Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



serted on lateral wall of crop just above posterior lateral stomodeal 

 ganglion {PLGiig), the others on tii)s of the gastric caeca (GC) of 

 same side. 



THE MECHANISM FOR MOVING THE HEAD 



The head of the grasshopper is freely attached to the prothorax by 

 a membranous neck, but its movements are somewhat limited by the 

 overlapping anterior edges of the protergum, and by the pair of 

 cervical sclerites on each side (fig. 45 B, icv, 2cv) which link the head 

 with the concealed episternal plate of the prothorax (Eps^). 



The cervical sclerites, however, constitute an important part of the 

 mechanism for moving the head The two plates of each pair are 

 articulated end to end, and ordinarily they are bent downward at an 

 angle to each other (fig. 36 A, cv). The first is articulated anteriorly 

 to the posterior margin of the postoccipital rim of the head (fig. 45, 

 g), the second posteriorly to the anterior edge of the prothoracic 

 episternum (EpSi). The neck plates thus constitute a fulcrum on each 

 side between the head and the thorax, giving a leverage to the dorsal 

 and ventral muscles extending from the postoccipital ridge and ten- 

 torium to the prothorax and the first thoracic phragma. Moreover, 

 upon each plate are inserted strong levator muscles (fig. 45 B) arising 

 on the back of the head and on the prothoracic tergum, and the con- 

 traction of these muscles, with the consequent straightening of the 

 angle between the two plates of each pair, must cause the protraction 

 of the head. From each anterior plate a horizontal muscle extends to 

 the prosternal apophysis of the opposite side (fig. 45 A, B, 5^). Be- 

 sides the muscles that connect the skeletal parts of the head, neck, 

 and prothorax, there are two muscles on each side inserted directly 

 upon the neck membrane (A, 5<5, 57). 



It is difficult to give names signifying function to the neck muscles, 

 for it is evident that the function will depend on whether the two 

 muscles of any pair act in unison, or as antagonists. The neck muscles 

 of Dissosteira are as follows, on each side : 



4/. — First protergal muscle of the head (fig. 45 A). — A slender 

 muscle arising dorsally on prothoracic tergum ; inserted dorso-later- 

 ally on postoccipital ridge of head (PoR). 



48. — Second protergal muscle of the head (fig. 45 A). — A larger 

 muscle arising on principal ridge of protergum (e) ; inserted with 4/ 

 on postoccipital ridge of head. 



4p. — Longitudinal dorsal muscle of the prothorax (fig. 45 A). — • 

 Extends from first thoracic ])hragma ( rPh) to postoccipital ridge 

 (PoR) just l^elow 48. 



