March fly

These are march flies, Plecia pealei, from the famous Green River fish beds of SW Wyoming. They were found near Kemmerer, and date to around 50-52 million years old. Insects don't tend to preserve well as fossils, being delicate creatures, but the detail on these flies is pretty good.

The size of the slab varies, but is roughly 6cm square, and the insects themselves perhaps 1cm long.