50 THREE TYPES OF MOUTH AMONG INSECTS. 



This larval character is typical for these insects ; and 

 we cannot fancy winged species interspersed amongst 

 them in the same way as apterous species occasionally 

 occur in the families of typically winged insects." 



In short, there are among Hexapods three principal 

 forms of mouth : 



First, the mandibulate, in which the mandibles and 

 maxillae are attached externally, and are more or less 

 powerful and adapted for biting and chewing. 



Secondly, the suctorial, in which the mandibles and 

 maxillge are attached internally, and are not opposite, 

 but parallel, and intended for pricking ; and — 



Thirdly, the type presented by Campodea and the 

 Collembola; in which the mandibles and maxilla 

 are attached internally, and are far from strong, but 

 still have some freedom of motion, and can be used for 

 biting and chewing soft substances. 



This description of the mouth agrees in all essential 

 points with that which I gave in my first memoir on 

 the group, ^ but I quote it here as the account given by 

 an independent observer. It seems to me to throw no 

 little light on the possible course of insect evolution. 

 Assuming that certain representatives of this parent 

 group found themselves in circumstances which made 

 a suctorial mouth advantageous, those individuals 

 would be favoured by natural selection, in which the 

 mandibles and maxillae were best calculated to pierce 

 or prick, and their power of lateral motion would tend 

 to fall into abeyance. Such forms, however, might 

 have a continuous development without any interruption 

 of activity, as is the case with the Hemiptera. 



Again, imagine that other representatives of the 

 original type found themselves in circumstances which 

 supplied abundance of food both palatable and nutri- 

 tritious, but requiring mastication. Under such con- 

 ditions natural selection would favour those specimens 

 in which the mandibles and maxillge possessed the 

 greatest strength and most freedom of action. Thus, 



' ' Trans. Linn. Soc.,' 1862. 



