Ah, OK. Still, it needn't be a problem - just use a specimen jar to trap a male, and then gently turn the pot over and photograph the underside. See my related thread:
[ID] Insects, please ()!, and look at the last species - the Trichocerid, where I photographed a male's genetalia ventrally.
My male ended up being
annulata (
typcal 
), the only one of our 11-or-so
Winter Gnat species that can be ID'd dorsally from good photos (
where the species has a banded abdomen, clear when the wings are held apart which is infrequent). Still, I suppose my photo could have done with being closer, but with your set-up I should think you could as long as you can focus through the plastic sufficiently.
Let us know if you get one!