AND THE LOWEE ANIMALS. 69 



CHAPTER VIII. 



METAMOEPHOSES OE INSECTS GENEEALLY. 



We have studied the complete life-history of an insect. 

 By examining several species of each group from the 

 same point of view, we are enabled to form a notion 

 of the ideal plan upon which this class is constructed. 

 The typical insect would be an annulose animal, 

 breathing by tracheae, with a body composed of three 

 distinct segments, of which the middle bears three pairs 

 of legs and two pairs of wings, and which arrives at its 

 perfect condition after undergoing two metamorphoses. 

 Hence, independently of the time spent in the egg,, 

 its lifetime is divided into three separate periods, the- 

 first of which is characterized by both an external and 

 internal activity, whose only object is the growth of 

 the individual; the second by an internal activity alone,, 

 whose object is the modification of the individual; and 

 the third by an internal and external activity, the solfr 

 object of which is the perpetuation of the species.. 

 Some insects are constructed exactly upon this ideal 

 plan, and we need not go beyond the Lepidoptera to 

 find an example. From the egg of the wood-eating 

 cossus (Cossus ligniperda), a caterpillar is produced 

 which spends two years or rather more in this stage, 

 and then becomes converted into a chrysalis ; the latter 

 is transformed into a moth, which never eats any- 

 thing, has no proboscis, and spends the few days of its 



