84 THE LARVA OR NYMPH [CH. 



the median lobe slightly cleft, not prominent. Lateral lobes with 

 the outer and inner borders reduced, the movable hook and distal 

 borders greatly enlarged. Two or three strong setae on the hook ; 

 usually one seta on the lateral lobe, and a row on the mentum. 

 A huge apical tooth, supported by a strongly projecting serrated 

 ridge on the distal border. This ridge breaks up secondarily into 

 either (a) an outer tooth and an inner ridge (H), or (6) three teeth 

 of varying sizes (j). 



The beginnings of a concave formation are found in : 

 (vii) Petalurinae (fig. 32 K), a reduced archaic side-branch of 

 very feeble structure. As in all concave labia, the movable hook 

 is reduced in size, and the median lobe is prominent and triangular. 

 It differs however from the typical concave mask in having the 

 inner border elongated, and the distal border short ; also, no setae 

 are present. This labium is merely a mud-scoop, and probably 

 helps the larva to shovel out its mud-canals. 



Passing on to the true concave forms, we come next to 

 (viii) Cordulegastrinae (fig. 32 L). Here we see the reduced, 

 slender hook and the projecting, triangular, median lobe, which 

 however is bifid at the tip (an archaic character). The inner 

 border is now reduced, and the distal border becomes much 

 elongated and irregularly toothed. By this arrangement, the 

 lateral lobes are enabled to meet together along their distal 

 borders, above the median lobe, in an irregular median line, 

 formed by the teeth, which interlock like cogs. Mental and 

 lateral setae are present. 



(ix) Libellulidae (fig. 32N-Q). A single type, with many 

 minor variations, characterizes this huge family. The sub- 

 mentum is narrow, the mentum wide and sub -triangular. The 

 median lobe is projecting and triangular. The lateral lobes are 

 huge, very concave, and roughly triangular in shape, owing to the 

 extension of the distal border to a length about equal to that of 

 the inner border. Mental setae are generally numerous; in the 

 higher forms, they divide up into two sets (primary and secondary). 

 Lateral setae are present along the strongly ridged outer border. 

 The hook is small and slender. The dentition of the distal lobe 

 shews three principal types : (a) the Macromian, with deep regular 

 incisions (fig. 30) ; (6) the Eucordulian (fig. 32 o), with shallower 



