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APPENDIX 
but before passing on to discuss the biological significance of the sexual 
spots it will be well to describe briefly another organ, peculiar to the male 
sex, which appears to have hitherto escaped the attention of entomologists. 
If one strongly compresses the abdomen, there is everted on each side of 
the last segment a finger-shaped membranous tube with closed apex (PL A, 
Figs. 6 and 12), which is covered with dark hairs, erected as the tube passes 
out of the abdomen, and exhaling at the same time a somewhat strong 
odour in B. gilippus , and one less strong, though still perfectly distinct, in 
D. erippus —a difference which evidently depends upon the fact that the 
hairs are far more numerous, as well as thicker and longer, in the first- 
named species. On being withdrawn into the abdomen, the tube is turned 
outside in or introverted, so that the surface which was external becomes 
internal, forming a sheath or case round the hairs, which then appear to 
spring, in the form of a tuft, from the bottom of the tube. 
Such are the facts. It remains to discuss them. The wings of many 
species of butterflies bear, in the male sex only, scales of peculiar form, 
often collected together in well-defined patches, and in some cases concealed 
in furrows or folds of the wings—scales and patches which undoubtedly 
act as scent-organs. It appears probable that the modified scales concealed 
in the cavity of the sexual spot in D. erippus and gilippus serve, or may 
have served, the same purpose. It may perhaps be possible to find among 
the different species of Danais intermediate forms, linking the pockets of 
the American species with the patches which are seen on the hind-wings 
of the males of Amauris. 
Further, not only is there no perceptible scent exhaled from the wings 
of the male D. erippus and gilippus , but it appears extremely unlikely that 
such a function would be exercised by a cavity communicating with the air 
only by a narrow slit, and, in addition, having apparently no mechanism 
on the wing by means of which it could be opened; and as there are, at 
the extremity of the abdomen, organs whose scent-distributing functions 
cannot be doubted, it is natural to conjecture that the sexual spots of 
D. erippus and gilippus are scent-organs reduced to a rudimentary state by 
the development of the other organs at the apex of the abdomen, which 
better fulfil the same functions. We might cite in support of this conjec¬ 
ture certain analogous facts which exist in other families of butterflies. 
On the other hand, the blood in the walls of the sexual cavities—present 
in an amount that is rare in the wings of these insects—appears to forbid 
us to consider them as rudimentary organs, and the facts may perhaps be 
met by the supposition that the development of the wing-organs is in 
inverse ratio to that of the abdominal organs, and that they may show 
themselves sometimes more, sometimes less developed. Whether this is 
correct or not remains to be seen. 
In D. gilippus these organs, both of the wings and of the abdomen, are 
far larger than in D. erippus , in spite of the latter species being much the 
larger of the two. 
