Mayflies, or Up-winged Flies as they are sometimes called, are primitive insects that spend the greater part of their unique four-stage lifecycle in the water, first as an egg and then as a nymph. During this aquatic period the nymphs go through numerous moults (instars) increasing each time in size until they're ready to emerge, a process that may last from just a few weeks to as much as a couple of years dependent on the species and environment. When growth is complete, the nymph emerges as a dull-coloured dun that sheds its skin to become a fully-winged sub-adult insect. This is the unique part of their lifecycle, called the subimago stage, when they will rest in a sheltered spot for a few hours before shedding their skin for the last time. The appearance of the subimago is different to that of the adult (imago) form as the wings are generally opaque and tinted, often with heavy pigmentation along the veins, which may gives them a mottled appearance. Once out of the water mayflies develop quickly as neither the subimago or the imago (sometimes called a spinner) feed. Some species may only last a few hours, whilst others live rarely more than a day. Most are weak flyers, so unless they've been caught on the wind, will be found in close proximity to the river, stream, lake or pond from where their eggs were laid. The short adult life necessitates co-ordinated emergence to maximise the chances of mating, which results in the swarms seen on some rivers.

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The fully developed adult form of the species are characterised by having large forewings that are held together above the body when at rest. The hindwings are much smaller or, in the case of a few species, absent. They have large compound eyes, short antennae and reduced mouthparts. The slender abdomen tapers at the tip where it has two or three cerci, commonly known as 'tails'.

   

There are around 50 species in Britain, separated into ten families that can be determined by a combination of factors such as the size of the wings, wing markings, and whether they have two or three 'tails'. Many species are of interest to fly-fishermen, which has resulted in most forms having common names that, much like butterflies, are more frequently used than their actual scientific name. The following five families probably cover any mayflies that I'm likely to see locally as, unfortunately, I have limited access to suitable habitats.

 

Baetidae (14 spp.), most are common and widespread, small to medium-sized (> 23mm wing span) with two 'tails', hindwings small or absent; the running water species include several known as 'iron blues' including both the Small Dark Olive [Baetis scambus] and Large Dark Olive [Baetis rhodani], while the still or slow-moving water species include both the smaller Pond Olive [Cloeon dipterum] and Lake Olive [Cloeon simile].

 

Heptageniidae (11 spp.), small to large (> 38mm wing span) with two 'tails', five-segmented tarsus; although well-known to fly-fishermen, identification to species is difficult; one of the more common species is the Yellow May Dun [Heptagenia sulphurea] that occurs on larger rivers.

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Ephemeridae (3 spp.), large (30-50mm wing span) with three tails, hindwings large, forewings with dark markings, abdominal markings help distinguish species; two may be found locally, the Drake Mackerel [Ephemera vulgata] and the common Green Drake [Ephemera dancia], the latter predominantly on fast-flowing waters.

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Ephemerellidae (2 spp.), small to medium-sized (> 23mm wing-span) with three 'tails', hindwings large, forewings with short cross veins between the long veins; although two species, only the Blue-winged Olive [Serratella ignita] in the south, along fast-flowing rivers and streams.

   

Leptophlebidae (6 spp.), small to medium-sized (> 27mm wing span) with three 'tails', hindwings large, forewings with no regular cross veins between the long veins. One of the more common species is the Sepia Dun [Leptophlebia marginata] found along streams and on ponds and lakes.

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