NO. 3 INSECT HEM) SNODGKASS 4I 



is typically a pair of lateral neck plates, or cervical sclerites, hinged 

 to each other. The first is articulated anteriorly to a small process, 

 the odontoidea (Yuasa, 1920), or the occipital condyle (Crampton, 

 1921), on the rear margin of the postoccipital rim of the head 

 (B, C, g) just above the base of the labium. The posterior neck plate 

 articulates with the anterior margin of the prothoracic episternum. 

 Other cervical sclerites of less constant form are sometimes present in 

 the ventral wall of the neck, and occasionally there are chitinizations 

 also in the dorsal wall. The lateral neck sclerites are important ele- 

 ments in the mechanism for moving the head on the thorax. Upon 

 them are inserted muscles from the postoccipital ridge of the head, 

 and from the inner surface of the prothoracic tergum (fig. 45 A, B). 

 The uncertainty of the morphology of the insect neck, and con- 

 sequently of the neck skeleton, furnishes a problem still to be solved. 

 As already pointed out, the status of the neck and of its sclerites 

 will depend upon that of the postoccipital rim of the head : if the 

 latter is an anterior remnant of the labial segment, the neck sclerites 

 may belong to the labial segment, or also to the prothorax ; if , however, 

 the postoccipital ridge of the head, upon which the anterior ends of the 

 dorsal prothoracic muscles are attached, is the infolding between the 

 head and the prothorax, then the neck can only be a part of the pro- 

 thorax. The second assumption looks improbable in view of the 

 position of the labial articulations in generalized insects (fig. 18 B/). 



THE LABRUM AND EPIPHARYNX 



The labrum is a characteristic feature of the arthropod head, and 

 probably corresponds with the tip of the annelid prostomium. In the 

 embryo (figs. 8 D, 13, 22 A, D, Lrii), it appears at an early stage as 

 a median ventral lobe of the prostomial region, lying just before the 

 point where the stomodeal invagination will be formed. In the mature 

 head the mouth opening (figs. 18 D, ig, Mth) is immediately behind 

 the base of the labrum (Lm), and the posterior, or epipharyngeal, 

 surface of the latter is continued directly into the dorsal wall of the 

 pharynx (fig. 19, Phy). The adult labrum takes on various forms 

 in different insects, but it is typically a broad flap freely suspended 

 from the lower edge of the clypeus (fig. 18 A, Lin). When movable, 

 the labrum is provided with muscles inserted on its base, having their 

 origin on the inner surface of the frons. Typically, there are two 

 pairs of these muscles, one pair (fig. 19, 2) inserted anteriorly on 

 the labral base, the other (j) posteriorly on the chitinous bars of the 

 inner face of the labrum known as the tormae (figs. 37 B, 42 A). 



