between lepidopterous larve and thew surroundings. 857 
experiments, undertaken in order to attempt to ascertain 
the susceptible stages of this highly-sensitive larva. 
The experiment started on May 10th, when the young 
larvee were first fed upon the leaves of Popwlus nigra, all 
dark twigs and branches being at first rigidly excluded. 
It must be remarked, however, that for a few days after 
May 10th the leaves had only just expanded and were 
somewhat brownish. Various sets of larvee were withdrawn 
from the stock, and the transference experiments were then 
conducted in the following manner. All measurements 
were taken when the larve were stretched and straight. 
A convenient summary of the results of the following 
experiments will be found in the table on page 319. 
A. A. betularia. 
May 16th. Twenty betularia larvee at the end of the 1st 
stage, and nearly all changing the first skin, and 5°0 mm. 
long, were transferred from green to black surroundings 
(the twigs of the Turkish oak). Up to this date the green 
leaves of P. nagra upon which they had been placed on 
May 10th were somewhat brownish, because the buds had 
only just opened. 
May 20th. The larvee were examined from time to time 
between the 16th and this date, and had always been found 
upon the leaves and never upon the twigs. On the 20th 
every single larva was found upon the leaves. They even 
avoided the stem of the food-plant. Ten larve changing 
the 2nd skin, and 8°5 mm. long, were re-transferred into 
green surroundings (A!). The 10 remaining larve had 
attained various degrees of development in the 2nd stage, 
3 being at the end of it, but not yet changing the 2nd 
skin. From this date to the 29th these 10 larve were fed 
upon Balsam Poplar, but from the latter date onwards 
upon P. nigra. 
May 25th. Only 1 on the twigs, the rest on the leaves. 
One larva was changing the 2nd skin, and none had yet 
entered the 3rd stage. 
May 29th. Two larvee on the twigs, and both these 
were changing the 3rd skin. Of the rest 1 was changing 
the 3rd skin and 12°5 mm. long, and 2 had just changed 
it. These 5 were re-transferred to green surroundings 
(A?). The remaining 5 were in the 3rd stage, 4 nearly at 
the end of it, and 1 very small. 
May 31st. Only 1 on twigs. Four changing 3rd skin, 
