It comes from the Greek word ephemeros, and you know what it means.
What? Has everybody forgotten about the dainty little ephemeræ, that live but a day?
That is what ephemeros means, lasting but a day.
The stone flies have four wings, but they are not like those of the Odonata, or of the Ephemerida.
Do you remember how the hind wings are folded?
Yes, May, in plaits, so these are the plaited wings, or Ple-cop-te-ra, from pteran, a wing, and plecos, plaited.
The little silver fish, as you remember, has no wings at all, so its order is called Thy-sa-nu-ra, from its bristle tail, thysanos, in Greek, meaning a tassel, and oura, the tail.