HOW THE WINGS ENLARGE 



137 



fly issues from the chrysalis, these folds in the pupal wings flatten 

 out, and it is this flattening which causes the expansion of the 

 wings. ... It is evident that the wings after emergence undergo 

 a great stretching and flattening. The mechanics of the operation 

 appears to be as follows. The hsemolymph, or blood, within the 

 wings is under considerable pressure, and this pressure would 

 naturally tend to enlarge the freshly emerged wing into a balloon- 

 shaped bag ; but the hypodermal fibres (/i) hold the upper and lower 

 walls of the wing-membrane closely together, and so, instead of 

 becoming a swollen bag, the wing becomes a thin flat one. And 



FIG. 150. Diagrammatic reproduction of Pig-. 149 FIG. 151 . Section of the wing-germ, the 

 showing the wing-germ in its peripodal cavity (p): upper and lower sides connected by spindle- 

 h' (I i-in, hypodermis ; ti\ trachea; eta, cuticula; a, like hypodermic cells (A), forming- the rods 

 anterior end. After Mayer. of the adult wing ; mbr, ground-membrane 



of Semper. After Mayer. 



thus it is that the little thick corrugated sac-like wings of the freshly 

 emerged insect become the large, thin, flat wings of the imago. . . . 

 The area of the wing of the imago of Danais plexippus is 8.G times 

 that of the pupa. Now, as the wing of the young pupa has about 

 60 times the area of the wing in the mature larva, it is evident 

 that in passing from the larval state to maturity the area of the 

 wings increases more than 500 times." 



/. The primitive origin of the wings 



Farther observations are needed to connect the mode of formation 

 of the wings in the holdmetabolous insects with the more primitive 

 mode of origin seen in the hemimetabolous orders, but the former 

 mode is evidently inherited from the latter. Pancritius remarks 

 that the development of the rudiments of the wing in a hypodermal 

 cavity is in the holometabolic insects to be regarded as a later 

 inherited character, the external conditions causing it being un- 

 known. 



