MAYER: DEVELOPMENT OF WING SCALES. 281 
acquire the mature coloration are the outer and costal edges of the wings 
and the nervures. 
2. I here present the first satisfactory proof of the fact, that the scales 
are formed from modified Aypodermis cells, and are therefore truly homol- 
ogous with the hairs,of Arthropods. This has been a matter of inference 
by Semper (57), Landois (71), Schäffer (89), and many others. 
3. Most of those hypodermis cells which do not contribute to the 
formation of the scales become elongated, stretching from one wall of 
the wing membrane to the opposite, with which they finally fuse ; thus 
it is that the two walls of the wing are bound together by a great num- 
ber of bundles of fibres derived from the hypodermis cells of both upper 
and lower walls. Dr. Mark observes that in some respects these fibres 
resemble the muscle fibres of many invertebrates, and he therefore 
suggests that at first they may be muscular, although they afterwards 
become tendonous in their nature. 
4. The membrane of the pupal wings exhibits two sets of corruga- 
tions, or foldings, one being parallel to the trend of the nervures, and 
the other at right angles to it， In either eross or longitudinal sections 
these corrugations appear as a regular series of ridges, and a single scale 
arises from the crest of each ridge. 
The expansion of tbe wings after emergence is caused by the pressure 
of the “blood ”or hemolymph within them, and is accompanied by a 
flattening out of the ridges. This pressure would naturally have the 
effect of distending the freshly emerged wing into a balloon-shaped bag, 
but the hypodermal fibres hold the upper and lower walls of the wing 
closely together, and so, instead of becoming a bulging sac, the wing be- 
comes a thin flat one, which has an area more than five hundred times 
that of the wing pad in the mature larva. 
5. Very large scales are found along the nervures and upon the outer 
edges of the wings in Danais plexippus. In faet, these scales are so 
large that, after the protoplasm has withdrawn from them, a single 
leucocyte enters each one. These leucocytes soon degenerate, and 
finally disintegrate, without, however, contributing directly to the pig- 
mentation of the scale. The fact that the leucocytes degenerate after 
entering the scales indicates that the hemolymph within the scale is 
not in a normal condition. 
6. After the protoplasm has withdrawn, and the scales are com- 
pletely formed, the nuclei of the cells which formed the scales often 
go through several amitotic divisions. This has been observed only 
in the case of Danais plexippus. 
