Down on the Thames at Kingston, large mayflies, are known at this stage as sub-adults or duns, lift off the water with weak fluttering flight. Their aim is to reach bank-size vegetation where they can shed their skins for a final time to become perfect adult insects called ‘spinners’.

However, their chances of reaching cover are slim as they face a whole array of predators taking advantage of their slow progress to catch them in mid-air. Swallows, swifts and martins flash past picking off mayflies on the wing. Even if the vulnerable insects reach the shore, many blunder into spiders’ webs with no chance of escape, while trotting along the towpath, a pied wagtail appears with a beak stuffed so full of mayflies I’m surprised it can see ahead.

Any survivors form large swarms in the evening, rising and falling in mating dances. Once mated, males quickly die leaving females to alight on the water once more to lay eggs. They live for barely a day, having no need to feed and mouth parts are non-functional.

Sometimes the water surface is covered with egg-laying mayflies attracting trout and dace that gorge on the hapless insects. Anglers call the short mayfly season ‘duffers fortnight’ because even the least experienced fly fisherman will be able to catch trout as they indulge in a feeding frenzy.

Eggs that survive will hatch as nymphs that live for a year or two on the river bed feeding on detritus.