The Comparative Morphology of the Genitalia of Insects.*

SOMITES.—The third and caudal region of those into which an insect is divided is called the abdomen. Here, for the most part, the segments of which it is composed show a simple and almost primitive structure. In the adult a typical segment, more properly designated a somite, consists of a dorsal chitinized portion or sclerite, called the tergum, and a ventral sclerite, or sternum. These are connected laterally by arthrodial membranes, conjunctiva or pleura, in each one of which is a spiracle. There are exceptions to this arrangement at both cephalic and caudal ends, with the greater complexity at the latter. No spiracles are ever present caudad of the eighth somite, and, in most cases, where the somites caudad of the seventh are retracted, spiracles are lacking in the eighth. Therefore it is evident that the adult abdomen always presents, to the eye at least, a far more simple composition than do the head and thorax. In reality, in many larvæ an abdominal somite is an entire ring without differentiation into tergite or sternite, an arrangement often seen in the lower thoracic somites, and a more nearly primitive one than that above mentioned.