_An Angler’s Etymology_ by the Irishman J.R. Harris, is one of the classic books for a fly fisherman; “Classic” in this context, as in others, means “I’ve never read it”. It concentrates on _Ephemeroptera,_ which are the “prettiest”:http://www.fishing-in-wales.com/wildlife/insects/upwing/yellomay.htm trout food, though less important than the sedges, _Trichoptera_ and midges, _Chironomidae._ My copy of the book has all the colour plates reproduced in black and white, which reduces still further its usefulness.
But then you come across a passage like this, which bring back the golden age of obsessive naturalists.
bq.. Then purchase a copy of Mr. Kimmins’ _Keys to the British Species of Ephemeroptera with Keys to the Genera of the Nymphs,_ which is published by the Freshwater Biological Association, Wray Castle, Ambleside, Westmorland, and costs only 2 6d.
This is an excellent publication and it contains drawings of the genitalia of the male spinners of all British and Irish species of the Ephemeroptera. It also has illustrations of the wings, and, where necessary, other parts of several of the species. All of the species can be identified by the genitalia, although descriptions of additional features may make identifications more certain.
p. Every species of _ephemeroptera_ may be distinguished by its genitalia. There are two penes, sometimes fused into one; claspers, and large, curved, pseudo-penes called styli, whose purpose escapes me.
That these differ between species suggests vast Darwinian perspectives. It’s really extraoridinarily difficult to tell the difference between some similar species of tiny _ephemoperoptera_ unless you have a microscope. It’s much quicker, for another fly, to try to mate with them. Since different species tend to share the same environment, I think this bizarre genital elobrations must have evolved primarily as a means of species differentiation. They are the ephemeropteran equivalents of theology.
Whatever their motives, they have been astonishingly successful. Because the winged and sexually active stage of a mayfly’s life, as a spinner, lasts only a day, we think of them as fragile and short-lived insects. But they can live for a week in the preceding stage, as winged, asexual duns; and before then, some live underwater for two or three years as nymphs. This four-stage lifestyle is unique among insects. It is also very old. The Mayflies have been around for 500,000 years. They are “extremely fragile.”:http://www.fluefiske.biz/sider/Midtframesider/Entomologisider/Dognfluer.html Yet the brief wisps of life you sometimes see hatching are older than the river that they float on.