78 THE ENTOMOLOGIST. 



Second experiment loitli napi. — A number of the pupae were 

 placed in the refrigerator at approximately + 9° C. immediately 

 after pupation. 



A. Four of the same, placed in on June 28th, remained there 

 from June 29th to July 23rd, generally at 9° C. ; the temperature, 

 however, fluctuated between 7 and 11° C, but only rarely reached 

 either extreme. None of these pupae emerged in the refrigerator, 

 although they remained there 24-25 days ; and the longest pupal 

 period of the pupae of lot A, kept in the same refrigerator, 

 amounted to 26 days, the least 11 days, and with the majority 

 less than 20 days. All 4 pupae were still, on July 23rd, of a 

 beautiful green, as immediately after pupation, i. e. the colours 

 of the butterfly were not yet formed. After they had first been 

 brought for some hours into a room at 22° C, and into the 

 incubator at 30-31° C, they developed rapidly, and emerged 

 after 3 days (on July 26th and 27th), all four as not very sharply 

 defined tvinter forms (the powdering of the veins of the under 

 side of the hind wings feebler than usual). 



B. Twelve recently-formed pupae and 2 larvae, spun up ready 

 for pupation, were placed in the refrigerator on July 2nd, at an 

 average of + 9° C. These also remained unchanged in appear- 

 ance, partly green, partly straw-yellow, up to July 20th. On 

 Aug. 18th a male emerged as a decided winter form ; all the rest 

 of the pupae hybernated in a cold room, and emerged in the 

 spring of 1888, all as pronounced ivinter forms, viz. :— 



2 males on April 3rd. 

 1 male on April 22nd. 

 1 male on May 10th. 

 1 female on May 20th. 

 1 female on May 23rd. 



1 female on May 27th. 



2 females on June 2nd. 

 1 female on June 7th. 

 1 female on June 26th. 



11 butterflies. 



Results of the experiments with Pieris napi. — The first experi- 

 ment shows, in the first place, that low temperature no longer 

 brings about the transformation into the winter form, if the 

 pupae are first subjected to it a short time before their emergence. 

 But it proves further, what was already known for other season- 

 ally-dimorphic species, through many observers, Edwards, Merri- 

 field, myself, and others, that there are individuals which cannot 

 be forced by heat. A small portion of the pupae (12 specimens) 

 did not respond to the heat of the incubator, but hybernated, 

 and then produced the winter form. 



The second experiment proves once more, that the decision as 



