92 THE ENTOMOLOGIST. 
THE GENUS PdAICILOPSIS (Harrison). 
By J. W. H. Harrison, B.Sc. 
Part I.—Preuimrmnary REMARKS. 
As I have pointed out elsewhere, the ‘“‘ genus’”’ Biston, as 
represented in Staudinger’s ‘ Catalogue,’ isa very heterogeneous 
collection, comprising elements from no fewer than six distinct 
genera. These are :— 
Biston (Leach); type stratarius. 
Lycia (Hub.); type hirtaria. 
Ithysia (Hub.); type zonaria. 
Pecilopsis (Harrison) ; type pomonaria. 
Apocheima (H. 8.); type hispidaria. 
Microbiston (Stgr.); type lanarius (Ev.) (= tartaricus (Stgr.) ). 
It was originally my intention to take these genera in turn, 
and to deal with each of the species in all its stages in detail. 
The imagines of the genus Jthysia have already been discussed, 
and the treatment of the other forms postponed in order to give 
time for the completion of the life-histories of I[thysia grecaria, 
I. alpina, and I. italica, but it has been found impossible to rear 
(even in a hothouse) these exclusively Southern forms. In the 
meantime, however, I am glad to say that I have been able to 
secure, and describe at length, the early stages ofall of the species 
in the genus Pecilopsis, and therefore propose to complete my 
work in that section now. This genus was described in Lepid. 
Comp. fase. vii. p. 344, and I have but little to add to the 
description given there, except that one very important observa- 
tion has been made which justifies further my separation of 
these species from Ithysia. This is the fact that, whilst the 
chromosome number in Ithysia is 112, in Pecilopsis it is 56, and 
in Lycta 28. 
It has become imperative that I should take up this genus 
now, because I have discovered in the course of my studies that 
the Central European form that passes for P. lapponaria is not 
that insect at all, although, fortunately, our Scotch insect is so. 
Not only is this true, but, in addition, the two forms fall into two 
different sections of the genus, which contains four species form- 
ing two closely allied groups of two. These groups are :— 
The two species Pacilopsis lapponaria and P. rachele. 
The two species P. pomonaria and P. isabelle. 
The first group is probably Northern in its origin, for P. lap- 
ponaria ranges from Lapland to Livonia, and then reappears in 
Scotland; whilst P. rachele is widely distributed in America, 
from Montana and Manitoba along the Mackenzie Valley to 
Alaska. The other group is of Central European origin, pomo- 
naria having its headquarters in North Central Germany, but 
extending, although sparsely, to Eastern France, Scandinavia, 
