94 THE ENTOMOLOGIST S RECORD. 
and is not distinct enough. Tutt adds, (1) The dark shade on the 
costa between the stigmata differs in the two species. (2) The trans- 
verse line parallel to the hind margin is distinctly doubla in tridens, 
but the inner edge is to a great extent lost in psi. (8) Lower part of 
this line generally nearly vertical or slightly turned back in psi. (4) 
Line in the fringes, inner half ochreous and outer half whitish in 
tridens, in pst the inner half is not ochreous but darker gray than outer. 
(5) Hindwing of psi darker and more or less traces of the dark trans- 
verse line, which is rarely, if ever, developed in tridens. (6) A. pst has 
the black lines running through the fringes better developed. Here 
again the points of difference are all put forward with qualifying terms 
as to universal application, and must be taken for what they are worth. 
In 1906 Dr. Chapman read a short paper at the City of London 
Entomological Society on this question (Trans. City Lond. Ent. Soc.) 
of which the following is ashortsummary. Although his own experi- 
ence and breeding of the two species has enabled the Doctor to separate 
them with practical certainty, he expresses his “ absolute inability to 
lay down any characters by which someone unfamiliar with these two 
species’ can do so. The following points are then dealt with. 
1. Colour of the hindwing.—“ In tridens the hindwings are without 
any dark scales, except in the actual hind margin, pst has dark shading 
along the veins in the spaces, and often has a dark central shade.” “A 
good many tridens resemble pst very much in this matter.” “A speci- 
men of pst with pure hindwings is certainly very rare.” <‘‘ A male 
specimen with quite white hindwings is almost certainly tridens, with 
very slight dark scaling is probably tridens, with moderately dark 
scaling is probably psi, and with very dark scaling is certainly pst.” 
2. Form of the wing.—“ Psi has altogether a broader wing.” ‘The 
measures I have compared are the length of the wing (from centre of 
thorax to apex) with the breadth (from anal angle to costa). This is 
quite a definite difference and can be seen without measurement.” But 
there is great difficulty in actual measurement as the results overlap. 
3. Difference in marking.—‘‘I have no hesitation in saying that 
the markings present no differences.” Yet ‘some are unquestionably 
very constant throughout whole broods of one or the other species; but 
then they may equally be found to be similarly constant in the other 
species.” ‘‘T'he separation of the marginal dots from the anal dagger 
in psi, and their junction, especially the upper one, with it, in tridens, 
is more constant than any other in the markings; still it has not 
infrequent exceptions.” 
4. Coloration.—‘ Psi is pure- black and white; tridens has red, 
green, brown, and yellow. The pale form of psi, with white pre-. 
dominating, is probably always unmistakable. So in trédens, when 
richly suffused with pink, brown, or olive.” ‘‘ Tridens very commonly 
has the interior of the orbicular stigma coloured, or definitely of a 
different tint from the rest of the wing; ps?, I think, almost always 
has it of the same colour as the rest of the wing.” ‘‘ Some dark speci- 
mens of psi have a series of pale patches down the hind margin. Itis 
never so pronounced in tridens.” ‘Still, all these matters of colour 
are, in fact, questions of degree rather than absolute difference.” 
Referring to his own and Mr. Tutt’s remarks in brit. Noct., quoted _ 
above, Dr. Chapman says, “‘ Whilst they are equally valid now as when 
they were written, they are open to the same observation ’—‘ that, 
