Astute observation.
Out there just now, I poured a bit of beer around 3 of the holes where I've seen the critters today, that are within about 12" of each other
The flies descended, and were all over the beer dirt...
a few even went down into the holes briefly...but were not attacked...apparently, the critters
must be in position, at the top of the burrow, to attack...none were situated as such then, because the dogs were running around, keeping them hidden in their burrows...AND, they must be able to shoot back down into the burrow in a split-second, so body length may play a role in just how far away they can lurch...is my guess
I'll keep an eye on them....in the meantime, I've got some WC's to do...hope all are having a good weekend
Cheers