^r,•u■. .900.] Mercer: Development of Wings in Lepidop.era. it 



In some cases the formation of a distinct vein-cavity follows .the 

 development of the trachea within. This is shown in the section 

 represented in Figure 14. Here there is one large vein-cavity con- 

 taining a trachea ; but the other tracheae are closely surrounded by the 

 hypodermal tissue of the wing. It is probable that this surrounding 

 tissue is forced away later by blood pressure thus forming the large 

 vein cavities characteristic of later stages. 



T/ie Formation of (he Hypodermal Pillars. — During the prepupal 

 period, that is between the time at which the wings pass out from the 

 body cavity and the time of moulting the last larval skin, there is a 

 great increase in size of tTie wings. This e.xpansion is doubtless 

 brought about by blood pressure. If there were no provision to 

 prevent it, the forcing of the blood into the lumen of the wing would 

 distend it like a filled bag. But the upper and lower sides of the 

 wing are tied together by fibers of hypodermal origin, which have 

 been termed the hypodermal pillars. The result is that the expand- 

 ing of the wing is in a single plane. 



According to the ol)servations of Mayer ('96) the formation of 

 these hypodermal pillars takes place in a complicated manner: but I 

 have not been able to confirm his observations. In Pieris it takes 

 place as follows : 



During the period in which the two basement membranes are 

 closely applied to each other, thus forming the so-called middle mem- 

 brane, these membranes fuse in places, thus uniting hypodermal cells 

 of the opposite sides of the wing. Later, at these points of fusion 

 tie fused basement membranes disintegrate, or at least are not to be 

 distinguished from the bodies of the united cells, which form a fiber 

 passing from one cuticle to the other. 



When the wing is expanded, the hypodermal cells, each of which 

 must cover a much larger area of the surface of the wing, are greatly 

 shortened ; and thus the two basement membranes are pulled apart ex- 

 cept in those places where they have fused. Thus is presented the 

 appearance shown in Figure 12, Is, which represents a section of a 

 vein cavity and several spaces l)etween hypodermal pillars. 



Later in the pupal stage, the hypodermal pillars appear to contract, 

 for the two surfaces of the wing are brought even closer together 

 (Figs. 31 and 35). In fact, in sections of wings of the adult very 

 little hypodermal tissue can be found ; and in many places the two 

 cuticular layers are closely applied. The details of the disappearance 

 of the hypodermal pillars have not been worked out. 



