a 
858 THE AMERICAN NATURALIST.  [Vot. XXXIII. 
ontogeny, we desire explicitly to state that we believe these 
changes to accompany certain conditions under which cells 
are placed, rather than any particular stages. 
IV. THE TRACHEZ AND THE HYPODERMIS. 
After discussing these two separately, there remain a few 
interesting features of their correlated behavior to be noticed. 
In all insect wings the two plates of hypodermis constituting 
the wing fold are at first separate, z.e., not fused internally. At 
the time when the trachez enter the fold the two layers become 
approximated along lines midway between the trachez, result- 
ing in actual fusion of the internal ends of the cells. We 
have already shown in Psocus! (and have seen in several other 
insects) the external evidences of the gradual lateral extension 
of the fused area to delimit definitely the channel through which 
the tracheæ pass. The term “cuticular thickenings ” has been 
used hitherto to designate the pale bands along the tracheæ, 
and the veins to be formed here will be, of course, cuticular 
thickenings ; but until the veins are formed (and this does not 
occur until the final molting) the term is inaccurate and mis- 
leading. A glance at Fig. 84, A, will discover that the hypo- 
dermis is in the earlier stages actually thinner here than 
elsewhere. It is only at transformation to the imago that 
the cells become aggregated about these channels and form 
there the dense chitine of the veins. The pale color of the 
bands, indicating the extent of the vein cavities when viewed 
by transmitted light, is doubtless due to the fact that the 
hemolymph filling these cavities is more translucent than the 
hypodermal tissue which completely fills the wing elsewhere. 
But, returning to the earlier stages, we have seen that, in 
wings: developing externally, the hypodermis encloses the 
tracheze in channels which ultimately become veins. It is 
now to be noted that there are often channels present which 
do not contain tracheze. This is oftenest true of two large 
channels at the lateral margins of the wing (Fig. 84, A). Of 
these the costal remains abundantly lined with cells, which 
1 American Naturalist, vol. xxxii, p. 241, Figs. 11, 12. 
