The last couple of times I have been by lake or river in Kent there have been substantial hatches of very small cream or white flies, I assume caenis or very small midges. However, what caught my eye is that later in the hatch you see some of the adults sporting a long 'tail' which looks like a very fine white thread about 30mm long hanging below them, almost like a strand of spider web. What is this?

No expert at entomology but my guess would be a tube the females use to lay eggs

I expect I'm way off and someone with a bit more knowledge will be along to correct me shortly

Neil

Caenis spinners have very long tails, could be that, not generally 30mm though

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Don't worry, be happy.
Sandy
Carried it in full, then carry it out empty.

Eggs
richard

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Who resides on the right bank of the Derbyshire Wye and is lulled to sleep each night by the mutterings of a weir, dreaming that "When the rivers and their inhabitants come first, we ALL win..."

its for laying eggs ovipositor i think i work in pest control most flies hav them

As Richard says it is eggs.
Niether upwing flies or midges have ovipositors or if they do they are not long tube like ones that the casual angler is going to notice.
Generally midges and upwings carry the eggs as a ball but some of the upwinged flies have them hanging as a long thin thread and this is what you are seeing.


Andy

Ithink Richard is probably correct. Caenis have three tails, but 30mm is way too long, surely. The ephemeroptera, I am certain, do not have ovipositors, if the last contributor is thinking about the likes of woodwasps, ichneumon flies etc. then they need to inject their eggs into wood for the larvae to develop, or into a live caterpillar n order to parasitise them, respectively. Upwinged spinners dump their eggs in clusters, ones, twos and threes, on the surface, or they crawl down partly-submerged stones and lay them on the leeside, or on the substrate,all depending upon species. I haven't noticed the phenomenon described, but eggs are sticky, so perhaps they do form a string. ja.