478 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 J 



the opposite jaws thus pull against each other, the whole structure 

 forming a zygomatic mandibular adductor. Those of the other 

 group {KLt) arise on a pair of sternal processes {HA)^ wliich are 

 clearly the prototypes of the anterior tentorial arms of pterygote 

 insects. The ventral muscles, considered as; a single functional 

 group of fibers, evidently represent the ventral promotors and re- 

 motors of a generalized limb (fig. 14, /f, Z), and hence are col- 

 lectively designated KL^ as in Figures 18, 19, and 20. 



In the pterygote insects, and in some of the Apterygota, the man- 

 dibles have a quite different type of mechanism from that character- 

 istic of an apterygote mandible. Each jaw has a broad base, and, 

 instead of a single dorsal point of articulation, it has a long hinge 



-KLt 



KLz 



FiGUKE 20. — Diagrams of typical apterygote (A) and pterygote (B) 



mandibles 



a', primary articulation with cranium ; a'—c, secondary longitudinal axis of 

 movement on cranium ; HA, hypopharyngeal apophyses (anterior tentorial 

 arms) ; /, dorsal promoter; J, dorsal remoter; KLt, tentorial adductors; 

 KLz, zygomatic adductor ; z, ligament of zygomatic muscle. 



line on the lower lateral margin of the cranium (fig. 12 A, Md) 

 between strong anterior and posteinor articulations with the latter 

 (fig.8,c,a'). The posterior articulation (fig.20B,«') represents the 

 primary dorsal articulation (A a'). The anterior articulation (B, c) 

 is a secondary one ; its acquisition limits the movement of the appen- 

 dage to that of a hinge with a longitudinal axis {c-a^). By this 

 change in the articulation of the mandible, the muscles assume al- 

 tered functions. The primitive dorsal promotor (A, /) becomes 

 a dorsal adductor (B, /), and the primitive remoter (A, /) 

 becomes a dorsal adductor (B, /). The ventral adductor muscles 

 are either greatly reduced or are entirely obliterated in the Ptery- 

 gota. Remnants of them (B, KL) persist, however, in some of the 

 more generalized pterygote insects, as in the mandibles of the cricket 

 and some other Orthoptera, where the ventral adductors are repre- 



