35 
exception. This is the colour of the hindwings. In tridens the hind- 
wings are without any dark scales, except in the actual hind margin, 
psi has dark shading along the veins in the spaces, and often has a 
dark “ central shade. Well, you will find a good many tridens that 
resemble psi very much in this matter, that have, in fact, more dark 
scaling than occurs in a fair proportion of psi. On the other hand, a 
specimen of psi with pure white hindwings is certainly very rare, 
but does occur. A male specimen, therefore, Avith quite white hind¬ 
wings, is almost, if not quite, certainly tridens, Avith very slight dark 
scaling is probably tridens, Avith moderately dark scaling is probably 
P s h aQ d Avith very dark scaling is nearly certainly psi. 
. There is one character which affects both sexes, and which, I 
think, is fairly distinctive, but then, nevertheless, there is some 
overlapping, and, what places a greater difficulty in the Avay, it is very 
difficult to define and apply. This is the form of the wing. Psi has, 
altogether, a broader Aving; I have tried to find some way of defining 
this and have measured some specimens. The measures I have 
compared are the length of the wing (from centre of thorax to apex) 
Avith the breadth (from anal angle to costa). In eight specimens 
taken as they came, I find, in tridens, the breadth is 40 per cent, of 
the length, Avhereas, in psi, it is 42 per cent. This is quite a definite 
difference and can be seen Avithout measurement, but it is quite, 
indeed, very much less than differences that may occur in measure¬ 
ments by different persons at different times, owing to Avant of precise 
agreement as to points of measurement, etc. It has, however, a 
further defect, and that is that among so few as these eight specimens 
of each, tridens varied from 39 per cent, to 42 per cent., and psi from 
41 per cent, to 44 per cent. I think the eye suggests that there is a 
difference in the relative lengths of the costa and inner margin, but I 
have failed to verify this by measurement. 
When AA r e consider markings, I have no hesitation in saying that 
the markings present no differences. I do not know how many 
distinctions in markings betAveen the tAVO species I hav r e not read 
or heard of, and of these some are unquestionably very constant 
throughout Avhole broods of one or the other species ; but then they 
may equally be found to be similarly constant in the other speeies. 
The point on the costa from which the outer line starts, the distance 
between the orbicular and reniform stigmata, the lengths and breadths 
of the se\ 7 eral branches of the seA^eral daggers, the marginal dots, etc., 
vary, I think, in practically the same Avay in both species. It must 
be admitted, hoAvever, that the separation of the marginal dots from 
the anal dagger in psi, and their junction, especially the upper one, 
Avith it, in tridens, is more constant than any other in the markings ; 
still, it has not infrequent exceptions. 
A more trustAvorthy character is coloration. Psi is pure black and 
white ; tridens has red, green, broAvn, and yellow. The pale form of 
psi, Avith white predominating, is probably ahvays unmistakable. So 
in tridens, Avhen richly suffused with pink, brown, or olive. On the 
other hand, it is difficult to feel sure that some darker specimens of 
psi hav r e not some brown tinting, and tridens does not ahvays present 
quite definitely an^ ohing but white and black. Tridens very commonly 
has the interior of the orbicular stigma coloured, or definitely of a 
different tint from the rest of the wfing; psi, I think, almost always 
