Structure of the Wings of Butterflies. 359 
Lepidoptera exhibits an advance upon that of the fore- 
wings as regards simplification. Similarly an examina- 
tion proves that the change of the median series is 
almost invariably accentuated, however slightly, in the 
secondaries. A scrutiny of the course which the three 
branches of the media take necessitates a clear recogni- 
tion of the primitive generalized condition and arrange- 
ment of these veins. This is illustrated again by 
Hepialus as well as by the position of the veins in the 
ante-imaginal stage.* 
The position, so far as the middle branch (vein iv) is 
concerned, is a median one, and those forms in a group 
in which this is retained, that is, in which the middle 
branch is equidistant from its two companions, are 
clearly the most generalized. 
We are now concerned with the shifting of the middle 
branch from its original median position. Evidently, 
as a comparison of the entire series of butterflies and 
moths shows, it may move in one of two opposite 
directions. Sometimes it moves towards the radius, and 
sometimes it is attracted by the cubitus. The amount 
of its movement in either direction is the test of the 
specialization of the insect in this particular, just as the 
extent of the absorption of the radial veins is there the 
measure of progress. ‘Thus, this movement of the 
median branches tends to break up the series, with the 
object of dividing the residue between the radius and 
cubitus. In this distribution of the former property of 
the media, the upper branch (vein iv,) from its prox- 
imity falls generally and naturally to the share of the 
radius, and the lower branch (vein ivs) as naturally to the 
share of the cubitus. But a contest arises as to the 
middle branch (veinivz). As long as it keeps its original 
position, it sustains the relative integrity of the median 
series. When it yields in either direction, it abandons 
its primitive character, but in thus yieiding it preserves 
its own existence. In some groups it has resisted the 
attraction of the dominant veins of the present lepido- 
* “Dr, Sharp points out that, although the trachex in the pupal 
wings have much the same arrangement as the nervures in the 
imagines, the trachez are not changed into nervures ; but that the 
latter probably have their origin from string-like structures, which 
run near the trachez, called Semper’s rods.”—Ent. Rec., 1896, p. 112. 
