THE INSECT HEAD SNODGRASS 463 



With most of the more generalized insects the frontal aspect of 

 the head is directed forward, and the mouth appendages hang 

 downward from the subgenal margin. A head having this position 

 (fig. 12 A) is said to be of the hypognathous type. The hypognath- 

 ous type of head undoubtedly preserves the primitive relation of 

 the cranium with the body, because the mouth appendages are modi- 

 fied legs, and in the pendent position they correspond with the legs, 

 and retain the embryonic position of the primitive appendages. 



There are many insects, however, in which the frontal aspect of 

 the head is turned upward, and the mouth appendages are directed 

 forward. When the cranium has this relation to the body (fig. 12 

 B), the head is of the prognathous type. The prognathous position 

 of the head is unquestionably a secondary one, as is shown in the 

 structure of the cranium. The back of the head usually maintains 

 the primitive relation with the neck (B, Cv).^ but the forward 

 position of the jaws has involved a lengthening of the ventral head 

 wall and the basal region of the labium (Smt). In many prog- 

 nathous insects, particularly in Coleoptera, the posterior tentorial 

 pits (pt) have been drawn forward on the ventral head wall, and 

 the lower ends of the postoccipital suture (pos), which terminate 

 in the pits, have been correspondingly lengthened by a forward 

 extension on the ventral side of the cranium. The suture continued 

 anteriorly from each tentorial pit is the subgenal suture {sgs), which 

 ends at the anterior tentorial pit (at) in the usual manner. 



The position or structure of all the mouth appendages is more or 

 less affected by the transformation from the hypognathous to a 

 prognathous condition. The hinge line of the mandible (fig. 12 B, 

 Md) comes to approach a vertical position. The maxillae are car- 

 ried forward on the ventral side of the head, since they retain the 

 normal articulation with the hypostomal margins of the head im- 

 mediately behind the mandibles. It is the labium that is most 

 affected by the change. Its basal region becomes greatly elongate 

 between the ventral extensions of the postoccipital suture and the 

 posterior, or hypostomal, parts of the subgenal sutures, and appears 

 to be a plate of the ventral wall of the head. The part of the labium 

 posterior to the tentorial pits {pt) is now called the gula {Gu). 



A concrete example of the structure of a prognathous head in the 

 Coleoptera is well shown by the head of a blister beetle (fig. 13). 

 The postoccipital rim of the cranium is here almost obliterated, 

 except laterally where it bears the large occipital condyles {occ). 

 The ventral parts of the postoccipital sutures (pos), however, are 

 extended forward on the ventral side of the head to the posterior 

 tentorial pits {pt, pt), and they separate the enlarged gular area 

 of the labium {Gu) from the postgenal regions of the cranial wall. 



