No. 376.] THE WINGS OF INSECTS. 24I 
unsolved, although it has been attacked by such writers as 
Hagen, McLachlan, and Kolbe. 
But when it is approached by the ontogenetic method the 
difficulties vanish, and it is hardly necessary to do more for its 
solution than to refer to the accompanying figures representing 
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Fic. 11. — Psocus, fore wing of a nymph. 
three stages in the development of the fore wing of Psocus 
venosus. When one understands this wing, the working out of 
the homologies in the hind wing, which is more reduced, and 
in the wings of other genera is a comparatively simple matter. 
Fig. 11 represents the wing of a nymph which was not yet full 
grown. The lettering of the figure indicates the homologies 
2 
Fic. 12.— Psocus, fore wing of a full-grown nymph. 
of the tracheæ. The formation of the wing-veins has begun, 
but in most cases the outlines of these are vague. It will be 
observed that the wing is much smaller than the enveloping 
sheath. 
Fig. 12 represents the wing of a full-grown nymph. Here 
the forming veins are much more definite in outline, and there 
is no difficulty in tracing the venation of the adult wing. The 
