Our Man In Canada
April 2nd, 2001

The Absolute Beginner
A Fly Fisher's Basic Entomology, Part Two

Chris Marshall

By Piscator

In our last issue (Winter 2000) we introduced entomology for the beginning fly fisher with an explanation of the stages of development of mayflies and what these mean to the fly fisher. In this issue, we take a look at the other major order of flies which is important to fly fishers - caddis flies.

Although they're prolific and constitute a significant part of a trout's invertebrate diet, caddis flies have been relatively neglected by fly fishers. Unlike the order of mayflies, few caddis species have been immortalized with with popular names. This means that the vast majority of fly fishers have no common names to refer to each species (and there are many more species of caddis flies than mayflies), so the only way we talk about what species is hatching, we're obligated to describe it - "You know, that brown mottled caddis about size #14".

CADDIS FLIES (Order Trichoptera)

Nymph

Some caddis flies lay their eggs by fluttering above the water and depositing them on the surface, after which, they sink to the bottom and stick there among the gravel, detritus, or weeds. In some species, the adult fly actually swims to the bottom to deposit the eggs.

After a week or two, the eggs hatch into tiny worm like grubs, mainly in shades of green, grey, and dirty white. Some species build protective cases out of gravel or bits of stick. Most of you have seen these crawling slowly along the bottom. As they grow bigger, they enlarge the cases. Other species do not build cases, but hide among the cracks and undersides of structure on the bottom. Some anchor themselves permanently and build funnel-shaped nets to catch stream-borne morsels of food. While trout will pick up the cased species, the others are usually too well hidden to be readily available as food.

Despite this, fly fishers do catch trout on caddis nymph imitations bumped along the bottom, not so much because the trout are used to encountering them, but because they're just being opportunistic.

Pupa

Unlike the mayfly, the caddis has a pupal stage. All species enclose themselves in some form of pupal case (a chrysalis) anchored to the bottom, in which the nymph gradually transforms into the pupa. When the metamorphosis is complete, the case opens, and pupa emerges, swimming rapidly to the surface. The key workd here is "rapidly" - in fact, the pupa is literally jet-propelled, squirting itself up through the water column by ejecting the gases produced as a by-product of the metamorphosis. As soon as it reaches the surface, the pupal case splits and the adult fly bursts immediately into flight.

This means that the trout never have the chance to feed leisurely on hatching caddis flies as they do on hatching mayflies, which frequently drift downstream drying their wings. When they target hatching caddis flies, trout have to be quick. You'll often see a hatching fly erupt from the surface, just millimetres from the jaws of a fish lunging out of the water after it.

This emergence stage can be very productive for fly fishers. The basic technique is to cast a weighted nymph upstream, let it drift naturally along the bottom until it's just downstream, and then check the drift so that the nymph rises from the bottom - just like an emerging pupa. This "lift" presentation is more important than detailed imitation of the natural insect, as trout have little time to inspect their target closely. Therefore, simple, impressionistic patterns are usually all you'll need.

Dun

Spring 2001 issue

While caddis adults do sometimes drift at rest on the surface of a stream, they're much more likely to be fluttering about in the air just above the surface. Unlike mayflies, which mate and die a day or two after emergance, adult caddis flies can live for week, returning to the water to deposit eggs at intervals. You'll see them, especially at dusk, swarming just above the surface, occassionally dipping into the water. Again, trout have to be quick to catch them, and their rises will be abrupt and splashy, unlike the deliberate swirl they make when taking a drifting mayfly dun.

Afriticials with hair wings and stiff hackle are popular patterns for imitating fluttering caddis duns, and because the naturals are moving about on the surface, maintaining a drag-free drift isn't so critical. In fact, giving the artificial a few twitches is often a good tactic.

Spinner

Like mayflies, caddis flies die when they're finished mating. However, because they deposit their eggs over a much longer period than mayflies, you don't get the same concentrations of of spent flies on the water. Instead, the spent flies tend to be mixed in with others which are vigorously fluttering about. Consequently, there are rarely selective rises to spent caddis. Nevertheless, some fly fishers do tie and successfully fish imitations of spent caddis flies, although I suspect trout take them, not because they're targeting them specifically, but out of sheer opportunism.

Coming in future issues: stoneflies, midges, and specialised mayfly variants. ~ Piscator

If you just can't wait until the next issue of the Canadian Fly Fisher, you will find more on the specific insects in the Not Quite Entomology section - or Fly Fishing 101.

We thank the Canadian Fly Fisher for re-print permission!

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