Creepy fly photos [UPDATED]

Fair warning, though my intent is not to be creepy. Rather I'm trying to identify some flies that have been all over the Charles River Peninsula this week. (By all over, I mean close-your-mouth-and-eyes all over).

As you can see, not terribly distinctive. Maybe an inch long. When I peered down to investigate, I found that each had its own blade of grass to perch on. When I'd walk through the grass some, though not all, would erupt into the air.

Here was my surprise. Mingled among the fly-folk were insects with more of a beetle shape.

And, yes, they appeared to be mating, end-to-end, with the fly-folk.

I know sexual dimorphism is true among many species of insect, but this seems pretty extreme. I've gone through many online insect identification keys, but I haven't come across the answer yet. [UPDATE: March Fly]

Oh, also, as long as I've got my junior naturalist cap on--take a look at this huge egg I found trail-side. I do believe there will be one less wild turkey chick this spring. I wonder who the nest robber was.