358 



The imaginal discs of the wings are seen even in tthe larva 

 ot the first instar as two pairs of rather pronounced thickened 

 areas in the second and third thoracic segments. Cellular 

 proliferation takes place during larval life, and, just as in 

 the legs and other appendages, the excessive growth of the 

 surrounding "larval" cells produces an invagination of the 

 discs; rapid mitotic cell division during the resting period 

 results in an evagination of the disc, the underlying mesoderm 

 being, as usual, dragged into the structure. The epithelium 

 of the wing consists of a single layer of elongated cells; these 

 cells, in order to present a greater surface area, frequently 

 develop on their free surface in the larva shortly before pupat- 

 ing, distinct hammer-like thickenings. 



The secretion of a cuticle, simultaneously with cuticle 

 development over the rest of the animal is followed by the 

 pupal moult. 



The contraction of the wings, as already described, results 

 simply from a closer packing of the cells. In the early pupa 

 a cell of the fat-body often passes into the cavity of the wing, 

 helping to nourish that structure; the mesodermal cells are 

 seen undergoing mitotic division. 



After twenty-four hours the cavity of the wing has been 

 almost obliterated. This is the result of at least two factors : 

 firstly, the cells shorten somewhat, the peripheral pull drag- 

 ging the two surfaces nearer together; secondly, they begin 

 to undergo a remarkable process of wrinkling on their free 

 surface, as a result of which the lower part of the cell becomes 

 forced backwards. This wrinkling can already be seen com- 

 mencing in the four-hour pupa ; in the thirty-six hour pupa it 

 is very far advanced, and the free surface of the cells which 

 now present great folds, begin to secrete chitin which is itself, 

 therefore, strongly folded. Many of the cells, however, in 

 addition to forming folds, have also developed a hair-like 

 process on their free surface ; the cuticles secreted on these 

 processes are the fine hairs of the insect's wing (fig. 37). 



Other cells, again, on the anterior part of the wing 

 lengthen greatly, and extending beyond the surface of the 

 wing form bristles. Greatly hypertrophied cells on the hind 

 wings produce the clinging hooks (fig 38). 



Towards the end of pupal life the greatly folded cells 

 have lost the whole of their cytoplasm, this having become 

 transferred, api^arently, into the thin chitinous cuticle. The 

 cell walls, however, are still visible, as are also the cell nuclei; 

 some of these, indeed, show no visible signs of degeneration ; 

 others, however, are distinctly abnormal, having lost prac- 

 tically the whole of their chromatin contents (fig. 37). By 

 the time the insect emerges from the pupa all the nuclei have 



