SEASONAL DIMORPHISM OF LEPIDOPTERA. Ill 



The sixth experiment, in which, from pupae of the third 

 brood at 27-28° C, 14 prorsa emerged. 



The seventh experiment, in which, from pupae of the third 

 brood at the room temperature, three prorsa emerged at the 

 beginning of September. 



But one dare not assume from these experiments that every 

 pupa of the third brood assumes the prorsa-iovm, in case it is 

 brought into a high temperature on its pupation. The following 

 facts speak to the contrary : — 



In the first experiment only one prorsa emerged from about 

 60 pupae, which were aJl kept at the same temperature, i.e. in a 

 warmed room. In the third experiment only eight prorsa 

 emerged in the incubator out of 65 pupae, but there were besides 

 seven j^rorsa of the same brood, which had pupated in the room. 

 In the fourth experiment I bred five prorsa butterflies out of 241 

 pupae at the ordinary temperature at the end of August ; all the 

 rest hybernated. 



These facts admit of no other explanation, as it appears to 

 me, than the assumption, that the tendency of the pupce to the 

 development of the prorsa-/or??i is of different strength with the 

 different individuals of this third brood. It might be objected to 

 this conclusion, that the temperature of the incubator was fluctu- 

 ating, and that the different individuals might have been sub- 

 jected to unequal temperatures just at that period of develop- 

 ment in which the decision is given, whether the prorsa or 

 levana rudiment is to be active. On the other hand, it is proved 

 by Experiments 5 and 6, in both of which the temperature only 

 fluctuated very slightly, and in which it is equally certain, that 

 the critical time of this high temperature was struck while the 

 larvae had been reared in the heat, and pupation took place in 

 the incubator itself. Yet in both experiments together 35 pupae 

 hybernated uninfluenced by the heat, and eight of them produced 

 the levana-ioYm. in the spring. 



This conclusion of the different individual tendency towards 

 the assumption of the summer form might be contrasted with 

 my experiment of the year 1869 ; since then all the pupae of the 

 third brood assumed the summer form under the influence of 

 the exceptionally hot July sun. But first there were only 18 

 specimens ; secondly, there were five porima among them, i. e. 

 intermediate forms between levana and prorsa ; and thirdly, it is 

 still indeed dependent on the strength and duration of the 

 warmth excitant whether the prorsa rudiment i: active ; and it 

 is conceivable that, with very great heat, such as then prevailed, 

 this is always the case. Such a high temperature as a hot 

 summer brings about in the open, cannot be produced artificially, 

 without running the risk of destroying the pupae by an atmo- 

 sphere which is too dry or too damp, or by the prevalence of 

 vegetable parasites. After all it is never to be forgotten that we 



