There are five major orders of insects that are important to fly fishermen. Mayflies (Ephemeroptera), Caddisflies (Trichoptera), Stoneflies (Plecoptera), Midges, Mosquitoes, Aquatic Gnats and Flies (Diptera) and Damselflies or Dragonflies (Odonata). There are over 6,000 different species and sub species of these insects in North America north of Mexico.

drake_g caddisstageA


life cycle: egg, nymph, adult, spinner 
size range: 
#10 to #24  (The size range refers to the hook sizes normally used to tie the imitation.)
All adult or dun mayflies have upright wings & two or three tails.  They are the only trout insects with upright wings.  On the water they look like little sailboats floating down the stream.  The Nymphs emerge to the surface of the water where they hatch into adults or dun.  Spinner refers to the stage when the female has dropped back onto the water to drop or oviposit  her eggs and dies.


life cycle: egg, larva, pupa, adult
size range: 
#8 to #24
The adults have four wings held over the back in a V or tent shape when at rest.  They are related to moths and fly in a similar manner.  Most caddis larva build a case or house that they live in.  Cases can be built out of  sticks, sand, bits of leaves and bark or almost anything accessible.  Some caddis pupa crawl out of the water, others emerge to the surface to hatch.

stone4 cranefly


life cycle: egg, nymph, adult
size range: 
#2 to # 20
The adults have four wings held flat over the back when at rest.  The stoneflies flight is usually much smoother than that of the caddis.  The nymphs can live up to five years in the water before hatching into adults.  The lifespan as adults is normally about one day or up to five weeks depending on the species.  Most stonefly nymphs crawl out of the water onto rocks, grass, branches or trees to hatch.


life cycle: egg, larva, pupa, adult
size range: 
#8 to #28 or smaller
The adults have two wings with a slight V over the back when at rest and have no tails.  Some species leave the water to hatch, others float to the surface and hatch.  Diptera are the true flies.  They are also one of the largest orders of insects in North America with over 3,500 species.



life cycle: egg, nymph, adult
size range:
 #2 to #8
The adults have four wings that they cannot fold flat over there back when at rest. The wings are held outspread or somewhat vertically above the body. Nymphs crawl out of the water onto  grass or twigs to hatch.


This is a rough estimate on when the major hatches may occur on the Bitterroot River.  It always varies due to water level and weather conditions. Our high water flow normally peaks around the last of May to the first part of June.  In low snow pack years this can be some of the best fishing of the year.  With a heavy runoff you may not even be able to fish the Bitterroot at that time. Even on a year with heavy runoff our river is normally fishable by the last week in June.  In the early spring and late fall our river, at the gauging station near Darby, is running at a flow between 180 and 500 C.F.S. (cubic feet per second) . In high water it can peak out at over 10,000 C.F.S.  We also have a reservoir at the head of the West Fork of the Bitterroot that regulates the stream flow in the summer months. The reservoir was built to hold water back for irrigation, trout unlimited also owns some of the water in the reservoir. The increased flow in July – September is very beneficial to our fishery especially in the dry drought years.  All of these factors can greatly effect how, if, and when a particular hatch may come off.

Trout also feed on terrestrials; ants, crickets, grasshoppers etc…    On the Bitterroot  river  we don’t always need a hatch for good dry fly fishing.    The trout seem to key in on large attractor patterns whenever they are feeding.