Saddle-Case Caddis

By Jeff Morgan

Glossosoma caddis--commonly referred to as saddle-case caddis --are the Rodney Dangerfield of the caddis world: they get no respect. Even Ernest Schwiebert devoted only half a page to this insect in his classic book Nymphs.

This second-class status is hard to understand, for this genus occurs in almost every stream in the West. From headwater creeks to broad valley rivers, they are often present in large numbers. On many small streams without dense overhead canopies, they are the most populous aquatic insect other than midges. Trout eagerly feed on them when they are available, which is often since this diverse genus has species emerging from February through November.

Their small size and relative abundance immediately warrants comparison to their Ephemeropteran equivalent, the blue-winged olive . As with BWOs, trout will focus their feeding on Glossosoma caddis even when larger insects are available.

Larvae

Glossosoma larvae create small dome-like cases and scoot around on underwater rocks, grazing on algae and diatoms. Larvae often inhabit the top surface of rocks, where algae and diatom growth is at its maximum. Their rocky cases are a great defense mechanism, and protect them from all but the most persistent antagonist (such as an insect-collecting flyfisherman).

Glossosomatids often share habitat with blue-winged olive mayflies, feeding on the same film of algae coating the upper surfaces of most rocks. However, their case protects them from the creature that all "algae-feeders" fear most, the mighty sculpin! The Glossosoma larvae get to eat from the algae-rich surface areas of rocks, while the BWOs are regulated to the less fertile sides and bottoms of rocks.

So where there are good sculpin populations, look for the glossosomatids to be more prevalent than the much more loudly trumpeted Baetis It also should be noted that in the presence of sculpins, the drift rate for BWOs is much higher. So if you cast a tandem nymph rig with a size 20 dark brown CDC Caddis and a size 20 olive Hares Ear , you'll cover both insects.

The rocks that form the cases of glossosomatids are immense relative to their body size, the equivalent of us moving desk-sized boulders. Their cases are open at the head and tail end, and from the rear end of the case the anal hooks at the end of the abdomen help anchor it to the substrate. If you ever try to pry one of these insects off its rocky perch, you'll appreciate its strength and tenacity. This strong anal hook is necessary to anchor these insects in their often-precarious residences. Glossosomatids often hold in the most exposed currents; glossosomatids were the only insect group in a British study in which over 45% of the individuals were found in "exposed" locations.

When these small, tan, 3-10 mm larvae grow, they do not shed their cases; they simply add more rocks to it. Thus they are almost permanently protected from trout.

So does that mean the larvae are not important to anglers? No, because they will abandon their cases in times of stress. (For example, populations of Glossosomatids decreased greatly following the ash deposition of the Mt. St. Helen's explosion, where many abandoned early-instar larva cases were found.) When they do this, they become quite vulnerable to trout, for as they amble naked along the bottom they often lose their grip and end up drifting downstream. Tan uncased larvae imitations can be deadly at these times.

Pupae

The pupae mature in a cocoon attached to rocks in the same moderate currents that the larvae inhabit (unlike many other types of caddis that migrate into relatively slower water before beginning the pupal phase). Though they stay in the same section of the river, Glossosoma will migrate to the less intense currents behind rocks when preparing to pupate.

The pupae are about the same length as the larvae, and have a tannish-cream abdomen and a dark brown thorax. At hatch time, many pupae crawl towards shore rather than swimming to the surface.

Adults

In most of the West, glossosomatids emerge May through September. In Oregon they emerge almost exclusively between May and August, with the notable exception of G. pyroxum, which can come off in good, fishable numbers on the upper Willamette in November and December. Adults are between 4-10 mm long with dark olive or brown bodies and dark speckled wings.

The adults are only briefly available during emergence, but are especially targeted by trout when the females dive underwater to lay eggs on the substrate. This is the most important stage of the life cycle for the fisherman. About three hours before I wrote this piece, I had over 50 Glossosoma females crawling down my legs into the water, even attempting to lay eggs on my boots, while I was fishing on the Yellowstone River. I made many trout happy every time I moved my feet!

One scientific study revealed that brook trout in a small Wyoming stream consumed three times more female adults than male adults--a likely result of the underwater ovipositing of the females. Fishing a tiny Diving Caddis or a small, sunk Elk Hair Caddis is strongly advised when egg-laying behavior is observed.

Summary of Fly Patterns and Tactics

  1. Larval Stage. Larvae are not generally available, but sometimes they leave their cases and drift. A simple uncased caddis nymph can be fashioned by wrapping a hook with rabbit fur dubbing. Tan or cream on a size 18 hook covers most situations, but before tying on a fly you should check a few larvae to determine the size and color that are prevalent. Dead-drift the fly near the bottom.
  2. Pupal Stage.  A Sparkle Pupa with tan-cream body and shroud or Z Wing Caddis with tan-cream abdomen and dark brown thorax. Again, size 18 is most common, but sizes 16-22 can be encountered.
  3. Adults. The most vulnerable stage is egg-laying females. Try a size 16-22 (size 18 most common) Diving Caddis or Soft Hackle presented on a wet-fly swing.

Jeff Morgan has written many articles for Westfly, mostly on entomology and fly tying. He is the author of An Angler's Guide to the Oregon Cascades and Small Stream Fly Fishing.