II] EXTERNAL, FEATURES ANU SKELETON 19 



lateral lobes (II). The changes in form of these lobes are correlated 

 with those of the median lobe. Normally, each lateral lobe has 

 its outer border more or less strongly convex, its inner border 

 either slightly concave, waved or nearly straight. The inner 

 border ends distally in a sharp end-hook (eh) which is fixed. Just 

 external to this hook is a small articulated piece, called the movable 

 hook (mh), much resembling a small palpal joint, curved, hairy, 

 and only moderately pointed. The whole lobe is more or less 

 hairy. The end-hook varies in shape. It is most sharply pointed 

 in the Zygoptera (fig. 6 A), and in the older Aeschninae. In 

 Austrophlebia it is strongly bifid. In Cordulegaster it has a set of 

 supplementary teeth. In the higher Aeschninae it tends to become 

 reduced in size. The movable hook also tends to become reduced 

 in size along with the end-hook. In the Libellulidae (fig. 6 B) the 

 end-hook has disappeared, while the movable hook has lost its 

 articulation, and is only represented either by a small spine or 

 a thickened chitinous spot. The lateral lobes themselves are 

 greatly enlarged, with both inner and outer borders convex. As 

 already described, they meet together above the reduced median 

 lobe. Thus the closed mouth of the Libellulidae shews the very 

 characteristic appearance of having a large T-shaped slit, the 

 stem of the T being formed by the line of meeting of these two 

 lateral lobes. 



The labium carries internally, projecting from its base, a pro- 

 minent tongue-like organ projecting into the mouth. This is the 

 hypopharynx or lingua. This is dealt with in chap. vn. The 

 salivary ducts open close to its base. 



THE NECK, OR MICROTHORAX (fig. 7). 



In the Dragonfly the neck is exceedingly small and slender, 

 pivoting the head almost, as it were, on a point, and allowing of 

 turning movements to some degree in every direction. Although 

 the neck is firmly fixed to the prothorax, yet it is morphologically 

 part of the head itself. The plates of the neck, or cervical sderites 

 (1-4 mi), together with the gula, represent the skeleton of the last 

 head-segment. The neck is supported by four pairs of these 

 plates. Two of these (1 mi, 2 mi) are rather large and irregular 



22 



