NO. 2 DRAGONFLY LARVA — SNODGRASS II 



ordinarily is deeply inflected into the cibarial lumen (Cb). The sus- 

 pensorium of the hypopharynx consists of a transverse bar on the 

 base of the dorsal wall of the hypopharynx (B,w), and a pair of 

 lateral arms (y) going posteriorly and dorsally along the line of union 

 between the cibarium and the stomodaeum (A) to give attachment 

 to a pair of short frontal muscles (/j). These suspensorial arms of 

 the hypopharynx mark the site of the true (embryonic) mouth of the 

 insect, which is the opening into the stomodaeum (Stom), and lies 

 behind the base of the clypeus (Clp). The functional mouth of the 

 dragonfly larva (Mth"), opening into the cibarium over the hypo- 

 pharynx behind the labrum (Lm), is therefore a secondary mouth. 

 A full description with illustrations of the cibarial and stomodaeal 

 musculature of the larva of Ischnura verticalis is given by Grieve 

 (1937). Since his study was made entirely from microtome sections, 

 this author did not observe the importance of the suspensory arms 

 of the hypopharynx as landmarks for separating the cibarial pouch 

 from the stomodaeum, but he noted that the cibarial muscles are sepa- 

 rated from the stomodaeal (pharyngeal) muscles by the frontal gan- 

 glion (A, frGng) and its brain connectives. 



Labium. — The larval labium (fig. 3 A, L6) consists of two major 

 parts hinged on each other by an elbowlike joint. The two parts are 

 the prementum and postmentum of other insects, commonly called 

 "mentum" and "submentum" by writers on odonate anatomy, except 

 Corbet (1953). In most insects the postmentum is a mere plate on 

 the posterior or under side of the head ; in the dragonfly larva it is 

 produced into a hollow stalk supporting the prementum, and the whole 

 labium swings back and forth on the base of the postmentum. In an 

 aeschnid larva the labium hangs below the center of the head (fig. 

 5 C) ; in the retracted position (fig. i) the elbow lies against the ven- 

 tral surface of the mesothorax and the prementum is pressed against 

 the under side of the head, from which position it can be thrust out far 

 beyond the head by a forward swing of the postmentum. 



The large, spatulate prementum of an Anax lai-va (fig. 3 A, Prmt) 

 bears a pair of lateral apical lobes (Pip), each armed with a long, 

 sharp hook. These lobes with their hooks are the grasping organs of 

 the larva, but they represent the palpi of an ordinary insect labium 

 since each is provided with an abductor and an adductor muscle (fig. 

 5 A, 46, 42). Between their bases is a partially divided median ligular 

 lobe (fig. 3 A, Lig). The palpal lobes vary much in size and shape in 

 different genera ; in some anisopterous larvae they are large flaps that 

 form a mask covering the whole face when applied against the head ; 

 in an agrionid they may be slender and elongate and the ligula split 



