Flies in the sand

Sometimes, as one walks along the beach,
a swarm of insects will fly out of an old crab hole.
They look like house flies, but are slightly smaller and lighter in color.
We've never been able to get a good photo of them or an ID.

We bought a new book yesterday with a beach insect page.
There they were! STABLE FLIES (or dog-flies).

Photo courtesy of the University of Nebraska Ext. Lab.