Mayflies are synonymous with flyfishing. Since the history of the sport, the two have been closely linked. The reasons why are obvious; no other insect is more available to the trout, and more noticeable to us humans, than the mayfly. Mayflies live almost their whole life under water. They are available to the fish as nymphs year round. When they finally emerge, they resemble small sailboats being blown around by their upright wings, lazily going where the wind and the current take them. This, the time of the hatch, is when the mayflies are the most vulnerable, and is the time that is treasured by flyfishermen the world over. Once they have left the water, the mayflies most go through one more change that will make them capable of reproduction. The insects then return to the water to breed and die, once again becoming food for the trout.

The stages of life for a mayfly are egg, nymph, dun and spinner. Eggs can be deposited by the spinners in several ways; one is ovipositing, where eggs are dropped into the stream from above. Another form of egg laying takes place when the mature adult crawls onto aquatic vegetation (or your waders) and attaches their eggs to the plant. When the nymphs or larvae hatch, they crawl around the stream or lake bottom; it is usually a year before they are ready to hatch. Hatching (the metamorphosis from the juvenile stage to the first flying stage) can also come about in different ways. The first is for the nymph to swim or float to the stream's surface, where it will break open its nymphal shuck; a fully formed dun, or subimago, will crawl on to the water's surface, dry its wings, and fly away. The second form of emergence is for the dun to crawl from the nymphal shuck onto the stream bottom, then swim to the surface as an adult fly. A few days after the duns (subimagos) have hatched, they shed once again, their body color changes, and their wings become clear; most importantly they become mature adults capable of reproduction. This reproductive stage is called the spinner or imago. The spinners will mate in huge swarms over the top of the water. Once they have mated, they fall dead onto the water's surface, spent winged, like hundreds of little crosses. Due to the sheer number of flies, a spinner fall can be one of the most productive, but often frustrating, times to fish.

Identifying Mayflies
A nymph can be identified as a mayfly if it has all of the following characteristics:
1) Gills along its abdomen
2) A single claw at the end of its legs.
3) One set of visible wing pads.
4) 2 or 3 tails

Mayfly adults have the following characteristics:
1) 2 sets of wings that are upright at rest.
2) 2 or 3 tails.
3) A long, usually slender, body.

Types Of Mayflies

There are approximately 570 North American species of mayflies. Since it would be very difficult to identify them all on this web page we will break them down into groups based on the behavioral characteristics of the nymphs. The four basic groups are Swimmers, Clingers, Crawlers and Burrowers. The following keys should give you a good start on identifying and understanding the basic types of mayfly nymphs and adults.

Mayfly Swimmer Nymph

Mayfly Swimmer Wing

Identifying Characteristics of Swimmers:

Nymphs

Round and streamlined body.

Tails are fringed with fine hair.

Oval gills are positioned on the side of the abdomen.

Adults

Small body size.

Base of veins MP2 and CuA do not diverge from the base of MP1.

Two or three tails are present.

Mayfly Crawler Nymph

Mayfly Crawler Nymph

Mayfly Crawler Wing

Identifying Characteristics of Crawler :

Nymphs

Plate like gills on top of abdomen.

Forked gills with no fringes.

Adults

Series of short intercalary veins present along the outer margins of the fore wing and not attached to other veins.

Mayfly Clinger Nymph

Mayfly Clinger Wing

Identifying Characteristics of Clinger :

Nymphs

Head is larger and wider than abdomen excluding gills.

Body is very flat.

Plate like gills on the side of the abdomen.

Adults

Two sets of cubital intercalories.

Mayfly Burrower Nymph

Mayfly Burrower Wing

Identitifying Characteristics of Burrower :

Nymphs

Forked gills along the abdomen which are fringed ( covered with small hairs).

Large mandibles (mouth parts) that form tusks.

Adults

Veins MP1, MP2 and CuA strongly diverge from the base of the fore wing.

Very large mayfly with large wings.

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