THE INSECT HEAD SNODGRASS 457 



value. This suture is a groove lying very close to the posterior rim 

 of the cranium (fig. 8, pos)^ where it forms an inflection, or internal 

 ridge, around the dorsal and lateral aspects of the foramen magnum. 

 Since this suture lies behind the region of the head commonly called 

 the occiput {Oc), we may term it the post occipital suture (pos). 

 The narrow marginal rim of the cranium {Poc) set off at the base 

 of the neck (fig. 12 A, Cv) by the postoccipital suture may be desig- 

 nated, therefore, the postocciput {Poc). The postocciput is well 

 developed in the cricket (fig. 9 B, Poc)^ and its internal inflection, 

 the postoccipital ridge (PoR), is particularly large. 



One reason for regarding the postoccipital suture as an inter- 

 segmental groove is the fact that the dorsal head muscles from the 

 thorax are always attached on its internal ridge, just as the other 

 longitudinal muscles of the body are attached on intersegmental 

 ridges of the thoracic and abdominal skeletal plates. We might, 

 therefore, regard the postoccipital suture as the true separation 

 between the head and the prothorax, the neck being thus included 

 in the latter; but evidence derived from embryonic development 

 (Riley, 1904, Eastham, 1930) suggests, rather, that the postoccipital 

 suture is the line of separation between the two maxillary segments 

 of the head. The usual attachment of the maxillae on the head 

 (figs. 8, a'', 12 A, Mx) before the lower ends of the suture, and that 

 of the labium, behind the suture (figs. 8, a"\ 12 A, Lh) is in accord 

 with this theory. If the second view is correct, then the postocciput 

 of the cranium is a sclerotic remnant of the dorsal arch of the labial 

 segment. Dorsal muscles of the labium, in this case, should arise on 

 the postocciput, but since the labium ordinarily has no dorsal muscles, 

 a crucial point in the evidence is missing. The region of the neck 

 may be supposed to include a posterior membranous part of the 

 labial segment, and the anterior part of the first thoracic segment. 



We have already observed that in the Crustacea the dorsal plates 

 of the two maxillary segments are always intimately fused, as in 

 Euhirmchipus and Anaspides (figs. 2 A, 16, V + VI), in which 

 respect the crustaceans appear to differ from the insects. In both 

 of these crustacean forms, however, the mandibular segment is 

 separated from the maxillary segment by a distinct suture (y), 

 which suture is possibly represented in the thysanuran insect Machilis 

 (fig. 2 B) by the suture on the back part of the head (?/) that ends 

 ventrally between the bases of the mandibles and the first maxillae. 



The sutures of the head. — Aside from the postoccipital suture, 

 the surface of the cranium is usually marked by other impressed 

 lines, collectively termed "sutures," which are characteristic fea- 

 tures of the head, though they all appear to be of secondary forma- 

 tion. The so-called sutures of this type have no significance in 



