Brief life of magical mayflies

LAST WEEK I was sitting in my canoe bailing out rain water when a large and beautiful insect floated by on the river --- a mayfly. 

The fact that it was nearly June and this was the first one I’d seen this year wasn’t really surprising. Mayflies can emerge at any time from April through to the autumn but the principal hatches are normally in late May and early June. The reason the name doesn’t coincide accurately with the month is because they were named before the Pope changed the calendar and at that time May was a couple of weeks later in the year.

The larvae of mayflies, which anglers call nymphs, live for two, occasionally three, years on the bottom of suitable lakes and rivers. There they burrow into silt or sand and filter organic matter out of the water. They grow continuously and this involves regular shedding of their external skeleton and producing a new and larger one. Biologists call each of these stages between moults an ‘instar’.

When they eventually rise to the surface and the last larval skeleton breaks open to reveal a winged insect they do not have a great life expectancy — days at most. They are doomed because they have lost their mouth parts and can’t eat. Their only priority is the production of fertilised eggs to start a new generation. But mayflies are unusual among aquatic insects in that they undergo one more moult as a winged insect before they become sexually mature. Biologists call the two stages of winged insect the sub-imago and the imago. But mayflies are also very important to fly fishermen who have their own vocabulary for these things. The sub-imago and the imago become the dun and the spinner. These words are applied to all members of this family of flies. The common large mayfly, Ephemera danica, becomes a green drake and a grey drake.

Mating takes place in mid air with swarms of insects performing a graceful aerial dance as they select a partner. The fertilised female drops back to the water surface where she deposits up to eight thousand slow sinking eggs. Both males and females then die.

The fact that mayflies are important to anglers obviously shows that they are even more important to fish. In fact they’re a vital component of the overall ecology of the freshwater systems they still survive in. The number of these systems diminished considerably in the second half of the twentieth century. The main cause for this reduction was water pollution which mayfly nymphs are particularly sensitive to. They are carefully monitored by anglers and bodies like Inland Fisheries Ireland and there are some indications the downward trend has bottomed out. This is probably due to the enforcement of water quality directives from the EU.

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