NO. 3 



INSECT HEAD SNODGRASS 



123 



the tentorial arms have moved forward to a position above the mandib- 

 ular articulations, and their external openings, the anterior ten- 

 torial pits, appear in these positions (fig. 46 B, at, at). 



In some of the more generalized insects, the epistomal ridge and 

 its suture are lacking, as in the roach, and there is then present only 

 a single fronto-clypeal sclerite (fig. 47 A, Fr-Clp). In such cases, 

 the tentorial pits (at) lie in the anterior extremities of the subgenal 

 sutures (sgs), above the anterior articulations (c) of the mandibles. 

 Where an epistomal ridge unites the subgenal ridges across the face, 

 separating the clypeus from the frons, the tentorial pits may retain 



LmMcls /A ^ Fr-Clp 



A 



Fig, 47.— Diagrams showing variations in the position of the epistomal 

 suture (es), and the relations of the frons and the clypeus. 



Aclp, anteclypeus: at, anterior tentorial pit; c, anterior articulation of 

 mandible ; CIp, clypeus ; es, epistomal suture ; Fr, frons ; //-, " adfrontal ; Fr-Clp. 

 fronto-clypeus ; fs. frontal suture ; h, line of secondary ridge across lower part 

 of clypeus ; Lin, labrum ; LmMcls, labral muscles, with origm always on frons ; 

 O, median ocellus. 



their positions above the mandibular articulations (fig. 46 A, B, at, 

 at) ; but more commonly they move into the epistomal suture (fig. 

 47 B). In any case, the tentorial pits identify the epistomal suture, 

 when this suture is present. The mandibular articulations (c, c) are 

 carried by the ventral margin of the epicranium and are not true land- 

 marks of the epistomal suture, as has been pointed out by Yuasa 

 (1920), and by Crampton (1925). 



As long as the epistomal suture maintains its direct course across 

 the face, no complications arise ; but the suture is frequently arched 

 upward, and this shift in the position of the suture extends the clyp- 

 eus into the facial region above the bases of the mandibles, and re- 

 duces the area of the frons (fig. 47 C). A modification of this kind 



ft 



