Mar. ijoo.] Mercer : Development of Wings in Lepidoptera. 9 



boundary of the wing-bud (■<('), and the internal wall of the perii)odal 

 membrane (<'). 



T/ie Parts of tlic Wiui^. — These can be best studied from the sec- 

 tions of the wings in various planes. Figure 13 shows a cross section 

 of a wing through a vein cavity ( /r'). This vein cavity contains a 

 trachea (/) which is not functional, is filled with lymph, and contains 

 no chitinous intima. This trachea consists of a single layer of epi- 

 thelial cells which will finally change somewhat upon their internal 

 margin and form the chitinous intima in a manner to be described 

 later. In this vein cavity sections of tracheoles, both cross and lon- 

 gitudinal, are found; those in cross section appear as holes through a 

 mother-cell (//). Leucocytes (/) are also found. 



In a frontal section (Fig. 16) the lateral trachea ( Z") is cut in 

 longi-section, and the razor has passed through a wing trachea (/) in 

 longi-section. This trachea has passed into the lumen of the wing 

 (/7t'). The tracheoles (//) are connected with the lumen of the main 

 trachea at a point (w/) while at the same time the chitinous intima (///) 

 extends across the mouth of the trachea (/') which, as yet, has no 

 chitinous intima and is not functional. 



Figure 17 is a section of a wing in a slightly different plane than 

 Figure 14 ; enough so that a mass of tracheoles (/"/) appears instead 

 of the bases of the developing tracheae (Fig. 14, T). 



iJuring this stage the wing has been growing very rapidly and the 

 ventral margin has extended toward the feet, and at the latter part of 

 the period the wing changes from the inside of the body to the out- 

 side. This is all brought about while the caterpillar is searching for 

 a suitable place to hang itself up. 



The basement membrane (/v//) is not easily distinguished in some 

 sections in this period though quite distinct in others ; it is shown in 

 Figures 16 and 23. In many places between the vein cavities in the 

 wing the two layers of the basement membrane appear as coming to- 

 gether and are somewhat fused. In some sections (Fig. 13) the line 

 of union is seen with difficulty, but even in this section the line of 

 contact is faintly made out by careful focusing. The union of these 

 two layers forms the so-called middle membrane (Fig. 23, bin) which 

 will be discussed more fully later. 



The Cuticle of the \\'ixg-Bud. 

 There has been a long dispute as to whether or not a cuticle is 

 formed around the wing-bud in the younger stages. 



