204. Rev. G. A. Crawshay on the 
for the larva than the natural sickly tree standing in a 
plantation, in which the sap must have been gradually 
failing before the insect will attack it. 
The reason for this is that, in a tree of luxuriant growth, 
recently felled, the bark is full of sap, which, because it 
has ceased to flow, admits of the larva advancing in it 
without being smothered and so thriving. 
I have never found abnormally large individuals in a 
standing tree, but in a few instances in felled timber I 
have known the larva attain the size of 24 mm. and the 
brood average only a little less, while, under less favourable 
conditions, they frequently occur as small as 10 mm. 
VARIATION: (2) IN CoLour.—This is marked in the femora 
especially, some series in my possession embracing all intergradations 
of colour from red to black. What governs the variation I have 
failed to discover. 
At first I was incltned to seek the causes in the condition of the 
food-supply. It seemed possible that abundant and sappy bast 
might account for the black colour, and the scanty and drier food 
the red. This supposition was based on the fact of a very long series 
exclusively of the black form having been bred in sappy food. How- 
ever, experiment disproved this so far as any visible change taking 
place in the imago in one generation, for I subsequently reared series 
of the black form from logs in which the sap had been exhausted by 
a previous brood. 
Nor does temperature account for the variation, for, in the same 
glass tube and subject to the same temperature, I have reared both 
forms at the same time. 
Where the red-legged form comes from ; whether it is gradually 
asserting itself over the black, which, at present, largely predominates 
in Britain, or the black over the red, are interesting problems which 
will repay investigation. The following data bear on this subject. 
(1) Two gf and two 92, type form, taken from their 
pupa cells in Larix europea containing imagines with 
femora of all intergradations of colour from red to black, 
mingled, were isolated on a 4-ft. log of the same conifer 
on May 18th and 24th, 1906, and produced between 
August 13th, 1906, and May 1907, 65 of the type form 
and 31 var. 0. 
(2) Two $f, type form, and two 9, var. b, selected and 
treated the same way, May 18th and 25th, 1906, pro- 
duced between August 13th, 1906, and May 1907, 69 type 
form and 71 var. b. 
