THE INSECT HEAD SNODGRASS 459 



which when fully developed extends forward from the posterior 

 tentorial pit (pt) in the lower end of the postoccipital suture to a 

 point just above the anterior articulation of the mandible (c). The 

 part of the subgenal suture between the two mandibular articula- 

 tions {c, a') is sometimes distinguished as the pleurostomal suture 

 (ps), and the part behind the mandible as the hypostomal suture 

 {hs). Very commonly the anterior ends of the subgenal sutures are 

 connected across the face by an epistomal suture {es). The subgenal 

 sutures are well developed in both the cricket and the cockroach 

 (figs. 9, 10), but in the roach the epistomal suture is absent. 



The anterior tentorial pits of pterygote insects (fig. % at) are 

 always located somewhere in the pleurostomal or epistomal sutures, 

 but their position in the sutures is subject to much variation in dif- 

 ferent insects. In the cricket (fig. 9 A) each " pit " {at) is a long 

 slit occupying almost the entire length of the pleurostomal suture 

 and extending a considerable distance into the epistomal suture. 

 More commonly the pits lie entirely within the epistomal suture, 

 and are often carried upward on the face with the dorsal arching of 

 the suture common in many insects. 



Extending across the back of the cranium there is in some insects 

 an occipital suture (fig. 8, ocs), which may reach downward on the 

 lateral head walls to the subgenal sutures. An occipital suture is 

 well developed in most Orthoptera, as in the cricket (fig. 9 B, ocs)^ 

 along the line w^here the dorsal and lateral walls of the cranium are 

 inflected into the posterior wall. The postoccipital suture (Pos) 

 has already been described. It is always present, but if the post- 

 occiput (Poc) is absent, the postoccipital suture becomes merely a 

 groove marking the line of attachment of the neck membrane to the 

 posterior rim of the head. 



Still other sutures frequently occur in the head wall, but they are 

 less constant than those described above. Often an ocular suture 

 (fig. 8, os) surrounds the compound eye; and generally the antennal 

 socket is encircled by an antennal suture (a^), the internal ridge of 

 which strengthens the rim of the socket. In the cricket a suhocular 

 suture (fig. 9, A, sos) extends from the compound eye to the subgenal 

 suture, and in the roach a subantennal suture (fig. 10, sas) extends 

 from the antennal socket to the subgenal suture. These sutures are 

 sometimes called " fronto-genal " sutures, but it is doubtfid if the 

 part of the head wall immediately before them belongs to the area 

 of the true frons (fig. 8, Fr). 



The areas of the head. — The head areas are merely the spaces be- 

 tween the head sutures. They are often called " sclerites," but they 

 must not be thought of as plates united along the sutures ; they are 



