68 INSECT TRANSFORMATION 



them giving off branches and penetrating further as the 

 structure increases in size at each moult. These air-tubes 

 convey the oxygen needed by the growing tissues and also 

 prefigure the scheme of nervuration of the perfected wing, the 

 main nervures following generally the tracks of the air-tubes, 

 in some cases coming to enclose both them and fine nerve-cords 

 which follow their courses, and being distinguished by the 

 same names. The two layers of the wing-fold come into 

 contact with each other except along the courses of the air- 

 tubes to which the blood-spaces thus become restricted (Fig. 36) . 

 During the last larval or nymphal stage the wing-nervuration 

 begins to form definitely beneath the cuticle soon to be cast 

 off, the new cuticle showing thickenings along the courses of 

 the air-tubes. As these are often situated alternately nearer 



FIG. 36. SECTION THROUGH WING-RUDIMENT OF A NYMPHAL 

 DRAGON-FLY (Atiax). 



Showing the two folds approximated except where traversed by the 

 air-tubes ; ep, skin (epidermis) ; bm, basement membrane ; 

 c, cuticle ; C, costal; A, anal edge of wing, x 40. AfterComstock 

 and Needham, Amer. Nat. XXXIII. 



to the upper and lower surfaces of the developing wing, the 

 cuticular thickenings arise in correspondingly alternate 

 positions resulting in the formation of convex nervures on the 

 upper surface and concave ones on the lower aspect of the 

 perfected wing. Reference has been already made in the 

 preceding chapter (pp. 38, 64) to the manner in which the 

 growing wings of the adult are folded within the cuticle of 

 the wing-rudiments of the last nymph-stage (Fig. 37 a 6). 

 The process of unfolding and expansion has been carefully 

 traced 1 in various species of termites (the so-called " white 

 ants "). The growing wings (Fig. 37 cde) are closely crumpled 

 and rolled and their expansion works outwards from base to 

 tip; " the process may well be likened to the billowing out of 

 a crumpled and wet cloth by driving a blast of air below it ". 



1 Claude Fuller: "The Wing- Venation and Respiratory System of 

 Certain South African Termites ". Ann. Natal Mus., IV. 1919. 



