part of develops from posterior spiracle primordium embryonic/larval spiracle embryonic/larval posterior spiracle FBbt:00005604 fly_anatomy.ontology The main spiracle of the larva on the dorsal side of the eight (last) abdominal segment. During the first larval instar it is the only functional spiracle of the larva. A bilateral pair of these spiracles is housed in a bilateral pair of integumentary specializations - the stigmatophores. Each consists of a plate containing the spiracular openings. These connect to a spiracular chamber which in turn connects to the posterior end of the dorsal trunk. In the second instar there are three ovoid openings on the stigmatic plate and four alternating tufts of slightly branched hairs. In the third instar, the spiracles are somewhat larger and the hairs more markedly branched. larval posterior spiracle FBbt:00000424 stigmatophore