NO. I 



THE INSECT HEAD — SNODGRASS 



35 



From this primitive condition of the tentorium in Thysanura it is 

 only a step to that in the Pterygota in which the anterior arms have 

 become united with the posterior bridge (fig. 17 A). In an ephemerid 

 larva (fig. 16 E) the anterior arms still arise on the ventral surface 

 of the head, but in a lateral position from pits (at) just mesad of 

 the mandibles, which lie in lateral fossae of the head wall (mdFs) 

 but have no articulation on the clypeus. In Odonata, Plecoptera, 

 Dermaptera, and some Orthoptera, however, the roots of the anterior 



Fig. 17. — Variations of the pterygote tentorium, diagrammatic. 



at, anterior tentorial pit; AT, anterior tentorial arm; Cvx, neck (cervix); 

 CT, corpotentorium ; DT, dorsal tentorial arm; Poc, postocciput; pt, posterior 

 tentorial pit; PT, posterior tentorial arm; TB, tentorial bridge (united poste- 

 rior arms). 



arms (F, AT) are in the subgenal sulci laterad of the mandibles 

 (G, sgs). Evidently the arms have been transposed to this position 

 before the mandibles acquired their anterior articulations (c) on 

 the clypeus. Finally in most higher insects these arms have taken a 

 facial position on the head by migration into the epistomal sulcus, in 

 which their pits are usually located (fig. 6 A, at). It may seem sur- 

 prising that fixed structures should migrate in this apparent manner 

 from a ventral to a lateral and finally to a facial position. It is 

 hardly to be supposed, however, that the anterior tentorial arms, 

 carrying always the same muscles, have been independently rede- 

 veloped in each of their several positions. 



Once established as a unified composite structure (fig. 17 B), the 

 pterygote tentorium undergoes numerous variations. In the orthop- 



