The mayflies, Order Ephemeroptera, are primitive insects with two winged adult stages and several aquatic nymphal stages. Adults are easily identified by their bristle-like antennae, upright wings containing many wing veins, the presence of two or three tails extending from the posterior end, and their fragile nature (Figure 1). Nymphs can also be identified by the presence of two or three long tails at the end of the abdomen. In addition, nymphs have paired gills associated with some of their abdominal segments, and their tarsal claws are single rather than double (Figure 2). Two tails, double tarsal claws, and gills associated with the thorax rather than the abdomen are characteristics of stonefly nymphs, which superficially resemble mayfly nymphs and may sometimes be confused with them (Figure 3).

This publication will enable the user to identify the recorded mayfly nymphs of British Columbia to family and genus, with the aid of pictorial keys. Also included are short synopses of each family and genus in B.C., a glossary of taxonomic terms, and a reference list for further studies.

ADULTS

Mayfly adults usually have four wings; the front pair is always much larger than the hind pair, which is sometimes completely absent. These wings cannot be folded over the abdomen when the insect is at rest, giving the adult insect its characteristic profile.

The presence of two winged adult stages is unique among the insects. The first of these stages is known as the subimago, and can be readily distinguished from the second stage by the presence of hairs on the wings and sometimes on the body, making them appear dull. The subimago is non-flying, non-reproductive, and short-lived. The second winged stage is the imago, which has completely clear, shiny wings and is capable of reproduction. It may also be short-lived (a few hours to a few days), or it may survive up to several weeks.

Mayflies are very poor fliers; the two or three tails at the end of the abdomen in the adult are thought to act as stabilizers during flight. Adults in both stages have non-functional, strongly reduced mouthparts, so they cannot feed.

NYMPHS

Mayfly nymphs vary greatly in general body shape owing to their many different lifestyles. All live in freshwater, but some are burrowers, possessing large mandibular tusks for digging, and feathery abdominal gills for creating a water current through their underground burrows. Others live underneath stones in fast-flowing streams and rivers; these have flattened heads and bodies, and widespread legs for clinging. Still others are slender and elongate, adapted for crawling on the surface or through vegetation. These mobile nymphs have cylindrical bodies and long legs for holding their bodies up off the ground. All of these nymphal types are well-camouflaged against the background, and are therefore difficult to spot in their natural habitat.

Most mayfly nymphs are omnivores, browsing for tiny plants and animals on the surfaces of rocks or underwater plants. Feeding habits of many are not known, but it appears that a few at least are true carnivores.

The abdominal gills possessed by mayfly nymphs are used for obtaining both oxygen and salts from the water. Oxygen can be taken up across the gill surface itself, or across the general body surface from the water currents created by the movement of the gills. Maintaining a body concentration high in salts is difficult for freshwater animals, so gills of mayfly nymphs are covered in special cells, known as chloride cells, that aid in salt uptake in dilute waters. Gills vary greatly in shape and size between mayfly species, so are an excellent taxonomic character when trying to identify individuals within this Order.

LIFE CYCLE

Because mayfly adults are short-lived and are poor fliers, mating swarms have been observed in many species. Adults emerge simultaneously in large numbers and males then congregate to make the females' job of finding them easier. When a female enters this mating swarm, she is approached by a male from underneath. He grasps her thorax with his front legs, and then attempts to copulate with her by curving the end of his abdomen upwards, grasping her abdomen with his forceps, and inserting his penes into her oviducal openings. Sometimes these joined couples sink to the ground; other times they remain in flight during copulation. The female then flies off to find water in which to lay her eggs. Eggs can be laid on the surface of the water singly or in clumps, after which they sink to the bottom. However, females of a few species actually crawl under the water on submerged vegetation and lay their eggs directly on the surface of some submerged object.

Eggs of most species hatch within a few weeks. Nymphs then undergo several molts, growing larger with each successive stage. Nymphs may live a few weeks, a few months, or a few years, depending on the species. Water temperature plays a large role in the speed of nymphal development.

