Order Ephemeroptera - Mayflies

• Ephemera and Cloeon species

Soft-bodied insects with large eyes, very short antennae and atrophied mouthparts. Wings membranous with many veins, hind pair small or even absent. Wings held vertically upwards when at rest. Front legs of males are often very long and used to hold the female during mating. Abdomen terminated by a pair of very long, many-segmented, thread-like cerci, sometimes with a third median tail filament. Mayflies are unique among insects in moulting after reaching the adult winged state (the first winged stage is called the subimago and has a dull appearance due to a covering of very fine hairs - this outer skin is shed within a few hours of emergence, revealing the mature adult, recognisable by its shiny appearance and full colouration). Simple metamorphosis, with over 20 nymphal moults before the adult stage in some species. The nymphs are aquatic, with three tail filaments and several pairs of plate-like tracheal gills that grow out from the sides of the abdomen. These gills are used for breathing, by absorbing oxygen directly from the water. About 2,100 species have been described, of which 51 occur in the British Isles.

Description. Adults of Ephemera danica, which are about 15-20 mm long excluding the 'tails', have a brown thorax and pale yellowish-white abdomen with darker brown markings on most segments. The fore-wings have a conspicuous dark streak across the middle and smaller dark patches near the base, which together with the distinctive abominal colouring make this species one of the most easily recognised mayflies. Nymphs mainly brown, up to 30 mm long.

Biology. Adult mayflies are usually found close to rivers, streams and other bodies of freshwater. The adults do not feed and generally live for less than a day. During this brief existence they mate and lay eggs. Most species simply drop their eggs into the water, but some may descend into the water to lay their eggs on submerged plants. The nymphs of Ephemera live and burrow in gravel, sandy or muddy areas of rivers and lakes. Other species live under stones and amongst aquatic weeds in fast and slow moving rivers and streams, or in some cases in ponds and lakes. The nymphs of Cloeon (illustrated) are found mainly in still water, or sometimes in in very slow rivers, clinging to stones and vegetation. Mayfly nymphs are mainly herbivorous, feeding on plant debris and algae. Nymphal development may take up to two years, but most mayflies complete their life cycle in one year.

Distribution. Both Ephemera and Cloeon spp. are widespread throughout northern and central Europe, including the British Isles.

Photos: (Ephemera adult) C. Macadam © ; (Cloeon nymph) V.J. Stanek ©

Adult (Ephemera danica)