VI. SCENT-ORGANS OF ANTIRRHAEA 
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resemble the scent-scales of many other Lepidoptera. The shape of the 
scales on the smaller spot (PL E, Fig. 15) do not differ greatly from the 
ordinary incubi (Fig. 13 A) of the adjoining parts of the wing ; those of 
the larger spot (Fig. 16) are, on the contrary, so narrow as to be almost 
mistaken for hairs. The latter are 0T6 mm. in length ; those of the 
smaller spot 0T3 mm., by 0*025 to 0*03 broad. 
After removing the scales, the spots become much more conspicuous 
than before, differing not only by the amount of their opacity, but also 
by their horn-like colour, rather pale in the smaller spot, but darker at 
the base and still somewhat dark in the prolongation of the larger. The 
smaller spot contains the ramifications of a few slender air-vessels, 
springing from the two [internal] nervures bounding the spot. In 
the larger spot these air-vessels attain a much greater, and sometimes a 
truly enormous, development, although different individuals vary greatly 
in this respect. The majority of the air-vessels which run through this 
spot are more or less dilated at their bases, and tortuous, resembling 
varicose veins (PI. E, Fig. 8). In some individuals the dilatation of the air- 
vessels extends so far that they occupy nearly the whole area of the scent- 
spot, losing at the same time their capillary ramifications (Fig. 9). In 
Fig. 7 this same degree of varicosity is exhibited by the air-vessels between 
the two branches (6 and 7) of the subcostal nervure, and to the left of 
the upper discocellular, whereas on the right of the latter nervure there 
are various forms transitional between the ordinary air-vessels and those 
which are excessively dilated and destitute of capillary branches. 
The variability of the air-vessels also extends, though in a much less 
degree, to the nervures which traverse the spots and from which these air- 
vessels arise. The two branches of the subcostal nervure (6 and 7) may be 
either nearly parallel (PI. E, Figs. 4 and 8), or convergent (Figs. 7 and 9), 
sometimes almost meeting. In the females on the other hand (Figs. 3 
and 6) these branches diverge from their point of origin. The upper 
discocellular nervure traverses the spot either in a straight line (Fig. 8) 
or in a slight curve (Fig. 9). 
This extraordinary variability of the air-vessels of the scent-patches 
affords an excellent example of a law admirably discussed by Darwin 
(Origin of Species , 4th Ed. p. 177) that “ a part developed in any species 
in an extraordinary degree or manner in comparison with the same part 
in allied species, tends to be highly variable.” Although the scent-patch 
with its greatly modified veins and air-vessels is not confined exclusively to 
Antirrhaea archaea , it appears to be restricted to the three allied species 
united by Butler in the genus AnchiphleMa. 
Another notable point is the separation of the scent-organ into two 
parts, for it appears that to the hind-wings is reserved the production, to 
the fore-wings the emission and exhalation of the scent which is intended 
to attract the amorous females. As to the dark spot on the anterior 
wings, it seems to be a rudimentary scent-organ, which, though it is as 
