II. SEXUAL SPOTS OE MALE DAXAIS 
617 
black colour is not due to the covering of ordinary scales only—for it 
persists when they are removed—but to the membrane itself, which at this 
part is both darker and of a firmer consistence. The sexual spot is 
elliptical in shape, with the major axis parallel to the nervure. It is much 
larger in the smaller species, viz. D. gilippus , being about 4 mm. long, by 
from 1*5 to 2 mm. wide; while in erippus it rarely exceeds 2 mm. in 
length, by 6 [0*6 is evidently intended] in width. The sexual spot is 
shaped like a leaflet of wood-sorrel [usually called “obcordate”],and forms, 
as Doubleday described in some species of his third group of the genus 
Danais , a kind of pocket, opening on the upper surface of the wing, where 
there is, on the posterior margin of the spot, a narrow slit occupying half, 
more or less, of its length. The lower wall of this pocket or cavity is 
formed by the actual membrane of the wing; the upper wall is separated 
from the lower, a little distance from the nervure, at a very acute angle. 
The free or posterior margin of this wall curves or rolls round towards the 
interior of this cavity, as is well seen in transverse sections (PI. A, Figs. 3 
and 9). 
I have noticed that in the living insect the free margin of the upper 
wall is pressed closely against the lower wall, thus closing the cavity on all 
sides ; yet it is easy to pass through the slit which separates the walls some 
slender object, as may best be understood by means of Figs. 3 and 9. The 
membrane of the wings of insects is composed, as is well known, of two 
layers which generally adhere together. These two layers exist also in the 
walls of the spot, or, to express it better, of the sexual cavity, but may 
easily be separated, and, in fresh specimens, usually have between them a 
good deal of blood. The external layer, as already stated, is tough 
[indurated or hardened], black, and covered with ordinary scales. The 
internal layer, which is much thinner than the other, presents a somewhat 
different aspect in our two species. 
In D. erippus it exhibits small circles of about 0‘01 mm. diameter, a 
little more transparent than the rest of the membrane. From the centre 
of each rises a straight hair, about 0*06 mm. long. The circles are placed 
in regular lines, about 0*03 to 0*06 mm. apart. Alternating with these 
circles are opaque grey scales, distinguished from the ordinary ones by 
their smaller size and by their shape (PI. A, Fig. 4). 
In D. gilippus (PI. A, Fig. 10) the circles are much closer together—so 
much so that in places they almostftouch: although more transparent than 
the rest of the membrane, they are less so than those of D. erippus. The 
hairs are wanting, but one sees in the centre of each circle a small spot, 
the last vestige which proves their former existence. The scales are far 
smaller than those of D. erippus , being about 0*04 mm. in length, as 
compared with 0*08. Probably these little scales are the “grey dust” 
found by Doubleday in certain other species of Danais. 
It is not possible to detect any odour arising from the wings of the 
males of either of the two species found in the Province of S, Catharina, 
