the MAYFLIES (ephemeroptera) OF
BRITISH COLUMBIA

by

R. A. Cannings and G. G. E. Scudder

Copyright © 2005 - All rights reserved

Extracted from the forthcoming publication The Insect Families of British Columbia. 

Introduction

The Ephemeroptera or Mayflies are the most primitive flying insects. Like the Odonata they are Palaeoptera and cannot fold their wings down flat over the body. Mayflies are aquatic as larvae -- the larvae live in many kinds of running and standing freshwater habitats and the adults normally are found around water. Because they are so abundant, mayflies are a critical part of both aquatic and terrestrial food webs and an important indicator of pollution in many environments. Artificial flies used by fishermen are often modelled after these insects. Ephemeroptera are the only insects that have two winged stages; the larva moults into a flying subimago that then moults again into an adult. The larvae have biting and chewing mouthparts; although most feed on a variety of detritus and algae, some are true carnivores. Adults have atrophied mouthparts and do not eat -- they live only a few hours or days and their sole functions are reproduction and dispersal.

The order is cosmopolitan and about 2100 species are described worldwide. About 85 genera and 700 species are known in North America. Ten families containing 31 genera and 92 species are recorded in B.C., but many more likely occur. It is probably fair to say that the larvae of mayflies are better known than the adults in B.C. and many other regions. However, in some groups, the identification of larvae is difficult and often impossible without associated adults.

Adults are small to medium-sized insects (without the filaments, the body is from about 2 to 32 mm long), elongate and soft-bodied. The eyes of the female are rounded and separated dorsally; but in the male, they are enlarged, normally meeting above and divided into an upper part with large facets and a lower one with smaller facets. In the Baetidae, especially, the upper section is columnar or stalked (turbinate). These specialized eyes are related to swarming behaviour and mating. There are usually three ocelli. The filamentous antennae are usually short (shorter than the head width), but often are long (more than twice the head width).

The forewings are large and  triangular with many veins, but the hindwings are small and rounded, sometimes strongly reduced and strap-like, and sometimes absent. The front margin of the hindwing is often produced into a lobe or point (the costal projection) that helps link the fore and hindwings in flight. When at rest, mayflies hold their wings stiffly upright, together over the body. The front legs of most males are greatly lengthened and are used to grasp the female by the base of the forewings during mating. The tarsi have 3 to 5 segments; the tarsal claws are paired and similar (both hooked or both blunt) or dissimilar (one hooked and one blunt). The tip of the 10-segmented abdomen bears two (the cerci) or three (an additional central terminal filament) long and slender caudal filaments.

Larvae have large eyes and three ocelli; the antennae are filamentous, shorter or longer than the head. The well-developed mouthparts are the biting and chewing type; the labrum can be small and narrow, but often is prominent and wider than the head. A few groups have gills at the base of the maxillae. The thorax of the mature larva bears two (sometimes only one) pairs of wing pads; often with spines or other projections on the dorsal or ventral surface. In the South American genus Murphyella there are ventral thoracic gills. The legs are usually shorter and more stout than those of adults and the femora are often flattened. In some species, legs are modified for digging, filtering food or protecting the gills. The tarsi normally have only one segment; each with a single claw, which are variable in structure and useful in identification. A few genera have gills near the base of the coxae. There is a pair of leaf-like gills on the sides of some abdominal segments; the number and shapes of these gills vary and are important in classification. The abdomen normally has three caudal filaments, but the terminal filament is sometimes reduced or absent. The location and shape of gills and the three (rarely two) caudal filaments distinguish mayfly from stonefly larvae.

Immatures feed mainly by gathering and scraping algae and organic detritus from the substrate; some filter food particles from the water. A few genera are predacious -- in North America these are mostly in the Heptageniidae and feed on chironomid midge larvae. Baetid, ameletid and siphlonurid larvae are streamlined, strong swimmers; many other groups are sprawlers that move over the bottom surface; still others (for example, heptageniids and ephemerellids) are flattened and live under rocks or in other crevices; ephemerids burrow in silt and mud.

Although the egg stage can last up to a year, there are usually one or two generations per year; some larval mayflies develop in as little as three weeks or as long as three years. Growth depends on the species and environmental factors such as water temperature; some species moult up to 50 times (15 to 30 times is more usual).

The final larval stage moults to a winged form (subimago) on the water surface or on an object protruding from the water. The subimago is dull grey and, because the body and wings are covered with a dense layer of hair, it is rather waterproof. Emergence may involve huge numbers of individuals. Although it has functional wings, the subimago is not an adult; the genitalia and legs are not fully developed. It moults again, usually within a day, and becomes sexually mature. In a few genera, there is no adult stage in the female and the subimago mates and lays eggs. Adults normally mate and die within a day, but some may last up to a week. Males often engage in swarming flights, frequently over water on warm evenings, flying up and down in unison. Females enter the swarm, are seized by males, and mating ensues. Egg masses are washed off in the water or are attached to stones or other objects in the water.

The higher classification used is modified from McCafferty (1996).

References

Edmunds, G.F., S.L. Jensen and L. Berner. 1976. The mayflies of North and Central America. University of Minnesota Press, Minneapolis. 330 pp.

McCafferty, W.P. 1996. Ephemeroptera. Pp 89-117 in Nomina Insecta Nearctica, Vol. 4. Entomological Information Services, Rockville, MD. 731 pp.

McCafferty, W.P. and R.P. Randolph. 1998. Canada mayflies: a faunistic compendium. Proceedings of the Entomological Society of Ontario 129: 47-97.

Needham, K.M. 1996. An identification guide to the nymphal mayflies (Order Ephemeroptera) of British Columbia. Resource Inventory Committee, BC Ministry of Environment, Lands, and Parks, Victoria. 77 pp.

Scudder, G.G.E. 1975. An annotated checklist of the Ephemeroptera (Insecta) of British Columbia. Syesis 8: 311-315.

Illustrations by L. L. Lucas.  Copyright © 2005 - All rights reserved