Flesh Flies, stuck on a glueboard, were attracted to the malt-flavored cat furball medicine placed on the glueboard. The glue board was in a corner, under the house, so the flies could only approach from two sides. The house had a rodent problem. It appears a rodent died, and was colonized by flesh flies. The newly emerged flies were then attracted to the malt-flavored cat furball medicine, which we sometimes use as an attractant in traps. Supporting the conclusion that the flies walked to the glueboard is that two of the flies have wings that had not fully developed, one of which is pictured below. It appears they walked to the glueboard and got stuck. Specimen collected by Jose Rodriquez, July 2015. Arnold Ca. (Calaveras County).
What is especially interesting is that the flesh flies were so strongly attracted by the cat furball medicine.
Click on an image to enlarge it.