NO. 3 INSECT HEAD SNOUGRASS 55 



therefore, appears to be in an intermediate stage of development in 

 which the anterior and posterior elements are still independent of 

 each other. A two-branded fiber (q) extends downward in the head 

 from the middle of the posterior bar. A similar tentorial bar is 

 strongly developed in the crustacean, Gaimnarus (fig. 28B, PT). 



In all the pterygote insects the anterior and the posterior arms of 

 the tentorium are united with each other, and typically the lateral 

 elements are fused across the median line to form the central plate- 

 like body of the tentorium (figs. 21 F, 39 B, Tut). The median plate, 

 however, is not developed in all cases ; in the caterpillars the posterior 

 arms form only a slender bar through the back of the head, to which 

 the anterior arms are attached on each side (fig. 53 D, Tnt), and a 

 similar condition exists in adults of the higher Hymenoptera, where 

 the posterior bar appears as a slender yoke between the posterior ends 

 of the large anterior arms. In all insects of the orthopteroid branch 

 of the Pterygota, the roots of the anterior tentorial arms lie in the 

 fronto-clypeal suture (figs. 18 A, B, 36 A, B, 46 A, B, D, F, at, at). 

 So constantly do they have this position that they become diagnostic 

 marks of the suture, or of the fronto-clypeal line when a suture is 

 absent. In the Ephemerida (nymphs), however, the roots of the 

 broad anterior arms (fig. 21 F, at) lie at the edges of the inflected 

 ventral areas of the genae (Ge), before the bases of the mandibles. 

 Here, clearly, is a more primitive condition, dififering from that of 

 the Myriapoda and Apterygota only in that the bases of the hypo- 

 pharyngeal apodemes have moved outward from the hypopharynx 

 to the lateral walls of the cranium. In the Odonata the roots of the 

 anterior tentorial arms lie on the sides of the head, above the bases of 

 the mandibles, in the subgenal sutures. This is a second step toward 

 the orthopteroid condition, in which finally the tentorial roots have 

 migrated anteriorly into the fronto-clypeal suture on the facial aspect 

 of the head. 



The writer believes that the facts presented in the foregoing de- 

 scriptions solve the riddle of the insect tentorium, and explain the 

 seeming anomalies of the gnathal musculature, though he has not 

 shown the mode of union between the anterior and the posterior arms 

 in forming the characteristic tentorium of pterygote insects, and 

 though the method of the change in the connections of the anterior 

 arms from the base of the hypopharynx to the facial aspect of the 

 head may still be held as not exactly determined. The origin of the 

 anterior tentorial arms as apophyses of the sternal region of the 

 gnathal segments, however, shows that the adductor muscles of the 

 gnathal appendages, which arise on the tentorium in pterygote insects. 



