iv,] METAMORPHOSES OF INSECTS. 71 



species in which the immature forms have a separate 

 and independent existence. If an animal which, 

 when young, pursues one mode of life, and lives on 

 one kind of food, subsequently, either from its own 

 growth in size and strength, or from any change 

 of season, alters its habits or food, however slightly, 

 it immediately becomes subject to the action of new 

 forces : natural selection affects it in two different, 

 and, it may be, very distinct manners, gradually 

 tending to changes which may become so great as 

 to involve an intermediate period of change and 

 quiescence. 



There are, however, peculiar difficulties in those 

 cases in which, as among the Lepidoptera, the same 

 species is mandibulate as a larva, and suctorial as an 

 imago. From this point of view Campodea and the 

 Collembola (Podura, &c.) are peculiarly interesting. 

 There are in insects three principal types of mouth ; 



First, the mandibulate ; 



Secondly, the suctorial ; and 



Thirdly, that of Campodea and the Collembola 



generally, 



in which the mandibles and maxillae are retracted, 

 but have some freedom of motion, and can be used 

 for biting and chewing soft substances. This type is, 

 in some respects, intermediate between the other two. 

 Assuming that certain representatives of such a type 

 were placed under conditions which made a suctorial 

 mouth advantageous, those individuals in which the 

 mandibles and maxillae were best calculated to pierce 

 or prick would be favoured by natural selection, and 

 their power of lateral motion would tend to fall into 



