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Does anyone know what this insect is? I put a rubber ball on my lawn, and within seconds a bunch of these insects (maybe 15 or 20) landed on it. I took a look around and saw that there were lots of them flying low near the grass or sitting on the grass (they were not as obvious on the lawn as on an orange ball :). Location: Massachusetts (USA).

Zoomed in Insect on a rubber ball

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    $\begingroup$ Could you provide an estimated size of your specimen? $\endgroup$ – theforestecologist May 9 '17 at 18:03
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Looks like a Bibio, sometimes referred to as "March flies" and one species in particular known as "St Mark's Fly". Yours seems unlikely to be that one because it is found in England and it seems like nowhere else.

Most likely I think yours is a Bibio vestitus from the picture below (unfortunately I wasn't able to find multiple sources corroborating this, though, so take it with some caution), and although I had trouble finding definitive distribution data, it looks like the one in your picture has the copper-colored legs, and in addition the first image below claims to be from Massachusetts. There are many many species of Bibio though so I can't be certain of species.

Bibio vestitus

Bibio vestitus Reference

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This is certainly some species of fly (order Diptera) and appears to be a species of march fly (Bibionidae)

enter image description here

Bibio anglicus

Bibionidae (march flies) characteristics [Source]:

  • Dark colored

  • Eyes usually meet along the top of the head

  • Front legs often have strong spines on the tibia

  • Often with dark spots or “stigma” on the wings.

  • Wing patterns (from Wikipedia):

    The wings have two basal cells (posterior basal wing cell and basal wing cell), but are without a discoidal wing cell. R4+5 is simple or branched; at most, only three branches of R developed. The leading edge wing veins are stronger than the weak veins of the trailing edge.

    enter image description here

The march flies are known to hover close to the ground like you saw [Source].

Although IDing to species is difficult with the low quality of the photo, I would think that Hardy et al 1958 would be a good start for you to use:

  • Hardy, D.E. et al., 1958. Guide of the insects of Connecticut PartVI. The Diptera or true flies of Connecticut Sixth Fascicle: March flies and gall midges. Bibionidae, Itonididae (Cecidomiidae). Conn. Geol. Nat. Hist. Surv. Bull. 87, 218 pp., 15 pl., 29 figs.

Your specimen is likely in the genus Bibio (march flies or St. Mark's flies).

You can tell it's not a robber fly (family Asilidae) because the eyes of your specimen come together at the top of the head (like most march fly species [Source]), while according to here

All robber flies have a characteristic divot on top of the head, which is located between their especially prominent compound eyes.

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