300 INTRODUCTION TO ZOOLOGY CHAP. 



structure with three distinct parts : there is a long segment, 

 attached beneath the mouth, which when at rest extends 

 backwards below the head and throat; connected with this 

 by a kind of elbow-joint is another long segment which lies, 

 when at rest, below the first, but which projects forwards ; 

 finally, at its free end are two sharply curved teeth known 

 as the "forceps." The broad end of the second segment 

 of this labium, with its " forceps," lies, when not in use, over 

 the mouth, hiding the real jaws, and therefore the structure 

 is termed the "mask" (Fig. 221, m). 



When the larva has approached stealthily within reach of 

 its prey, it suddenly shoots forward the whole "lip arm," at 

 the same time opening wide the forceps and seizing the 

 animal with them. The " arm " or " mask " is then pulled 

 back with equal rapidity, so that the prey is held by the 

 forceps against the mouth, where it is quickly eaten by means 

 of the toothed mandibles and maxillae. 



Although at first the newly hatched larva has 

 morphosis. no trace * W i n s 5 a ^ ter three or four moults, rudi- 

 ments of them appear as two small backwardly 

 extending lobes ; these are attached at their bases to two 

 horny plates which have previously grown up from the sides 

 of the body, and which nearly meet in the middle line over 

 the back. A larva with wing rudiments is often called a 

 nymph (Fig. 220). Many moults occur before this nymph is 

 full grown. In fact, the larval stage usually continues for 

 more than a year. By this time the wings are about f of 

 an inch long, and the thoracic spiracles have become func- 

 tional, otherwise the larva has changed but little, except in 

 size. Now, however, the nymph ceases to feed, and climbs 

 up some weed or projecting stump right out of the water, 

 perhaps to a height of several feet above the surface. When 

 it has found a suitable spot, the nymph clings firmly, in an 

 upright position, to the support, and prepares for its last 

 moult, when the remarkable change from the dingy, slow- 

 moving larva to the brilliant, swiftly flying imago will take 

 place. It may remain thus, motionless, for some hours, or 

 even a whole day, before any further change occurs ; then it 

 will suddenly be noticed that the skin over the eyes, which 

 previously was dull and opaque, has become bright and 

 transparent, owing to the brilliancy of the eyes lying under- 



