Ill] THE WINGS 47 



the veins of the imaginal wing become alternately convex and 

 concave, according as they are laid down upon one or other 

 surface of the wing-rudiment. 



At metamorphosis the tracheae are withdrawn from the wing, 

 and shrivel up. During the rapid growth of the wing, chitin 

 is deposited w T ith amazing rapidity. As each vein becomes 

 stretched to its full extent, the hypoderm cells along its course 

 completely exhaust themselves with chitin formation, so that the 

 vein becomes built up to the necessary strength in the course of 

 a few minutes. When the construction of both veins and membrane 

 is completed, the hypoderm cells shrivel and die; the blood by 

 whose agency the wing was expanded gradually evaporates, as 

 the wing is exposed to the air and sunlight. Finally, the two 

 surfaces of the wing come into contact and fuse together, except 

 along the main veins, where small blood-canals are retained. 



Histology of the Developing Wing. The wing-bud is simply an 

 ectodermal evagination, in the form of a small bag lined internally 

 with hypoderm cells, and externally with the cuticle. Even before 

 any sign of an external bud can be seen, the hypoderm cells 

 increase in size, becoming distinctly columnar, and forming 

 a small fold or bag beneath the cuticle. At the next ecdysis, 

 the cuticle becomes folded also, so that the evagination is then 

 visible externally. At first, the layers of hypoderm bordering 

 the inner and outer surfaces of the wing-bud do not meet. 

 Between them runs a narrow prolongation of the haemocoele, 

 filled with blood. Into this space tracheae soon penetrate, taking 

 the courses already described. Fine nerves accompany the 

 tracheae 1 . 



By the time that the wing- bud has become a definite flap or 

 lamina, a change has occurred in the form of the hypoderm cells. 

 These have become spindle-shaped, with the nucleus lying in the 

 middle. Owing to this elongation, the two layers of hypoderm 

 come into contact along their inner ends. Here they unite, 

 forming a basement membrane, except in the places in which 

 the tracheae and nerves run, small blood-channels are left 



1 The presence or absence, in the wings of Odonata, of the peculiar structure 

 called Semper's rod, which accompanies the tracheae in the wings of certain 

 Lepidoptera, does not appear to have been determined with certainty. 



