I had never heard of a March Fly until I found hundreds of larvae in composted leaves.
It was the last weekend in March, and I happened to grab a handful of composted leaves and immediately noticed hundreds of little "worms" under the layer I removed. It was impressive how many were, I would say 500 at least. My curiosity lead me to take a couple to the house and clean them up and take some pictures so I could see the details and try to identify them.
After searching the web, I decided they are of the family Bibionidae or March Flies and their larvae are known to aggregate together before emerging . When they emerge in late April, it is often in large numbers at the same time. Descriptions say they are oblivious to humans and do not fly away when swatted. They do not cause harm and some species are pollinators of flowering plants. In other areas of the country, they are known to fly together in such numbers that they clog the radiators of cars on the highways. Their appearance is described as a small dark fly with large eyes. I hope to see some adults this spring because I don't know exactly what they look like. They tend to crawl about on east facing walls and mate and then the females lay eggs in the soil and the life cycle is complete.
--Ross Williams