THE DEVELOPMENT OF WINGS 117 



the prepupal stadia, is due to the retention of the wing within the narrow 

 limits of its hypodermal pouch ; for its small size renders possible its aera- 

 tion by tracheoles alone. In an externally developing wing it is necessary 

 that tracheae should grow with the wing, in order to carry the tracheoles out 

 within reach of the tissues ; but when a wing develops internally its length 

 for a long time does not exceed the length of normal tracheoles. In such a 

 wing, tracheae develop only when needed, at the approach of the time when 

 rapid extension is to take place. 



