84 
Leech (Ann. Mac). Nat. Hist. (6), xix, p. 570) considers his from 
Amur to agree well with the European, while blackened races exist in 
Western China (var. chinensis), and perhaps (cf. Fuchs, Jahrb. IS ass. 
Ver. Nat., liii., p. 62) about Krasnoiarsk. 
DIFFERENTIATION OF T. TRIDENS AND T. PSI IN THE 
IMAGINAL STAGE. 
(Read March 20th, 1906, by T. A. CHAPMAN, M.D.) 
When our invaluable Secretary, Mr. Bell, asked me in his most 
Napoleonic manner to open this discussion, I saw escape was im¬ 
possible, so at once acquiesced with the most cordial alacrity my 
innate churlishness permitted me to assume. A few years ago I paid 
some attention to the early stages of the Acronyctas, and so, though I 
possess the greatest ignorance of the imagines, nobody will give me 
credit for it. I reflected, however, that to open a good discussion, an 
exhaustive paper is the last thing to be desired, and that the intro¬ 
ductory remarks are more effective if they are stuffed full of omissions 
and seasoned with a few glaring mistakes. I have felt it to be 
necessary, however, out of respect for the Society, to try and get 
together a few observations hearing on the subject. 
There are certainly a good many differences between psi and 
tridens in the imaginal state; I will begin, however, by asserting that 
there are no differences whatever. If you ask me to lay down any 
characters by which someone unfamiliar with these two species may 
be able to say with absolute certainty to which species a specimen he 
has just taken belongs, I confess absolute inability to do so. There is 
no one character that always holds good, although it may furnish a 
correct conclusion in a large proportion of cases. A combination of 
characters, no doubt, will fail less frequently; nevertheless, it will fail 
sufficiently often to prevent entire dependence on it. 
It is, notwithstanding, unquestionable that, if the specimen he a 
male, an examination of the ancillary appendages leaves no shadow of 
doubt as to which species the specimen belongs to. 
The inner chitinous processes of the clasps are abundantly different, 
and though there are considerable variations in each species, they in 
no way approximate the two forms. I presume both species derive 
their names from the “ dagger ” on the wings, a three-pronged mark¬ 
ing that is like the letter psi (f) or like a trident, but it is curious 
that tridens may be distinguished from psi by usually having, in the 
pupa, three spines on a certain portion of the cremaster, instead of 
four, and in these ancillary appendages tridens is furnished with three 
branches to the inner spine of the clasps, and psi has only two. 
There is another male character that is less to be depended on, 
although in some of my Hereford broods it was practically without 
