POSSIBILITY OP CHANOES IN MOUTH PARTS. 51 



a typically mandibulate group would arise, sucli as 

 Nicoletia (if, indeed, Nicoletia be not a larval form) ; 

 but here, again, the development might be continuous 

 and the activity uninterrupted, as in the case of the 

 grasshoppers, locusts, crickets, cockroaches, &c. 



There is yet a third possibility, namely, that during 

 the first portion of life the power of mastication should 

 be advantageous, and during the second that of suction, 

 or vice versa. A certain kind of food misfht abound 

 at one season and fail at another ; might be suitable for 

 the animal at one age and not at another : now, in such 

 cases you would have two forces acting successively on 

 each individual, and tending to modify the organization 

 mouth in different directions. 



It will not be denied that the ten thousand varia- 

 tions in the mouth-parts of insects have special 

 reference to the mode of life, and are of some advan- 

 tage to the species in which they occur. Hence, no 

 believer in natural selection can doubt the possibility 

 of the three cases above suggested, and the last of 

 which seems to me to explain the possible origin of 

 species which are mandibulate in one period of life and 

 not in another. The change from the one condition to 

 the other would, no doubt, take place during a change 

 of skin. At such times we know that, even when 

 there is no change of form, the temporary softness of 

 the organs often prevents the insect from feeding for a 

 time — as, for instance, in the case of the silkworm. 

 When, however, any considerable change was involved, 

 this period of fasting would be prolonged, and would 

 lead to the existence of a third condition, that of the 

 pupa, intermediate between the other two. Since 

 other changes are more conspicuous than those relating 

 to the mouth, we are apt to associate the pupa state 

 with the acquisition of wings, but the case of the 

 Orthoptera (grasshoppers, &c.) is sufficient proof that 

 the development of wings is perfectly compatible with 

 continuous activity ; so that in reality the necessity for 

 rest is much more intimately connected with the change 



