CLASSIFICATION OF INSECT. 3 



other close parallel in other groups of animals. It is true that 

 the shells of molluscs, the tests of echinoderms, or the external 

 armor of reptiles present features in common, but they do not 

 exhibit in combination, the high degree of adaptiveness and the 

 great variety of modification seen in the exoskeleton of insects. 

 Crustacea make the closest approach to this condition, but these 

 animals are greatly restricted in many ways by their more uni- 

 form aquatic environment. 



It is, of course, in relation to their metamorphosis, with the 

 interpolation of such dissimilar preparatory stages, chat insects 

 find one of their greatest opportunities for functional and struc- 

 tural modification. In consequence, we find that the larvae 

 exhibit a greater degree of plasticity than do the imagines. 

 Apart from the comparatively short period during which the 

 adults exercise their reproductive functions, the important 

 activities of the higher insects are restricted to the usually much 

 prolonged period of larval development. Tlvs period is largely 

 concerned w r ith the growth through which adult size is reached, 

 and growth practically stops in almost all insects at the com- 

 pletion of the final larval instar. Although the beginnings of 

 the purely imaginal organs may appear often qu'.te early in 

 larval development, they do not assume much physiological 

 importance until the pupal stage, at which time the relations of 

 the insect and its external surroundings suddenly decrease almost 

 to the vanishing point, and all activities are closely bound up 

 in the profound internal changes necessary to bridge the gap 

 between larva and imago. 



It is, therefore, clear that the trophic functions dominate the 

 larva as most essential to its growth, and it appears that the 

 characteristics of the larval stages may usually be interpreted 

 with this in view. That the larva shows the greatest plasticity 

 will I think be evident from an examination of the structure of 

 the larvae of the several groups. The pupa on the other hand is 

 the most un.form stage, a condition w T hich may be readily attr - 

 buted to its much reduced relations with its environment and 

 the slight peculiarities of some pupae are usually directly corre- 

 lated with the few activities which they do exhibit toward 

 external conditions. 