The final nymphal instar is easily identified by the presence of wing pads on its thorax. This nymph floats to the surface by filling its midgut with air. Emergence to the subimago may occur just under the surface of the water, on the surface of the water, out of the water on a floating object or on the shore. Large numbers of sexually immature subimagos are often seen floating on the surface of the water while their wings expand and their skeletons harden. The hairs covering their wings and bodies are thought to provide waterproofing for the subimagos at this vulnerable time.

The molt to imago may occur within a few minutes in some species, or may take up to 48 hours in others. During this time, the subimago rests on vegetation, rocks, or walls, and rarely attempts flight. The sexually mature imagos of most species emerge synchronously, usually near dusk, and immediately begin the process of mating. In the Pacific Northwest, where winters are mild, adult emergence occurs throughout the year.

COLLECTING AND PRESERVING MAYFLIES

Mayfly nymphs can be collected by using an aquatic net with a fairly fine mesh. Place the net near the bottom of a stream or river, and then disturb the substratum or turn over some rocks just upstream from it. The disturbed mayflies will drift downstream into the awaiting net. An aquatic net can also be moved gently over submerged vegetation to disturb and capture mayflies clinging there. After each sweep, the net should be inverted and washed into a shallow, light-coloured pan, where the insects can easily be seen. Be sure to check for nymphs clinging to vegetation and debris in the pan. Remove each specimen from the pan and place it in a vial of its own, labelled with the date, collecting locale, and collector's name. If additional information is available, such as water temperature, air temperature, weather conditions, or altitude, this should be recorded in a field note book and cross-referenced to each specimen.

Adult mayflies are best collected using an aerial net. This can be swept through the air above a water body, or it can be used to gently sweep the vegetation near the water's edge, where subimagos or imagos may be resting. Again, place each specimen in a vial of its own and record all of the pertinent collecting data.

Both nymphs and adults should be preserved in 70% ethyl alcohol to prevent drying, and so that the specimens can easily be examined at a later date. Glass dram vials with lids that prevent alcohol evaporation are excellent for this.

EXTERNAL MORPHOLOGY OF MATURE NYMPHS

The external structures of a generalized mature mayfly nymph are illustrated in Figure 4. An enlarged view of the mouthparts of this nymph are illustrated in Figure 5. These features will aid in the use of the taxonomic keys to follow. Presence of wing pads on the meso- and metathorax identify a nymph as mature. The thorax also bears three pairs of legs, one on each thoracic segment. The abdomen of mayflies consists of 10 segments. Begin numbering these segments by counting forward from the 10th abdominal segment. Some of these segments will contain gills, which may be located dorsally, ventrally, or laterally. At the posterior end of the abdomen are caudal filaments of varying lengths; usually there are two lateral caudal filaments and one middle caudal filament.

USING THE KEYS

Begin by identifying your specimen to family using the key located on pages 12-14. Once you have placed your specimen in a mayfly family, turn to the appropriate family key and use this to identify your specimen to genus. Four families in British Columbia have only one representative genus; these are indicated on the Key to Families. Using the keys involves making a series of choices. Eventually, these choices will lead you to the end of a pathway where the specimen is then identified to either family or genus. Taxonomic characters chosen for the keys are those that were deemed easiest to recognize, and are illustrated wherever appropriate. Within each block of text in the keys, lower case letters in brackets refer to the illustrations directly below that block of text. Illustrations are by Launi Lucas, and are redrawn after Edmunds et al. (1976), Clifford (1991), and Merritt and Cummins (1996).

Once you have identified your specimen to family or genus, you may wish to compare its overall features with the whole animal illustrations and with the more complete list of taxonomic characters located in the tables following the pictorial keys). Here, you will also find specific information about distribution, habitat, habits, and miscellaneous facts for each British Columbia family and genus within the Order Ephemeroptera. A glossary of terms is available to further aid you in the use of the keys. A bibliography to other keys, more in-depth taxonomic information, and the biology and ecology of mayflies can be found at the end of this publication.