Distribution of Moths of the Sub-family Bistoninae. 275 
with those of the Alps and of the Baltic Ice. After the ice had 
finally vanished and the Post-glacial ‘ Dryas’ period had 
yielded successively to those of birch and pine, and these, in 
turn, had given place to the oak period the species came forth 
with many another Pontic refugee spreading westward and 
northward along the valleys of the Vistula, Oder and Elbe 
as the oaks advanced. Bound by no iron climatic barrier 
such as determined (and determines) the continued existence 
of P. pomonaria, the oaks went far ahead of the insect, for this 
reached its maximum westerly extension just east of Paris. 
Here the proximity of the Gulf stream, with its warmth, 
stopped it short, and its course was now southward down the 
colder parts of the Rhone Valley whence it passed into the 
valleys of Switzerland and the Piedmont until a milder climate 
once more forbade its occupation of new territory. 
Similarly, it passed northward into Scandinavia, Finland 
and Lapland, only bound in its northward career by the failure 
of oak and its powers of adapting itself to new foods. South- 
ward, but little progress could be made as milder winters stepped 
in; eastward it was more fortunate, a gradual advance 
being made, slow it is true, on account of the lack of food, but 
nevertheless sure, for the insect penetrated Asia across the 
Uralsk far into the steppes of the Kirghiz where, however, 
altered food and steppe conditions have transformed it into 
the elusive species called P. liquidaria—evidently but little 
more than a specialised or local race of P. pomonaria. 
Next we take up P. isabellae, the youngest species of the 
genus. The study of the geographical distribution of this species 
presents us with a very pretty problem as to the period of its 
origin—a problem that the specialised nature of its food very 
neatly solves. Unlike its congeners, it is rigidly attached to 
a special food and that special food is the common larch (Larix 
decidua) ; it absolutely rejects all other foods. The connection 
between the insect and the tree is much closer than this would 
suggest. Its larvae in their early stages mimic larch needles 
and in their later ones they imitate the curious markings 
of a larch twig. In addition, the female has acquired the 
curious instinct of climbing the larches and laying its ova 
under the scales of the cones of the previous year. Obviously, 
therefore, if we can elucidate the history of the larch in Europe 
that of the insect is known. 
The genus Larix comprises within its limits nine to thirteen 
species depending upon the value one attaches to certain forms. 
Adopting the minimum view, we have the following species :— 
Larix decidua (europaea) N. Asia, Central Europe, but 
absent from Scandinavia, France, Spain, Apennines, British 
Islands etc. 
Larix ledebour1a.—Siberia. 
1916 Sept. 1. 
