8 A BOOK OF INSECTS 



different ways, and upon different substances, in succes- 

 sive periods of their development is very general among 

 insects, and it has doubtless contributed largely to the 

 success of the race. The drain upon a particular food- 

 stuff is lessened, while the risk of lean years consequent 

 upon a period of wantonness is avoided. 



These, then, are the chief qualities that have made 

 insects successful. Does the reader ask for evidence that 

 they have truly succeeded ? Let him take his stand in a 

 field or a coppice on any spring or summer day. He will 

 see, perhaps, a bird or two, a rabbit, a mouse, and a 

 lizard. But for every animal of another class, scores of 

 insects will meet the eye. Flies hover in the air, bees 

 and butterflies pass from flower to flower, beetles climb 

 the grass stems or scuttle across the sun-dried patches of 

 earth. A similar comparison may be made within the 

 walls of one's own home. A few mice, some spiders, 

 possibly a rat — these are perforce our lodgers ; but in 

 addition we are liable to support at least a score of 

 different insects. There is the familiar cockroach — the 

 " black beetle " of our kitchen, the cheerful cricket on the 

 hearth, clothes moths in the wardrobe, beetles burrowing 

 in the woodwork, book-lice among old papers, and flies 

 upon the window-pane — to enumerate only a few of these 

 unbidden guests. Moreover, what is true of the house, 

 and of the English country-side, holds good the world 

 over. Insects predominate from the tropics to the poles, 

 wherever animal life can exist. It is computed that 

 500,000 different kinds of insects are actually known and 

 named ; hundreds more are being classified each year ; 

 while it is safe to estimate the total number of existing 

 species as at least a million. 



The geographical range of insects is truly astonishing. 

 Butterflies are found beyond the polar circle, and 



