It thus appears that the procephalic lobes of the embryo of insects, with the rudiments of the antennæ, constitute the primitive head, and perhaps correspond to the annelidan head, while gradually the antennal appendages were in the phylogenetic development of the class fused with the two segments of the primary head. That the second maxillary segment, the occiput, was the last to be added, and at first somewhat corresponded in position to the poison-fangs of centipedes (Chilopods), is shown by our observations on the embryology of Æschna (Fig. 36). [Illustration: FIG. 36.—Æschna nearly ready to hatch: 4, labium, between _T_ and _e_ the occipital tergite; 5–7, legs. ] [Illustration: FIG. 37.—Head of embryo Nematus, showing the labial segment: _occ_, forming the occiput; _cl_, clypeus; _lb_, labrum; _md_, mandible; _mdm_, muscle of same; _mx_, maxilla; _mx′_, second maxilla (labium); _oe_, œsophagus. ] The mandibular segment appears to form a large part of the post-antennal region of the epicranium on account of the great mandibular muscle which arises from so large an area of the anterior region of the head (Fig. 37). Judging from the embryo of Nematus (Fig. 37), the first maxillary segment is tergally aborted, there being no tergo-pleural portion left.[13] The second maxillary segment tergally appears to be represented by the occipital region of the head. All the gular region, including the submentum and mentum, probably represents the base of the labium or second maxillæ.[14] The so-called “occiput” forms the base of the head of Corydalus, a neuropterous insect, which, however, is more distinct in the larva. In most other adult insects the occiput is either obsolete or fused with the hinder part of the epicranium. We have traced the history of this piece (sclerite) in the embryo of Æschna, a dragon-fly, and have found that it represents the tergal portion of the sixth or labial segment. In our memoir on the development of this dragon-fly, Pl. 2, Fig. 9, the head of the embryo is seen to be divided into two regions, the anterior, formed of the antennal, mandibular, and first maxillary segments, and the posterior, formed of the sixth or labial segment. This postoral segment at first appears to be one of the thoracic segments, but is afterwards added to the head, though not until after birth, as it is still separate in the freshly hatched nymph (Fig. 4; see also Kolbe, p. 132, Fig. 59, _sq._ 5). A. Brandt’s figure of _Calopteryx virgo_ (Pl. 2, Fig. 19) represents an embryo of a stage similar to ours, in which the postoral or sixth (labial) segment is quite separate from the rest of the head. The accompanying figure, copied from our memoir, also shows in a saw-fly larva (_Nematus ventricosus_) the relations of the labial or sixth segment to the rest of the head. The suture between the labial segment and the preoral part of the head disappears in adult life. From this