112 HORTENSE BUTLER. 



and curiously shaped labium. This labium, which has been, not 

 inaptly, compared to a mask, in the Libellulidse completely covers 

 the remaining mouth parts, and in the other families very nearly 

 does so. At first glance it seems unweildy in size and shape, but it 

 is capable of remarkably swift and accurate movement. If one ob- 

 serve large nymphs of An.ax junms in proximity to which a few 

 Ischmira nymphs have been placed, it will be seen that directly the 

 latter are brought near, the Ahax nymphs remain perfectly quiet 

 until the prey conies quite close to them, then with a motion so 

 swift as to preclude observation the labium is darted out, the vic- 

 tim seized and instantly drawn up into a position in which the 

 mandibles can be used with advantage. The strength of the labium 

 is surprising : indeed, when the animal is held suspended in the air 

 by its extended tip, it can with ease lift the weight of its entire body 

 up to the support. 



The powerful muscles and hinges which constitute the niechanism 

 of the labium have never hitherto been investigated. The various 

 movements are, however, secured by a comparatively very simple 

 apparatus. For an understanding of the mechanism of the labium 

 it would be necessary, first of all, to consider what are the various 

 divisions composing it. 



The typical labium consists of a submentum and (sm. PI. II, Fig. 

 1) mentum {m. PI. II, Fig. 1), a ligula (Z/. PI. II, Fig. 1) which 

 varies greatly in shape in the various families; to lateral lobes (11. 

 PI. II, Fig. 1) each bearing a movable hook (»i,/i. PI. II, Fig. 1) 

 and terminating in an end hook. These various parts are hinged 

 together at four points, the movable hooks are hinged onto the lat- 

 eral lobes, the lateral lobes in their turn are hinged onto the men- 

 tum at the apical hinge, and there is a hinge at the joining of the 

 mentum and submentum, the middle hinge, and another at the junc- 

 ture of the submentum with the head, the basal hinge. 



The movable hook is without muscles. Four pairs of muscles are 

 so arranged in connection with the apical, middle and basal hinges 

 before mentioned, as to produce all the movements of which the 

 labium is capable. These are a pair of abductors {aa .PI. II, Figs, 

 1 and 2), a pair of adductors (66. PI. II, Figs. 1 and 2), a pair of 

 extensors {dd. PI. II, Figs. 1 and 2), and a pair of flexors {cc. PI. 

 II, Figs. 1 and 2). The adductors, which are fastened to the exter- 

 nal tendenous prolongation of the lateral lobe at its insertion into 



