12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



to the middle of the prementum. The abductor muscles of the palpi 

 (fig. 5 A, 46) lie entirely within the prementum as in other insects, 

 but the large adductors (^7) break with all the rules of labial anatomy 

 by taking their origins in the distal end of the postmentum. In so 

 doing the adductors acquire an increased power for closing the palpal 

 hooks in the body of the prey, but they disrupt our generalizations on 

 the labial musculature. 



The postmentum is supported in the membranous ventral wall of 

 the head behind the maxillae (fig. 3 A,Pmf). Anteriorly its base is 

 produced into a pair of triangular lobes strengthened by lateral mar- 

 ginal thickenings which are extended laterally in the head membrane 

 as a pair of articular rods (ar) that reach to the maxillary cardines 

 (C, Cd), but are not attached to the latter. The true hinge points of 

 the labium on the head are thus at the mesal ends of these rods where 

 the rods join the basal lobes of the postmentum (figs. 3 A, 5 B, /i). 



The labial musculature of an aeschnid larva has been described, for 

 the most part correctly, by several writers, including Amans (1881), 

 Butler (1904), Whedon (1927), and Munscheid (1933). Four mus- 

 cles from the head traverse the postmentum to be attached on the base 

 of the prementum (fig. 5 A). The first pair {20) are the long slender 

 muscles arising on the hypopharyngeal apodeme (D). They are in- 

 serted on the prementum anterior to the elbow hinge (A, F, 20) and 

 serve as flexors of the prementum on the postmentum. The some- 

 what thicker muscles of the second pair (A, E, 44) arise on the 

 tentorium {Tnt) and are inserted on leverlike arms (F, /w) of the 

 premental base that project beyond the elbow hinge ; these muscles, 

 therefore, are effective extensors of the prementum. The tentorial 

 muscles are readily identified with the posterior tentorial muscles of 

 the prementum in other insects ; the usual accompanying anterior pair 

 are absent in the odonate larva. The apodemal muscles, as already 

 noted, appear to be represented in the cockroach and other orthop- 

 teroid insects by the pair of short muscles going from the hypo- 

 pharyngeal fulcrum below the salivary orifice to the base of the pre- 

 mentum. There are, therefore, in the odonate larva no special muscles 

 developed for the unusual movements of the labium on the head. 



A special group of muscles not represented in other insects is pres- 

 ent in the elbow region of the larval labium, there being two of these 

 muscles on each side (fig. 5G). The first {k) is a relatively large, 

 flat muscle arising on the lateral wall of the enlarged distal part of 

 the postmentum, the fibers of which converge to their insertions on 

 the lateral margin of the base of the prementum. This muscle is 

 clearly a secondary flexor of the prementum, as it is termed by 



