NO. 3 INSECT HEAD SNODGRASS 39 



(1928a) has called the mandibulo-maxillary suture the " archice- 

 phalic " suture, since he calls the region before it the " archicephalon," 

 but the term thus applied denotes too much antiquity for a stage that 

 is clearly subsequent to several others in the head evolution. A simi- 

 larly-placed suture is present in the head of Japyx (fig. 30 B, PcR), 

 but the relation of the suture here to the bases of the head appendages 

 can not be determined. The occipital suture of the pterygote insect 

 head, ending laterally before the posterior mandibular articulations, 

 therefore, is probably not the mandibulo-maxillary suture of the 

 simpler crustaceans, or the homologue of the posterior suture in the 

 head of Machilis. 



The postoccipital suture (fig. i8B,C, pos) is a most important 

 landmark of the head because it is invariably present, and because of 

 its constant anatomical relations to other parts. The posterior tentorial 

 pits (pt) are always located in its lower ends, and if the pits migrate 

 in position, as in some of the Coleoptera and other insects, the lower 

 ends of the suture are correspondingly lengthened (fig. 49 C, pt, pt) . 

 Frequently the suture is inconspicuous by reason of its closeness to 

 the margin of the cranium, and for this reason, probably, it has not 

 been given sufficient attention by entomologists. Comstock and Kochi 

 (1902) believed that the suture is the groove between the pleurites 

 of the maxillary segment; but Riley (1904) claimed, from a study of 

 the developing head of Blatta, that the suture is the intersegmental 

 groove between the maxillary and the labial segments, and that the 

 postoccipital sclerite is a remnant of the wall of the labial segment, 

 which segment is otherwise obliterated or represented in the anterior 

 part of the neck membrane. This view is at least in harmony with 

 certain anatomical relations in the adult head, and is tentatively 

 adopted in this paper. 



Internally, the postoccipital suture forms a postoccipital ridge (fig. 

 39 A, PoR) just within the foramen magnum, and upon this ridge are 

 attached the anterior ends of the dorsal muscles of the prothorax 

 (figs. 45 A, 57A, B, C). The ridge, therefore, must be a primary 

 intersegmental fold corresponding with the ridges or phragmata sup- 

 porting the longitudinal muscles in the thorax and abdomen. If it 

 does not represent the fold between the maxillary and labial segments, 

 it should be that between the labial segment and the prothorax. If 

 the first possibility is true, as claimed by Riley, there is an interseg- 

 mental line lost somewhere in the neck, and the muscles going from 

 the first phragma of the thorax to the postoccipital ridge of the head 

 must be regarded as extending through the region of two primary 

 segments. If, on the other hand, the posterior ridge of the head is the 



