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That the head of insects is composed of four segments was shown on embryological grounds by the writer (1871) and afterwards by Graber (1879). The antennæ and mouth-parts are outgrowths budding out from the four primitive segments of the head; the antennæ grow out from the under side of the procephalic lobes, and these should therefore receive the name of antennal lobes. In like manner the mandibles and first and second maxillæ arise respectively from the three succeeding segments.

Fig. 34.—Embryo of Anurida maritima: tc. ap, minute temporary appendage of the tritocerebral segment, the premandibular appendage; at, antenna; md, mandible; mx¹, first maxilla; mx², second maxilla; p¹–p³, thoracic; ap¹, ap², abdominal appendages; an, anus—After Wheeler.

While the postoral segments and their appendages are readily seen to be four in number, the question arises as to whether the eyes represent the appendages of one or more preoral segments. In this case embryology thus far has not afforded clear, indubitable evidence. We are therefore obliged to rely on the number of neuromeres, or primitive ganglia. In the postoral region of the head, as also in the trunk, a pair of neuromeres correspond to each segment. (See also under Nervous System, and under Embryology.) We therefore turn to the primitive number of neuromeres constituting the procephalic lobes or brain.