needs hordes of Beastmen!
About the same time that I picked up that pack of plastic flies, I also picked up a pack of rubbery cockroaches.
I bought these bugs online, and I don't remember where they came from. They are rubbery, bendy, and somewhat translucent.
To prep the bugs, I used a sharp hobby knife to remove the thin, film-like plastic around some of the edges, legs and antennae. Then I used a permanent marker to darken the legs and antennae (to get rid of the translucency), as well as to darken the front of the body and those parts of the lower body that would be visible. So basically, I darkened everything except the wings!
I then superglued the cockroaches to previously prepared and flocked bases. I'm using Mighty Armies large bases (2" x 2") from Rebel Minis.
Since I mounted the flies on white-edged bases, I should have put the cockroaches on white-edged bases as well. So much for consistency! 
(Maybe they'll be in different armies…)
I sealed the bases with a clear matte spray before I added the cockroaches, but I did not seal the cockroaches as I liked the gloss look of the marker and didn't want to mess up the translucent wings. The only problem is that the natural rubber of the bugs seems to attract dust, which is resistant to removal by canned air or soft brush. (A damp paper towel would probably do the trick.)
So there you go – an inexpensive and somewhat repulsive element for your 15mm fantasy gaming! I thought three bases would be enough for my 15mm fantasy needs, so the rest of the cockroaches will be used elsewhere…