22 A BOOK OF INSECTS 



insects are divided. The simplest insects, typified by the 

 wingless " silver-fishes " of our larders, undergo no meta- 

 morphosis. They merely increase in size as they grow 

 from youth to maturity. Among the cockroaches and 

 their allies an incomplete metamorphosis obtains, marked 

 by the acquisition of wings after the final moult. But as 

 the environment and food habits of the individual remain 

 practically unchanged throughout life, the development 

 of wings scarcely affects its activities, and there is little 

 difference between the young and the adult forms. We 

 may contrast this with the case of the dragon-fly, of which 

 the nymph bears little or no resemblance to its parent. 

 Each obtains its livelihood in a different way, and among 

 distinct surroundings — the one capturing its prey in the 

 water, the other in the air. The change of habit is not 

 so great, however, as to render necessary an intermediate 

 period of quiescence. Yet it is possible to regard the 

 dragon-fly nymph, in the last stage of its development, 

 as an incipient pupa. If it were to remain inactive for 

 a few hours or days prior to its final transformation, it 

 would, indeed, foreshadow the true pupal state. Some- 

 thing of the kind actually occurs among the cicads. These 

 insects are very abundant in tropical countries, but only 

 one species has been found in Britain. The young nymphs 

 have large digging fore-legs, with which they burrow in 

 the ground. Their mouth-parts are formed for sucking, 

 and they obtain food from the soil humus, as well as from 

 the roots of plants. The perfect insect, on the other 

 hand, has large and powerful wings, and sucks the juices 

 of trees and shrubs. In the case of the American cicada, 

 known as the " seventeen-year locust," the nymph remains 

 underground for thirteen or seventeen years — a life-spell 

 greatly in excess of that enjoyed by any other insect. 

 The point which I desire to emphasize, however, is this : 



