little consequence to the fly fisherman.
Such an assessment, however, has been a serious mistake, underestimating one of the most important insects in cool, running waters. Not just among caddisflies, but among the entire fauna in many trout rivers....
With that in mind I would like to shed some light on these small “inconspicuous caddis.”

Glossosoma velona is one of six genera and 79 species of the family Glossosomatidae. Their common name, turtle case or saddle case caddis, refers to the turtle-shell like case the larvae construct of small stones. While easily overlooked, these small cased larvae are also easily recognized. Pick up any smooth softball sized rock from a riffle in a cool mountain stream and you will likely see small oval, limpet-like clumps of gravel stuck to the rock's surface. Under each of these turtle-like clusters is a glossosomatid larva.

These are primitive caddis, meaning they have been around a long time. The rough turtle-shell cases are considered the earliest attempt at making portable cases by caddisflies. The finely tapered tubular sand and plant cases of better known caddis larvae are the results of a hundred thousand more years of evolution.

The habitat of glossosomatids is restricted to flowing water, and their greatest abundance occurs in cool rapid streams of small to moderate size. Herbert Ross in the Evolution and Classification of the Mountain Caddisflies, the definitive study of the glossosomatids, states:
The larvae of the Glossosomatidae make saddle-like cases, and without exception inhabit cool or cold, rapid streams. Some groups such as Agapetus live chiefly in spring-fed rills or brooks; others such as Protoptila and Glossosoma live in cold rapid rivers of moderate size. Both adults and larvae are relatively uniform in appearance throughout the family, but there is a considerable range in size, from Glossosoma species measuring 10 mm. in length to some of the smaller Protoptila species, which are less than 3 mm. long.
This preference for cool mountain streams puts them in the very places where trout thrive.

Because of their need for cool, mountain streams the greatest diversity of species occurs in the western mountains. Over 90 percent of all known species are found in the Rocky Mountains west to the Pacific Coast and from California north to Alaska. But diversity does not necessarily mean abundance. Excellent populations have been documented in many cool trout streams of the Midwest and East. Some species have even adapted to warmer slower flowing streams. Some species of the genus Protoptila, for example, are found in thermal streams like the Firehole.

Behavior of the glossosomatids is similar wherever they are found. The larvae feed by scraping the fine layer of algae called periphyton off the surface of smooth rocks. Their case is pulled with them, so they are rarely exposed. Like most cased caddis they must make their case larger as they grow. Unlike many other cased caddis, which simply enlarge their current case, they must build a new case each time they molt. During these periods the larvae abandon their old cases and end up drifting in the current, sometimes in large numbers. Over 1600 larvae per square meter per hour have been measured in the drift. The stomach contents of trout likewise reflect this behavior.

Once the larvae reach full size pupation begins. First the case is securely anchored to the side of a rock in moderate to swift current. Larvae often seem to congregate at preferred spots resulting in the surface of some rocks to become completely encrusted with the pebbled cases of the larvae. After anchoring the case, the larvae spin a fine silk cocoon inside and transform into pupae.

Pupation takes three to six weeks. Once complete the pupae break out of their cases. The pupae are 3 to 10 mm. long, have yellowish brown bodies, and dark gray or black heads and wingpads. Their hind legs are fringed with fine hairs making them effective oars that they use to swim with a quick erratic darting motion. At the surface the adults struggle out of the pupal shuck, float or run briefly on the surface, then fly for the safety of the shoreline.

The adults are nervous creatures. If not for their small size and grayish black color, they would easily be seen running and skittering over rocks and shrubs along the bank. After mating females return to the water to lay their eggs, where they again become vulnerable to hungry trout. On reaching the water they dive under and swim to the bottom where they paste their eggs to a rock. They then let go and swim back to the surface where many die if not eaten first.

Emergence periods for these caddis often extend over a four to eight week period. The specific time of year depends on the species and water conditions (see attached emergence chart). In the West, with its diverse populations, a near continuous chain of hatches occurs from March through November. June through August appears to be the period of greatest emergence activity in the Midwest and East. A few species have a bimodal emergence, or two hatches per season, with peaks in the spring and fall.

Fishing success depends on timing and observation, not on being able to identify the specific species occurring in your streams. Observation is the biggest factor. These small caddis go unnoticed through their entire life cycle by most anglers, or if seen as larvae clustered on a rock or as adults skittering over the bank, they are not registered as important to imitate. If I hadn't seen the pupae crawling up my waders I too would have completely overlooked their emergence even though I saw numerous adults crawling on the bank.

The behavior of the larvae, pupae and adults results in all stages being available to trout and worthy of imitation. The larvae become available during periods of high drift rates when old cases are left behind to build new ones. Knowing when these periods are occurring can only be confirmed by putting a net in the stream for ten to thirty minutes then checking its contents for the larvae. However, as a rule the greatest growth, and thus drift, of these larvae occurs in the spring and early summer. Gary LaFontaine has this to say about the importance of the larvae:
From May to July, when the larvae are reaching full growth, this genus, with its caseless drift, creates a unique situation. The dominant species in an area can completely control the dawn and dusk activity of a stream. The larvae form a food supply in the current that concentrates both forage fish and gamefish in or below riffles that have high populations of the insect.

My suggestion is to make sure you have a few larval patterns in your fly box and give them a try in the spring and early summer where you have confirmed healthy populations of the natural exist. This is easy to determine. Just pick up a few rocks from a riffle and look for the small domed shaped cases. That's also a good time to pull a few of the cases off, and check to see if it’s the larval stage or pupal stage hiding underneath. Fish the larval pattern dead drift close to the bottom. Areas below a riffle will be your best bet. A strike indicator will also prove very helpful, as trout are in no rush to take these small, helpless, but enticing larvae.

If you find pupae under the cases pulled off a rock, and have seen some adults along the shore, its time to fish a pupa pattern. Emergence periods during the day typically occur in the morning or late afternoon. The wader crawling activity I experienced occurred at 10:00 in the morning. But the pupa pattern caught fish from 9 am until 5 in the afternoon with a period of slower activity from around 11 until 2. Fish may concentrate their feeding on the pupae at different depths so experiment to find the best depth to fish. If you're not seeing any surface activity, then more than likely fish are feeding close to the bottom.

Use the same setup you had for the larvae. Cast upstream and let the pupa sink. As it swings below you give it some light twitches so it seems to be darting to the surface. Strikes to the pupa can be gentle or violent. I find a strike indicator helpful in seeing the gentle ones. The violent ones are easy, just don't snap you're leader on the strike.

Adults are best imitated with small dries fished wet, or with the diving caddis patterns described by LaFontaine. Egg laying activity will generally be in the afternoon and evening. One useful method is to use a pupa and adult pattern together. Put the pupa on the end of your tippet and attach the adult to a dropper. Cast up and across and let them both sink slightly underwater. You will likely find that some fish take the pupa while others are hitting the adult pattern. The effectiveness of the adult pattern may increase as fish switch their feeding from pupae to the egg-laying adults.

This summer, while fishing one of your favorite mountain streams, take a good look around. Pick up a few rocks and check for the small turtle-like cases of these little, inconspicuous caddis. If you find them, chances are good trout will be finding them too.

PATTERNS

EMERGENCE CHART

The chart below lists only the species of major importance
to the angler.