A true insect, or member of the class Hexapoda, may be known by the grouping of its body-segments in three distinct regions--a head, a thorax and an abdomen--each of which consists of a definite number of segments. In the terminology proposed by E. R. Lankester the arrangement is "nomomeristic" and "nomotagmic." The head of an insect carries usually four pairs of conspicuous appendages--feelers, mandibles and two pairs of maxillae, so that the presence of four primitive somites is immediately evident. The compound eyes of insects resemble so closely the similar organs in Crustaceans that there can hardly be reasonable doubt of their homology, and the primitively appendicular nature of the eyes in the latter class suggests that in the Hexapoda also they represent the appendages of an anterior (protocerebral) segment. Behind the antennal (or deutocerebral) segment an "intercalary" or tritocerebral segment has been demonstrated by W. M. Wheeler (1893) and others in various insect embryos, while in the lowest insect order--the Aptera--a pair of minute jaws--the maxillulae--in close association with the tongue are present, as has been shown by H. J. Hansen (1893) and J. W. Folsom (1900). Distinct vestiges of the maxillulae exist also in the earwigs and booklice, according to G. Enderlein and C. Borner (1904), and they are very evident in larval may-flies. The number of limb-bearing somites in the insectan head is thus seen to be seven. All of these are to be regarded as primitively post-oral, but in the course of development the mouth moves back to the mandibular segment, so that the first three somites--ocular, antennal and intercalary--lie in front of it. In Lankester's terminology, therefore, the head of an insect is "triprosthomerous." The maxillae of the hinder pair become more or less fused together to form a "lower lip" or labium, and the segment of these appendages is, in some insects, only imperfectly united with the head-capsule.