418 Mr. 'Poulton' 8 further experiments upon 



Effect of cold upon duration of stages and colours of pupce. 

 Experiment 93. 1892. 



A-lthough the experiment described below was unsuc- 

 cessful, I think the method employed may be useful if 

 lower temperatures are obtained, and I therefore give a 

 brief description of it. 



It has already been pointed out that larvae (at any 

 rate of V. urticce) pass through longer preparatory stages 

 when they produce dark pupse. I was therefore anxious 

 to test whether dark pupae would be formed in bright 

 surroundings if the stages were protracted by some 

 other cause, such as cold. A number of similar bottles 

 were lined with black and with gilt paper, in the form of 

 a back-ground covering half the circumference and a 

 roof. The latter was made by covering the bottom, the 

 bottle being inverted when in use. Some of these 

 bottles were sunk beneath the surface of water in a 

 large glass vessel, the water being constantly changed 

 so as to maintain a uniform temperature of 16° C. This 

 was but little lower than the air of the room, but the 

 high specific heat of water would cause it to produce 

 more effect upon the larvae. If, however, the temperature 

 was sufficient, the effect would be beneficial rather than 

 the reverse, and this was probably the case. It would 

 be well to repeat the experiment, using ice to obtain the 

 lowest temperature compatible with the process of 

 pupation. This I was unable to carry out last summer, 

 being much away from home at the time. 



The results only serve to confirm those already 

 obtained by the use of gilt and black surfaces in a strong 

 light. Four larvae belonging to the same company were 

 placed in each bottle. The pupae were compared Aug. 

 19, 1892. 



In air, two black-lined bottles contained 7 dark pupae, 

 but they were dead, and it was impossible to state their 

 degrees of colour with precision ; one gilt bottle con- 

 tained 4 bright green pupae, evidently (5)s. 



In water, two black-lined bottles contained 4 dark 

 pupae, like those in air ; while five gilt bottles contained 

 11 green pupae, evidently (5)s. 



