Morphology of the Pleurosternal Region of the Pterothorax in Insects1

In the pterothorax of primitive insects the sternum was exposed. The precoxal suture was complete throughout the entire pleural wall, including the episternum and epimeron, and divided it into the outer anapleural and inner katapleural rings. The anapleural region was composed of the anepisternum, preepisternum, ventropleurite, and anepimeron; the katapleural area was composed of the katepisternum, trochantin, and katepimeron. From this probable primitive condition of the pleural wall the pleurosternal region of insects has undergone various modes of modification by fusion and loss of plates, accompanied by increasing sclerotization.