THE MAY-FLIES AND THE DRAGON-FLIES 29 



system of air-tubes, and water, deprived of its free oxygen, 

 can be ejected violently, thus forcing the nymph forward. 

 After successive molts the nymph develops rudiments of 

 wings, and finally crawls out of the water to some conven- 

 ient support, when the skin splits down the back and the 

 dragon-fly, with crumpled 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. 15 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. To this 

 type belong the best fliers of the group. The insects which 

 illustrate the other type, while they can easily enough be 

 recognized as dragon-flies, are of more slender build. Their 

 eyes are widely separated on opposite sides of the head. The 

 anterior and posterior wings are alike in size and shape, and 

 when not in use are folded against the abdomen. The flight 

 is less sustained and more erratic than with species of the 

 first type. Some of these dragon-flies, notably species from 

 the tropics, are most beautiful both in form and color. The 

 French call these insects " demoiselles," which we may trans- 

 late damsel-flies. 



Definition of Odonata (Gr. odons (odont], a tooth). The 

 dragon-flies constitute the order Odona'ta, a word meaning 

 " toothed," perhaps in allusion to the teeth on the second 

 maxillae of the larvae. The Odonata are distinguished by the 

 biting mouth-parts and the four equal or nearly equal net- 

 veined wings. The metamorphosis is fully as well marked 

 as in the preceding order. 



