2 ON THE ORIGIN AND [CHAP. 



says that, excluding certain very rare anomalies, 

 "we may observe four distinct periods of existence 

 in every insect, namely, those of the egg, the larva, 

 the pupa, and the imago, or perfect insect." In fact, 

 however, the various groups of insects differ widely 

 from one another in the metamorphoses they pass 

 through : in some, as in the grasshoppers and crickets, 

 the changes consist principally in a gradual increase 

 of size, and in the acquisition of wings ; while others, 

 as for instance the common fly, acquire their full 

 bulk in a form very different from that which they 

 ultimately assume, and pass through a period of in- 

 action in which not only is the whole form of the 

 body altered, not only are legs and wings acquired, 

 but even the internal organs themselves are almost 

 entirely disintegrated and re-formed. It will be my 

 object, after having briefly described these changes, 

 to throw some light on the causes to which they are 

 due, and on the indications they afford of the stages 

 through which insects have been evolved. 



The following list gives the orders or principal 

 groups into which the Class Insecta may be divided. 

 I will not, indeed, here enter upon my own views, but 

 will adopt the system given by Mr. Westwood in his 

 excellent " Introduction to the Modern Classification 

 of Insects," from which also, as a standard authority, 

 most of the figures on Plates I. to IV., when not other- 

 wise acknowledged, have been taken. He divides 

 insects into thirteen groups, and with reference to 

 eight of them it may be said that there is little 

 difference of opinion among entomologists. These 

 orders are by far the most numerous, and I have 



