TRACHEATES I 87 



As the air is cut off from the larvae it was useful for 

 them to have different adaptations from the imago. 

 While the latter flies easily out of reach of its enemies, 

 the creeping larva can be caught at any time, and so 

 urgently needed protective colouring so as to escape 

 notice in its surroundings. Thus the larvae and 

 imagines were selected in different directions according 

 to their different conditions of life, and came to differ 

 more and more. 



In the case of moths and grasshoppers, there is not 

 a very great difference in habits between the larva and 

 the imago ; the latter hardly use their wings except to 

 lengthen their leaps. Hence the larva does not differ 

 so much from the imago, and is merely without the 

 wings, or has shorter wings. 



It is otherwise with beetles, flies, bees, wasps, and 

 butterflies. The vital activity of these insects chiefly 

 centres about their power of flying; some of them 

 hardly move in any other way. But the air is only 

 opened to them after the last cast of the skin. The 

 gradual transition of the larva into the imago would 

 clearly be very much out of place here, as the two 

 stages are so very different from each other ; during 

 such stages of transition the animal would be neither 

 adapted to its larva - surroundings, which it cannot 

 leave because of the absence of wings, nor would the 

 imperfect characteristics of the imago be of any use to 

 it. We understand, therefore, why natural selection 

 has cut down these transitional stages as much as 

 possible, so as merely to let the larvae grow in the 

 first and most of the other casts, without changing 



