THE POSTANTENNAL REGION OF THE INSECT EMBRYO

Abstract

The postanteunal region of the insect embryo is generally interpreted as a postoral metamere because of the presence of coelomic sacs, minute appendages, and a presumed neuromere, the tritocerebral ganglion. These structures, however, are not necessarily evidence of the presence of a metamere because they may occur in the prostomium of the Polychaeta. Insects and crustaceans are presumed to have originated from a common ancestor; therefore this region is usually interpreted as the homologue of the second antennae of the crustaceans. More probably, however, insects arose from a proto-onychophoran stock and therefore had no second antennae in their ancestry. The prostomium of the Polychaeta, which lie near the ancestry of the arthropods, bears two or more pairs of appendages, one of which probably persists as the antennae of insects and are associated with coelomic sacs and a ganglion. It is suggested that the postantennal appendages represent the transient vestiges of a second pair. If the tritocerebrum is not the neuromere of a second antennal segment it is probably the homologue of the prostomial ganglion of the Oligochaeta. It unites with the protocerebrum and deutocerebrum, ganglionic concentrations developed in association with the eyes and antennae, to form the definitive brain.