NO. I THE INSECT HEAD — SNODGRASS 33 



bridge through the back of the head. The anterior arms are variable 

 in their points of origin, but they project posteriorly through the 

 head and in pterygote insects unite with the posterior bridge. The 

 term tentorium, meaning a "tent," seems curiously inappropriate for 

 this structure, but in some of the lower insects the arms are united 

 in a central plate, which might suggest a canopy supported on four 

 stays. Though the tentorium varies much in form and degree of 

 development, the name has become fixed in entomological nomencla- 

 ture. The points of ingrowth of the four tentorial arms are marked 

 externally on the head by depressions known as the anterior and 

 posterior tentorial pits. Functionally the tentorium gives attachment 

 to the ventral muscles of the mouth parts, and, when strongly de- 

 veloped, probably serves to brace the lower edges of the cranial walls. 



To understand the origin and evolution of the insect tentorium 

 we must revert to the myriapods. A comparable structure is not 

 present in the entognathous hexapods — Protura, Collembola, and 

 Diplura. In the chilopods a pair of plates in the ventral head wall lies 

 before the mandibles between the lateral cranial margins and the 

 hypopharynx (fig. i6A, B, hF). These plates are the kommandi- 

 bulares Geriist of German writers, but since their relation to the 

 hypopharynx is more intimate than that with the mandibles, they may 

 be termed the hypopharyngeal fulturae. From each plate is given 

 off at the side of the hypopharynx an apodemal arm (Ap) that ex- 

 tends posteriorly within the head. In Scutigera (A) the inner ends of 

 the arms support a wide sheet of soft tissue {Lg) from which are 

 given off the ventral muscles (mcls) of the mouth parts. In Litho- 

 bius (B) the apodemal arms are connected merely by a membranous 

 bridge {Lg), and most of the muscles have been taken over by the 

 apodemes. In the diplopods premandibular ventral sclerites are 

 present, but the apodemes are less developed than in the chilopods. 

 In Symphyla (C) the supporting sclerites are absent; the long muscle- 

 bearing apodemes (Ap) arise at the base of the hypopharynx, and 

 have no connection with each other. 



When we turn now to the Thysanura it is seen that in the Machili- 

 dae (fig. i6D) two long apodemes (AT) arise ventrally mesad 

 of the mandibles and extend posteriorly and dorsally in the head. 

 In addition, however, a transverse bar (TB) forms a bridge through 

 the back of the head. Here, therefore, are the elements of the 

 pterygote tentorium, and there can be little doubt that the anterior 

 arms (AT) are homologues of the ventral head apodemes of the 

 chilopods and symphylans. In the Lepismatidae the structure be- 

 comes more elaborate by the union of the anterior arms in a broad 



