Story by Harvey Thommasen with Photography by Mike Wigle

There is a predictable, orderly sequence of emergence among aquatic insects that is repeated throughout the seasons. Getting to know the detailed life cycles of insects can be a daunting task for a fly fisherman - and let’s face it, unless you want to spend more time studying insects than fishing, you’re never going to learn it all.

But there are some basic things that are relatively easy to pick up on, that will help any fly fisherman anticipate what might be happening in any given season, and be alert for signs.

Early in the season, before the snow has even had time to melt from the valley bottoms, aquatic insects are already hatching in coastal river systems. On warm days in February I watch for the first emergence of the little mayfly Baetis tricaudatus (Little Blue-wing= Olive), the caddisfly Psychoglypha alascensis (Snow Sedge), the tiny Winter Black stoneflies (eg Capnia sp, Zapada cinctipes, and Despaxia augusta), chironomids (eg Chironominae sp, orthocladiinae sp), and black flies (eg Simulium sp) .

The largest number of individuals and largest number of species of mayflies, stoneflies, caddisflies, and true flies will emerge in the months of April, May, and June.

During those spring months I particularly watch for the emergence of the Giant Stonefly (Pteronarcys californica), the Big Golden stonef ly (Hesperoperla Pacifica) , the Great Grey Spotted Sedge (Arctopsyche grandis), the Summer Flier Sedges (Limnephilus sp), and the Fishfly (Corydalidae sp).

Through the summer months I watch carefully for the emergence of the Giant Caddisfly (Dicosmoecus atripes), the Great Late-Summer Summer Sedge (Oncosmoecus spp) , and the Western Green Drake (Drunella spp) .

After July the number of different species which emerge declines rather drastically. By mid-September only a handful of mayfly (eg Ameletus validus, Epeorus deceptivus, Baetus tricaudatus, Acentrella turbida) , stonefly (eg Despaxia augusta, Naeviperla forcipata, Claasenia sabulosa) and caddisfly (eg Oncosmoecus , Rhyacophila sp) species are emerging.

By November the only aquatic insects one will see emerging will be Chironomid (Chironominae and Orthocladiinae) and a few blackfly species (simulium spp).

The nymphs and pupae of each aquatic insect species also have their own methods of emergence. Some shed their nymphal skins or pupal cases while underwater and then float or swim to the surface.

Some do not crawl out of their nymphal skin or pupal case until they are at or near the water surface.

Yet others crawl to shore or onto large rocks or logs sticking out of the water. Once out of the water they discard their nymphal or pupal skins and fly away.

There’s certainly no end of details to be learned about aquatic insects. Getting them sorted out as to seasons is a good start.

Many fly fishermen associate hatches with warm spring days, but as we’ve seen, there’s always something going on in the insect world, even in the coldest months.

On my way down to a favorite steelhead pool last winter, I was amazed to see a small, whitish, moth flying between snow covered branches of a fir tree.

It was a winter moth. There are two kinds on the north Pacific Coast, where I fish: the Large Winter Moths (Paraptera danbyi) whose larvae feed on Picea spp, and the Small Winter Moths (Oporophtera bruceata) whose larvae feed on Saskatoon (Amelanchier alnifolia) and snowberry (Symphoricarpus racemosus).

Winter moths are fascinating insects. At a time when most insects have settled into a state of suspended animation, winter moths are just emerging from their cocoons or pupae.

A dense "fur" coat, similar to that seen on bumblebees, is among the adaptations which help keep these moths warm through winter months. And when it gets particularly cold, they simply crawl under leaves and wait for a warmer day to be out and about.

To fuel flight in winter months, these moths feed on sap that oozes from broken branches, broken tree trunks, sapsucker holes, or cuts made by hungry red squirrels. Moths that are still alive in April can also be seen feeding on pussy willow flowers.

In late winter, mating takes place and eggs are laid near buds of host trees. The caterpillars hatch in early spring, at about the same time that the first leaves are emerging from the buds. They feed till the summer, and then build a cocoon in which the pupae will spent the rest of the summer and fall months in a kind of summer-time hibernation known as aestivation - waiting for the cold of winter to make their appearance as adult moths.

I have never seen a prolific winter moth “hatch”, but don’t doubt that if one of them fluttered to the surface, on a cold day, a cutthroat or steelhead would rise to intercept it.

If nothing else, seeing a few winter moths come off is enough to warm a fly fisherman’s heart - by reminding him of the more big hatches that lie ahead.