14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 1 23 



Whedon (1927) and by Munscheid (1933) ; when the prementum is 

 fully extended it is greatly stretched, as shown by these authors. The 

 Other elbow muscle is a small, fan-shaped group of fibers (/) arising 

 in the end of the postmentum laterad of the lever arm of the pre- 

 mentum and inserted on the edge of a dorsal lobe (F, G, H, c?) of 

 the lever. From the lobe on which this muscle is attached a strongly 

 plicated fold (G, H, e) of the inflected articular membrane of the 

 elbow extends around the anterior side of the joint to the correspond- 

 ing lever lobe of the opposite side (H). From the middle of the fold 

 a tubular membranous tongue (/) extends proximally between the 

 ends of the great adductor muscles of the palpi, and gives attachment 

 to a pair of thick, divergent, fibrous ligaments (g) attached on two 

 small apodemal knobs of the basal margin of the prementum close to 

 the lever arms. By this structure the palpal adductors are girdled and 

 strongly held in place where they curve upward from the prementum 

 to their origins in the postmentum (G, 4/'). The muscles (I) attached 

 on the lever lobes (d) are the "secondary extensors" of Whedon, the 

 "tertiary flexors" of Munscheid, but their function would appear to 

 be that of maintaining a tension on the folds (e) that girdle the palpal 

 adductors. The elbow muscles of the odonate larval labium are not 

 comparable to the usual median retractors of the prementum since 

 they lie laterad of the other muscles. 



The movements of the labium on the head have generally been at- 

 tributed to the same muscles that flex and extend the prementum, 

 since no other labial muscles from the head are present. Amans 

 (1881) regarded the muscles from the hypopharyngeal apodeme (fig. 

 5 D, i'o) as the propulseurs of the postmentum. Munscheid (1933), 

 on the other hand, attributed the protraction of the labium to the ten- 

 torial muscles {44). However, since both pairs of these muscles run 

 posterior to the hinge {h) of the postmentum on the head, it is not 

 clear how either of them could swing the labium forward. Munscheid 

 assumed erroneously that the Drehpunkt of the labium lies behind the 

 labial base. Either of the two pairs of labial muscles in contraction 

 might be supposed to effect the retraction of the labium. When the 

 postmentum swings forward (E) it is to be noted that the hypo- 

 pharyngeal apodeme turns downward, since its crossbar is firmly held 

 in the posterior lip of the postmentum. A downward pressure on the 

 end of the apodeme (arrow in the figure), therefore, turns the post- 

 mentum forward on its anterior hinge points (h), and would cause 

 the protraction of the labium if accompanied by an extension of the 

 prementum by the tentorial muscles {44). 



It was long ago suggested by Amans (1881) that blood pressure 



