There seems to be almost nothing to them: a gauzy hint of life, a diaphanous spectre on the periphery of your vision as you stroll by a river.
What is a mayfly and how big are they?
Mayflies are short lived winged insects from the order Ephemeroptera. Unless you’re an angler, mayflies tend to flutter by unnoticed, and what’s more, most of the 51 British species are tiny, with a body length fewer than 15mm.
How long does a mayfly live for?
Their winged life is short too – a matter of hours to a few days.
In May the green drake is on the wing. Not only is it the biggest British species, it also lives longest, so is the one most likely to be noticed – its visibility is what gives mayflies their name.
Over a healthy river on a languid spring evening, green drakes can form incredible dancing flocks as the females waft their heady perfume and attract the males, a dancing behaviour that earned them the anglers’ moniker of ‘spinners’.
Mayfly lifecycle
The nymphs of these mayflies, called naiads, spend 1–3 years developing in sediment at the bottom of clean rivers, streams and lakes. Here they feed on organic detritus and go through 30–50 moults. This life stage reminds me of Chinese dancing dragons. You’ll see what I mean if you excavate a nymph from its silty lair and witness the six pairs of hairy gills pulsating on its flanks, together with the horns and tusks on its head.
Keep nymphs in an aquarium and as they reach the end of their aquatic life you might notice them change behaviour. They burrow less, sit on top of the sediment and take on a silvery appearance, as if liquid mercury is running beneath their skin. This is air, and it’s causing them to become slowly more buoyant. The middle portion of their gut, which degenerated after they ate their last meal, has turned into a sausage-shaped flotation device that now fills with air. Eventually they bob to the surface.
Almost as fast as they hit the surface, like actors changing between scenes, they undress. It starts as a split along the back and then they unzip. In a matter of 30 seconds or fewer, they’re out of their old clothes and exposing their new look, with fully inflated, functional wings. These translucent visions pause on the water before flying off, leaving the shadowy husks of their former life to drift away.
The winged stages are known as duns, due to their dull, yellow-brown hue and the fuzz of unwettable hair on their wings and body. Now they perch on waterside vegetation to await the next transformation. Their sexual organs, among other things, still have some growing to do – and, unique in the insect world, they are about to moult from one winged form into another.
Mayfly second moult
After a few hours each dun ruptures, and out of the seam comes another. Its wings are the most surprising thing: they are crystalline and sparkle in the sun, with darker veins that give a dappled beauty. At the same time, the male’s front legs are longer than before and the triad of tails almost a third as long again.
Why have a second moult? The truth is nobody really knows. Perhaps the process has been carried over from more primitive forms of mayfly. Or maybe a mayfly’s adult life is too short for it to have been selected out of existence?
Since this stage lacks mouthparts and its body cavity is full of reproductive organs not digestive systems, it only has the fat reserves laid down as a nymph to power it through its brief love life, culminating in a clutch of 8,000 fertile eggs scattered by the female to the fate of the current.
Main image © Peter David Scott/ The Art Agency
