NO. 3 INSECT HEAD SNOIX^RASS 5E 



Besides bracing the walls of the cranium, the tentorium gives attach- 

 ment to muscles of the hypopharynx, of the mandibles (in some in- 

 sects), of the maxillae, of the labium, of the pharynx, and, when 

 dorsal arms are present, to muscles of the antennae. Such a com- 

 prehensive relation to the musculature of the head appendages, there- 

 fore, furnishes ample ground for the suspicion that the tentorium 

 includes in its composition more than is evident in its adult structure. 

 Janet (1899), after making a careful analysis of the muscles arising 

 upon the tentorium in the head of an ant, concluded that the tentorium 

 must be composed at least of three pairs of processes corresponding 

 with the antennal, the maxillary, and the labial segments. The antennal 

 processes, according to Janet's homolog}^ are the anterior arms, the 

 labial processes are the posterior arms ; the maxillary processes are 

 assumed to have lost their connection with the head wall, after their 

 inner ends had united with those of the other processes in the formation 

 of the central tentorial body. Janet's scheme, however, is not complete 

 without the assumption of mandibular elements in the tentorium, for, 

 in some of the lower insects, certain muscles of the mandibles are 

 attached upon the tentorium. Since these muscles were not then 

 known, Janet suggested that the homotypes of the mandibular ten- 

 torial processes are represented on the mandibular segment by the 

 points where the corpora allata have their origin in the hypodermis. 

 All the tentorial processes, both real and hypothetical, Janet regarded 

 as homologous with the f ureal invaginations of the thoracic sterna, 

 because the tentorium of the adult insect supports the adductor 

 muscles of the head appendages. This is sound reasoning, and the 

 conclusion probably comes as close to the truth in the matter as the 

 truth may be approached by induction from the facts presented by 

 the higher insects ; but a study of the Apterygota, the Myriapoda, and 

 the Crustacea throws an entirely new light on the origin and evolution 

 of the tentorium, and dispels the obscurity which has led to so many 

 theories concerning the nature of this head structure. 



The morphology of the tentorium, briefly summarized from facts 

 later to be described, is as follows : The anterior arms and the part of 

 the body of the tentorium on which the ventral adductor muscles of the 

 mandibles, the maxillae, and the labium have their origin are identical 

 with the hypopharyngeal apophyses of the Myriapoda and Apterygota, 

 and have their prototypes in the ventral apodemes of the gnathal seg- 

 ments in Crustacea. From their positions just laterad of the hypo- 

 pharynx, the bases of the apophyses have moved outward in the ventral 

 wall of the head before the leases of the mandibles to the lateral ventral 

 edges of the cranium, where thev come to lie in the subgenal sutures. 



