IV] THE LARVA OR NYMPH 79 



These are (i) the outer border (o), from the outer edge of the base 

 up to the movable hook ; it varies in length, but is always thickened 

 and often armed with spines; (ii) the distal border (d), or space 

 between the movable hook and apex; originally very short, but 

 probably toothed, it takes on a development of great importance 

 in the concave Libellulid mask ; (iii) the inner border (i), running 

 from the apex to the inner edge of the base; it may be short or 

 long, smooth or toothed. 



Setae (figs. 30, 32) may be developed both on the mentum 

 (mental setae) and on the lateral lobes (lateral setae, Is). The 

 mental setae often form two groups (fig. 32 N), an outer group 

 of long primary setae (msj) and an inner group of short secondary 

 setae (ms 2 ). In Lestinae (fig. 32 H) two or three large setae occur 

 on the movable hook. The border of the median lobe carries hairs, 

 small setae, and sometimes tubercles or spines. Small warts 

 (fig. 32 N) are very characteristic of the Libellulid mask, especially 

 on the surface of the lateral lobes. 



Mechanism of the Mask. The larva relies, for the capture of its 

 prey, almost entirely on the mask. Either it remains absolutely 

 motionless until the victim comes within reach, or it stalks it 

 slowly and stealthily until it is close enough to strike. Then the 

 mask is thrown out with lightning rapidity, by means of its strong 

 extensor muscles. At the same time, the extensors of the palps 

 throw the lateral lobes wide apart. Curiously enough, there are 

 no muscles connected with the movable hooks; yet these organs 

 are shot out with great rapidity at the moment of striking. The 

 ingenious mechanism of the hooks has been explained by A mans 

 [i]. A strong ligament on the inner side of the base tends naturally 

 to keep them tightly closed (as in the position of rest). Their 

 sudden extension is due to contraction of the diaphragm (p. 211) 

 situated between abdominal segments 4 and 5. The mask being 

 hollow, with a definite prolongation of the haemocoele right up to 

 the bases of the hooks, the forward pressure of the diaphragm 

 forces blood into the cavity, in such a way as to overcome the 

 elasticity of the ligament, and so shoots the movable hooks 

 suddenly outwards. The action is quite momentary, for the 

 pressure is relaxed almost at once, and the hooks spring quickly 

 back, fixing themselves in the body of the hapless victim. When the 



