MOVEMENT 



55 



covered by firm cuticle. Air- tubes, however, grow into the 

 pouch or wing- rudiment during its development, and these 

 become surrounded by the thickened tubular structures 

 which appear as the supporting nervures of the wing 

 (Fig. 1 8). As development proceeds the cells of the skin 

 (Fig. i8, B, e), which are active in forming the cuticle, 

 become attenuated and at length disappear, so that the 

 adult wing is entirely cuticular. 



A fully developed wing is jointed to the thoracic seg- 

 ment from which it arises, the forewings belonging to 



Fig. i8. — A, Forewing of nymphal Stone-fly {Taeniopteryx) showing 

 the tracheal trunks and their branches {sc, subcostal ; r, radial ; m, 

 median; c, cubital; a, anal). The pale tracks indicate positions of 

 developing nervures. X 12. B, cross-section through developing 

 wing of a Dragon-fly nymph {Anax). cu, cuticle ; e, epidermis ; rt, 

 radial trachea ; mt, median trachea. X 150. After J. H. Comstock 

 and J. G. Needham. Amer. Nat. xxxii, xxxiii. 



the second (mesothorax) and the hindwings to the third 

 segment (metathorax) of that region of the body. The 

 base of the wing, its region of attachment, is relatively small 

 and is supported by a series of sclerites which are connected 

 by flexible cuticle to the thoracic wall, so that the wing is 



