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Life in Freshwater

Mayflies (Ephemeroptera)

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Mayfly larva

This ancient group of insects is well known by anglers because of the "flies" produced to catch trout. Characteristically, they have 3 flowing tails at the end of the abdomen in the larval stage and this usually carries across to the adult. Sometimes the adults only have 2 tails. Male adults have long front legs for grasping teh female. They have soft bodies and a weak flight but at rest they always hold the wings vertically above the body. The eyes are large but there are no mouthparts.

Adult mayfly Ephemera danica

The Green Drake or Common Mayfly, Ephemera danica, (photos above) is found most frequently and is a beautiful and large insect, around 50 mm long.

The 200 species found in Europe are very widespread wherever water is plentiful. Around 50 species live in Britain. Despite the name they can be found during most months in spring, summer and even autumn. A common group of insects found in and around most ponds, lakes, streams and rivers.

ECOLOGY: This is an important group as both stages in the life cycle are important for fish development. The larvae take a year to develop in water, feeding on a variety of plant debris, algae and diatoms. They shed their exoskeletons very regularly - up to 40 times. The life cycle is unique amongst the insect. After mating the adult female lays her eggs on the surface of the water and then dies. The full-grown larva swims back to the surface where it emerges as a sub-adult or "dun". This process is synchronised as a mass emergence to increase the chance of some surviving as predators move in to feed on these feeble forms. They are very dull in colour (see photo below) and have difficulty in flying mainly because the gut is full of water.

Subimago mayfly
Subimago mayfly just emerging from the water on to a Potamogeton leaf

After an initial flight it soon ecdyses (sheds the exoskeleton) again. This is the unusual feature not found in other insects. This time a bright coloured adult is formed. After a "dance" to attract a mate eggs are laid and death results. Adult life is usually less than 24 hours and during this time it does not feed.

The larva, whilst under the water, uses the gills along the side of the body to pick up oxygen dissolved in the water. To increase the diffusion of the gases across the large surface area of these gills it often flicks them rapidly to produce a fresh current of water over the gills.

Larva of Ecdyonurus
Larva of Ecdyonurus genus of mayfly. Note the large head shield that provides excellent aerodynamics for water to flow over the head.

Larva of Leptophlebia
Larva of Leptophlebia genus note the forked gills along teh side











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