By Jeff Morgan
Fly anglers have imitated submerged adult insect for centuries. The original wet flies worked well because they mimiced, to some degree, a diving or drowned adult.
What are those insects doing underwater? In most cases, they're laying eggs. Most insects that lay eggs underwater crawl down rocks or logs to the bottom rather than dive-bomb the surface. Because the phrase "insects that lay their eggs by swimming or crawling down rocks or logs to the substrate" is unwiedly, the shorter, if less accurate, term "diving insects" is used here.
Many species of aquatic insects lay their eggs underwater, but there are three genera of "diving" genera that Western anglers should take special note of: the blue-winged olive (Baetis) mayfly, saddle-case caddis (Glossossoma), and the spotted caddis (Hydropsyche).
The three critical features to keep in mind when tying imitations of diving insects are: egg sac, flash, and weight. These are described below.
The egg sac is present on every caddis that dives. Why? Because like their human kin, male insects let the females bear the brunt of the reproduction process. If you take a look at stomach samples of trout feeding on adult insects (diving or not) at times other than during a hatch, you'll find that over 80% of the insects consumed are females.
The egg sac should be imitated with a few wraps of bright green, yellow, or reddish orange dubbing. Don't make it too large; the fly should just give the impression of a sac. A big tuft of Antron yarn is usually too flashy. Alternative approaches include a tiny butt of Krystal Flash or a small bright glass bead (must be very small).
A diving insect doesn't go under the surface without a survival plan. Most insects trap gasses under their folded wings. These gasses serve two purposes: they aid in respiration, and they allow the insect to float up after laying its eggs. This behavior makes the diving insect's descent even more perilous because it is more buoyant and more likely to lose its grip on the bottom.
These bouyant gasses shimmer in the light, and this is a visual feature that trout key on. Therefore it should be imitated on effective fly patterns. Krystal Flash tied over or under the winging material is one way to do this. A CDC underwing, flash ribbing, or a nickel or pearlescent bead in the thorax will also work.
The last thing to think about is the weight of the pattern. The fly should be fished near the bottom or have the "plop" effect of a natural insect diving through the surface.
Since two of the three major diving insects are small (Baetis and Glossossoma), this can be difficult to do without affecting the shape and proportion of the fly. My first choice is usually a small glass bead, though this doesn't offer a lot of weight and doesn't always get the fly to sink. My next choice is a 5/64" or 2mm brass bead; anything larger than these is far too bulky for small patterns. Some of the new "quick descent dubbings" can be useful for some patterns, though most lack the subtlety in color that I like.
I prefer to dead drift these flies along the bottom, like a nymph (indicator or tight line tactics), thus imitating the ones that get dislodged and are free-drifting. I will use them on a standard indicator rig in deeper water, or sightfish with them in shallow water, or fish them as a dropper off a dun or adult imitation. Some patterns can also be fished on a wet-fly swing. Because the naturals are almost never found in deep water, most of your fishing should be near the bank.
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.