40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



intersegmental fold between the labial and the prothoracic segments, 

 the muscles in the neck are all muscles of the prothorax, and the neck 

 itself is prothoracic. It is evident that nuicli mori)hological significance 

 hinges on this problem. The neck sclerites, for example, in the first 

 case, might belong either to the labial segment or to the prothorax, 

 or to both ; in the second case, they could pertain only to the prothorax. 

 The relation of the posterior arms of the tentorium to the postoccipital 

 suture and ridge will be noted under the special description of the 

 tentorium (page 50). 



The labrum, the appendages of the gnathal segments, and the 

 hypopharynx constitute the mouth parts of insects. The gnathal 

 appendages are the mandibles, the first maxillae, and the second 

 maxillae, which last are united in insects to form a labium. The 

 morphology of these appendages will be discussed in a later section 

 (pages 79-90), but it is important here to understand their relations 

 to the cranial wall. The mandible in biting pterygote insects is typi- 

 cally suspended from the lower edge of the gena and postgena, and 

 swings outward and inward on a longitudinal axis between anterior 

 and posterior articulations with the head wall. The anterior articu- 

 lation is with a condyle at the contingent angles of the gena and 

 clypeus (fig. 18 A, B, D, c), the posterior with a shallow facet on the 

 lower margin of the postgena (B, D, a). 



The maxilla hangs from the lower edge of the postgena, upon 

 which it is freely movable by a single articular point just before the 

 lower end of the postoccipital suture (fig. 18B, D, g). The labium, 

 in generalized insects, is suspended from the neck membrane, but each 

 lateral angle of its transverse base is closely attached to the postoc- 

 cipital rim of the head (B, C, D, /). The positions of the maxillary 

 and labial articulations relative to the postoccipital suture (pos) are 

 in harmony with the idea that this suture is the intersegmental groove 

 between the maxillary and the labial segments. In some insects, the 

 labium is shifted forward between the ventral edges of the postgenae, 

 and thus becomes removed from its primitive position. In such cases, 

 as in caterpillars (fig. 53 A) and adult Hymenoptera (fig. 48 B, C), 

 the ventral angles of the postgenae may approach each other medially, 

 or even unite into a ventral bridge (hypostoma) behind the labium. 

 In other insects, in which the posterior part of the head is lengthened, 

 the base of the labium is elongated between the postgenae, forming the 

 ]>late known as the gula. These modifications, however, will be dis- 

 cussed more fully in section VI of this paper. 



The head is attached to the thorax by a cylindrical, membranous 

 neck, or cervix (fig. 18 B. Cv). In each lateral wall of the neck there 



