26 GENERAL ZOOLOGY 



they are not only harmless but are even highly beneficial. 

 The head is made up almost entirely of the great, staring, 

 compound eyes, which shine like fire as the dragon fly moves 

 about. The mouth has strong jaws, somewhat resembling 

 the powerful mandibles of the grasshoppers. The wings are 

 large, with many veins, and are moved by powerful muscles ; 

 but the legs are slender and small, as the dragon flies are 

 preeminently creatures of the air. The long and slender ab- 

 domen is used to balance the insect in its headlong flight. 



The eggs, generally attached to water plants, hatch into 

 aquatic nymphs. The mouth parts of the nymphs are 

 unique in structure. The lower lip is enlarged and armed 

 with hooks at the extremity. This formidable organ is 

 hinged and folded, so that it can be extended to seize any 

 insect within reach. When not so engaged it covers the 

 entire face like a mask, giving a peculiar and comical aspect 

 to a front view. The nymphs breathe by means of tracheae, 

 which line the posterior portion of the alimentary canal. 

 When water is drawn into the canal, air is absorbed from 

 it by this system of air tubes, and water, deprived of its 

 free oxygen, is forced out again. When water is ejected 

 violently, the nymph darts forward. After successive molts 

 the nymph develops rudiments of wings and finally crawls 

 out of the water to some convenient support, when the skin 

 splits down the back (Fig. 13) and the dragon fly, with crum- 

 pled wings, slowly emerges. A short time elapses before the 

 body hardens and the wings expand, and then the imago 

 flies away to live its short adult life. 



There are two quite distinct types of dragon flies, both 

 widely distributed over the world. The form represented in 

 Fig. 13 is of comparatively robust build. The eyes touch 

 each other along the median line of the head. The posterior 

 wings are broader at the base than the anterior pair, and 

 both pairs are held horizontally when the insect is at rest. 



