626 THE AMERICAN NATURALIST. ` (VoL. XXXVI. 
DEVELOPMENT OF THE WING. 
Some time before the larva spins its cocoon the wing 
buds begin to appear, showing through the thin skin. These 
form a very convenient index of the age of the larve. Fig. 12 
represents a longitudinal section through a wing bud at an 


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Pm 
| j TAM 
Fic. 13. — Section of older wing. 4, basement membrane; c, cuticle; 7, trachea; 7, leucocytes. 


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bm 



early stage, before it is clearly visible from the outside. The 
two layers of closely packed cells have not yet united. The first 
formation of the trachez is also shown. 
The next stage figured is much later, when the two layers 
have united, leaving large openings through which pass the 
trachez, now fully developed (Fig. 1 3). Leucocytes are found 
frequently in these openings. The great stretching of the 
wing tissue by its growth both longitudinally and laterally has 
drawn the upper part of the cells into slender processes, all of 

Fic. 14. — Section of older larval wing. 5», basement membrane ; c, cuticle; 
“ £, trachea; 7, leucocytes. 
which join the basement membrane, separating the two cell 
layers. Many of the nuclei have been drawn up by the strain 
from their normal position along the layer of chitin, so that in 
some cases they nearly touch the basement membrane. The 
subsequent growth of the wing is nearly all in a longitudinal 
direction. 
