632 
APPENDIX 
femur is about 6 mm. long in the female, 7 mm. in the male, the tibiae 
5 mm. in both sexes, the first tarsal joint 3 mm. in the female, 4J mm. in 
the male, the other tarsal joints 4J mm. in both sexes. The mobility of 
the femur, evidently hindered by the scent-organ, may in some measure 
be compensated by the increased length of the first tarsal joint. 
The femur of the male (PI. D, Fig. 5), with a breadth of 2J mm., slightly 
more than one-third of its length (7 mm.), is at the same time greatly 
flattened, so that the dorsal and ventral surfaces approach closely and 
almost touch. The ventral surface is slightly convex, the dorsal concave. 
The scent-organ, which occupies the concave surface of the femur, is 
composed of an interior, strongly odoriferous, and an exterior protective 
part. The former is made up of innumerable very large scent-scales (PI. D, 
Figs. 9 b and 11), entirely covering the dorsal surface of the femur : they 
have the form of a narrow ribbon about 0’03 mm. wide and 2-3 mm. long, 
but longer still on the anterior or superior margin of the femur. The apex 
of each scale is expanded into a larger or smaller oval club (about 0*06 mm. 
wide and 0*25 mm. long). 
Many of the scent-scales adhere together in consequence of this ter¬ 
minal enlargement, and the surface of the compact mass thus formed is 
necessarily larger than its base, that is, than the area of the femur from 
which it springs (PL D, Fig. 9). When detached from the femur, the scent- 
scales have an appearance like that of some kinds of cotton removed from 
the capsule, forming a fluffy mass of incredible size : it seems impossible, 
indeed, that such a volume could have been contained in so limited a 
space. The scent-scales are everywhere protected and covered by an 
edging of large scales and hairs, inserted all round them on the margins 
of the femur. The innermost scales of this border, which lie immediately 
upon the scent-scales (PI. D, Fig. 12 6) are ovate, usually about 1*5-2 mm. 
in length by 0*6-1 *2 broad. The scales outside those just described have 
their base prolonged into a sort of petiole (Fig. 12 a ), outside these again 
the petiole becomes thinner and thinner, the club, at the same time, 
narrower and narrower (Fig. 12 c), until finally the scales are insensibly 
transformed into hairs (Fig. 12 d ), which betray their origin solely by a 
slight enlargement of the apex. These hairs, forming the outer layer of 
the covering of the scent-scales (Fig. 9 d ), are longest on the anterior or 
superior margin of the femur, and especially long at the base of that 
margin, where they exceed the length of the femur itself. 
There are thus in the family of the Erebidae some species whose males 
are provided with scent-organs on the tibiae of the hind legs; others 
possessing these organs on the tibiae of the front legs, others on the 
femora of the middle legs, and finally others whose legs show no trace of 
any apparatus which could serve as a scent-organ. From this we may 
conclude that the organs in question were not inherited from a common 
ancestor, but acquired later by the various species which now enjoy these 
attractive sexual possessions. 
