112 



THE DEVELOPMENT OF WINGS 



In insects with a gradual or with an incomplete metamorphosis the 

 external changes diiring growth are comparatively slight, with each molt 

 there is an increase in size, and at the assumption of the adult instar there 

 takes place a great expansion of the wings, the perfection of the veins, the 

 thinning of the spaces between the veins, and the perfection of the basal 

 articulations. 



The gradual increase in the size of the wings in the successive nymphal 

 instars and their great increase in size at the assumption of the adiilt instar 

 are well illustrated by the transformations of a capsid bug, Lopidea robinicB 

 described and figured by M. D. Leonard ('i6), (Fig. 112). 



In the change from the last nymphal instar to the adult, there occurs in 

 the jumping Orthoptera and in the Odonata the well known reversal of the 

 position of the wings by which that face of each wing which has been 

 exposed becomes the lower surface of the wing. 



"Figure 113 shows the relation of parts in a dragon-fly nymph one-third 

 grown. It represents a partial cross-section passing through the posterior 

 part of the basal attachments of the hind wings and through the fore wings 

 just before the arculus. In the hind wings are seen (a, b) the cut ends of 



m./nr 



Fig. 113. — Dorsal part of a transverse section of a nymph of Celithemis elisa, 

 one-third grown : d, dorsum; pi, pleurum; (/.z^., dorsal vessel; /,/, tracheae; 

 m, m, muscles in cross-section; ml, muscles in longitudinal section; m, 

 juv, developing muscles of the wing; hw, hind wing; f w, fore- 

 wing; a and b, the cut ends of the basal transverse trachea of 

 the hind wing; C, costa; Sc, subcosta; R M, the coal- 

 esced radius and media; Cu, cubitus; A, anal vein. 

 (After C. & N.). 



the transverse basal trachea. In the fore wings the tracheae in the vein 

 cavities are seen in section. The pleura (pi) are seen overlying the bases 

 of the wings. 



"It is interesting to follow the basement membrane of the hypodennis 

 throughout the section, noting how the hypodermal cells are elongated in 



