The spiracle disc of a terrestrial larva, at the posterior end of the abdomen.
These are somewhat simple in terrestrial larvae; in the aquatic species they get bizzare.
Full image: Olympus BHS:10x objecive,1.25x intermediate tube, 2.5x projection eyepiece.
Spiracle: as above but with 40x objective.
Crane Fly larva
Crane Fly larva
New Brunswick
Canada
Canada