mayfly Sorting out Mayflies mayfly

by Karen Christopherson

I don't think that it's just me who gets confused by the assortment of mayflies out on the water, or mentioned in a flyshop. References to nicknames, scientific names, or flies can get one mighty confused as to what fly we should be using as a certain hatch is on. Adding to the confusion is that, when you actually see a hatch, and maybe two hatches, you ask, "which mayfly is that?". Of course you can always throw on a fly that matches the size and color, but I sometimes actually want to know which mayfly it is that's hatching.

Mayflies make up the order Ephemeroptera, which is from the Greek "Ephemera" for "short-lived" and "ptera" for "winged". This aptly describes the mayfly's adult life. After emerging as an adult, the mayfly lives from one hour to one day, during which time its sole purpose is to mate and lay the eggs. Prior to this, it has enjoyed almost one year as a nymph (if it could avoid those trout)!

mayfly on branch by Weilenmann According to Ward and Kondratieff, 11 of the 17 known North American mayfly families are represented in Colorado. This includes 33 Genera and 71 Species. There are something like 570 species in North America. Yikes! As a flyfisherman, do you have to recognize 71 species of mayflies? Answer, "no", but it will be useful to know a few of the Genera. Telling the species apart is actually very difficult, especially when those mayflies are flitting around and you have more important things to do (like fish).

Life Cycle

The mayfly has three stage of insect life - egg, nymph and adult. It spends its life as an egg from an average of one to three months, then as a nymph for 8 to 11 months. When the adult emerges, it is first in a subimago stage (what we flyfisherpeople know as "dun") - it is winged but can't mate. Soon (like hours), the dun molts and becomes a true imago (translation "spinner"). The spinner mates, lays eggs, and dies. Falling on the water provides a welcome feast for trout.


Nymph mayflies have various habitats in the water as either swimmers, crawlers, burrowers, or clingers. They are mostly brown or black and have 2 or 3 tails.

Adult Mayflies are very distinctive in that they have relatively large upright wings (for their body size) and long, extended, upturned tail (actually 2 or 3 tails). They have a method of flying where they seem to dart up and down in the air. The males will swarm, and the female flies into this swarm.

The following table attempts to summarize some basic information about the various mayflies found in Colorado and Wyoming. Given are the scientific name (which many people will use to identify a hatch), the common name (which many other people will use for a specific mayfly), where in Colorado or Wyoming they are found, good dry fly imitations, the nymph behavior, the colors of the adult (to help you in identification), the time of year that they hatch, and the typical size of the adult mayfly (given by hook size).

Sci Name(Genus and species) Common Name State Dry flies Nymphs Adult colors Time of year Size
Baetis vagans+ many others BWO (Blue-winged olive) CO+WY BWO, Parachute Adams swimmer olive/brown belly; greyish wings mar-nov 16-24
Ephemerella/Drunella guttulata + grandis+ coloradensis Green Drake (western) western CO+WY green drake crawler grey-green, olive body; greyish wings jul-sep 10-12
Ephemerella inermis (smaller) + infrequens (larger) PMD (Pale Morning Dun) CO+WY PMD crawler greyish wings; body varies jun-aug 14-20
Rhithrogena hageni+ robusta+ others Red Quill WY+CO (parts) Red Quill   red/brown body; greyish wings jun-oct 12-18
Tricorythodes minutus+ stygiatus+ others Trico CO+WY Tricos, Parachute Adams, Royal Wulff crawler olive, black, brownish body; clear or grey wings jul-sep 20-26
Callibaetis americanus + many others Callibaetis CO+WY Callibaetis, cahill, swimmer light veins on dark wings; greyish body jul-aug 14-16
Ephemera simulans Brown drake nw WY+nw CO   clinger lt/dark brown body: splotchy wing may-oct 8-12
Rhithrogena morrisoni Western March Brown western CO+WY various Western March patterns clinger tan/brown body; mottled wing mar-apr 10-14
Drunella (Ephemerella) flavilinea Flav nw WY various Flavs (or small green dark patterns) crawler greenish body; greyish wings jun-aug 14-16
Heptagenia (various) Pale Evening Dun WY+CO (parts) PED clinger brownish body jun-aug 14-16

mayfly on branch by WeilenmannFlies

Almost all mayfly nymphs can be imitated by a Hare's Ear, Pheasant Tail, beadhead and flashback versions of these flies. The RS2 is used to imitate BWO's, especially at emergence. The more important aspect of nymphs is choosing the right size to match the mayflies on a particular river, and to present it matching how the nymph behaves (crawler, swimmer, clinger, burrower).

Adult mayflies can be imitated well by Adams, Royal Wulffs, Humpys for almost all species with attention paid to size to match the hatch. Else, most dry flies are named for the particular species or its common name.

Refer to the Fly Tying page for instructions on tying some of these patterns.


  • Ward, J.V. and B.C. Kondratieff, An Illustrated Guide to the Mountain Stream Insects of Colorado, Univ. Press of Colorado, 1992
  • Hafele, Rick and Scott Roederer, An Angler's Guide to Aquatic Insects and their Imitations, Johnson Books, 1995.
  • Mayfly Central
  • Earthlife

All information presented herein is to the best of my knowledge and research. If you have comments or corrections, please email me

All photos (except top left at title) copyrighted and courtesy of Hans Weilenmann