52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



Then, proceeding forward, they have migrated to the fronto-clypeal 

 suture on the facial aspect of the head. The primitive condition is 

 found in Chilopoda, Diplopoda, and Apterygota; intermediate con- 

 ditions occur in the Ephemerida and Odonata ; the final condition is 

 characteristic of all Pterygota, except the Ephemerida and Odonata. 

 The posterior tentorial arms are invaginations in the lower ends of 

 the postoccipital suture of the cranium, which is probably the inter- 

 segmental groove between the first and second maxillary segments. 

 These arms are absent in the Myriapoda and most Apterygota ; they 

 are present in Machilis and in some Crustacea, where their inner ends 

 are united to form a transverse bar through the back of the head ; 

 they are present in all Pterygota, where the anterior arms are united 

 with them to form the typical four-branched tentorium. The dorsal 

 tentorial arms are processes of the anterior arms and may secondarily 

 become attached to the dorsal or facial wall of the cranium. 



The muscles of the tentorium, with the exception of the antennal 

 muscles usually arising on the dorsal arms in pterygote insects, are 

 all muscles that primitively have their origin on the sterna of the 

 gnathal segments. They include two sets of median longitudinal 

 ventral muscles, one set going anteriorly to the hypopharynx, and 

 the other posteriorly to the sternum or sternal processes of the pro- 

 thorax ; they include also the transverse ventral adductors of the 

 mandibles, the first maxillae, and the second maxillae, and the ventral 

 dilators of the phaiynx. In the Chilopoda and Apterygota, all these 

 muscles arise from the hypopharyngeal apodemes, except some of the 

 mandibular muscles which may become detached from the apophyses, 

 or retain a direct connection with the base of the hypopharynx. The 

 hypopharyngeal apodemes are, therefore, paired apophyses of the 

 region of the gnathal sterna. There is no evidence that they are com- 

 posite structures ; each appears to be a single process invaginated 

 from a chitinous remnant of the mandibular sternum (the suspensorial 

 plate of the hypopharynx), but since it bears the sternal muscles of 

 the three gnathal appendages, either the bases of these muscles have 

 migrated forward, or each apophysis is a process of the three united 

 sterna. When the two apophyses move to the positions on the front 

 wall of the head characteristic of the orthopteroid branch of the 

 Pterygota, they retain the muscle attachments, and when they unite 

 with the posterior arms to form the typical tentorium, the head pre- 

 sents the aspect of having none of the ordinary sternal muscles of the 

 appendages attached on its sternal region, except for the small mandib- 

 ular adductors present in some of the lower Pterygota that have 

 retained their origin directly on the base of the hypopharynx. 



