50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8 1 



tentorium must have some relationship with the hypopharyngeal 

 apophyses of the Apterygota and the Chilopoda, and with the sternal 

 apodemes of the gnathal segments in the Crustacea. The nature of 

 this relationship will be shown following the anatomical description of 

 the tentorium. 



THE TENTORIUM 



The tentorium of orthopteroid insects is a horizontal, X-shaped 

 brace between the lower edges of the cranial walls (fig. 39 B, Tut). 

 It consists of a central body with a pair of divergent anterior arms 

 (AT) and a pair of divergent posterior arms (PT) . The arms are 

 hollow invaginations of the head wall. The roots of the anterior arms 

 appear as external pits, in most insects lying just before the anterior 

 articulations of the mandibles (fig. 18 A, B, at) in the epistomal suture, 

 when the latter is present ; the roots of the posterior arms form de- 

 pressions in the lower ends of the postoccipital suture (B, C, /^O- 

 Usually there is a pair of internal processes, or dorsal arms of the 

 tentorium (fig. 39 A, C, DT), arising centrally at the junction of the 

 anterior arms with the body, and extending dorsally and anteriorly 

 to the facial wall of the head near the bases of the antennae. Some- 

 times these arms are fused with the cuticula of the cranial wall, but 

 generally they are attached only to the hypodermis, and often their 

 outer ends are weak and tendinous. Riley (1904) says that the dorsal 

 arms of the tentorium of Blatta arise in the embryo as processes from 

 the inner ends of the anterior arms. The tentorium undergoes many 

 modifications of form in different insects, according as certain parts 

 become more highly developed and others reduced, but its typical 

 structure is seldom obscured. 



In its typical form, the tentorium is a simple " tent," as its name 

 implies, composed of the central plate, or body, suspended by the four 

 stays, or arms, from the four ventral angles of the head. Yet, mor- 

 phologists have always been suspicious of accepting the tentorial 

 structure at its apparent face value. Some writers would homologize 

 the arms with the apophyses of the thoracic pleura, others with the 

 apophyses of the thoracic sterna. Either disposition suggests, then, 

 that there should be a pair of such processes for each of the head 

 segments. Wheeler (1889) thought that he found in the embryo of 

 Lcptinotarsa (Doryphora) five pairs of tentorial invaginations, repre- 

 senting each head segment but the last. Other investigators have not 

 verified this, and most students of the development of the insect head 

 report the presence of only the two pairs of invaginations that form 

 the anterior and the posterior arms of the definitive structure. 



