Seasonal Dimorphism. 



By Dr. ¥. A. Dixey, M.A., F.ll.S.—Read April 8th, 1915. 



In man}^ parts of Europe there may be commonly seen, during 

 the months of spring, a small brightly-coloured butterfly whose 

 variegated mottling of tawny and black somewhat recalls the aspect 

 of our familiar "small tortoise-shell," to which indeed it is closely 

 related. Later on in the year the same localities are enlivened by 

 the presence of a larger butterfly of very different appearance. This 

 second insect shows a general ground-colour of very dark brown 

 with an obscure mottling of a still deeper shade, the total effect 

 being nearly black. Crossing the dark area of fore- and hindwmg 

 is a broken band of cream-colour or white ; and this combination of 

 dark ground-colour with pale median band gives the butterfly 

 an aspect not unlike that of our British " white admiral." Of 

 these two butterflies, the first received from Linnaeus the specific 

 name levana, the second was called by him }>yoriia. The generic 

 name given to these forms by entomologists at the present day is 

 Araschnia, the two insects being therefore known as Araschnia 

 levana and Araschnia ]>rorsa respectively. 



Araschnia levana, the bright little black-and-tawny butterfly, after 

 flying about for some time in the spring sunshine, deposits eggs, 

 just as our small tortoise-shell does, on plants of the common sting- 

 ing-nettle. From these eggs emerges a brood of spiny caterpillars, 

 which feed upon the stinging-nettle and in due course of time turn 

 into chrysalises. About the month of July these chrysalises disclose 

 the perfect insect, w^hich one would naturally expect to be like the 

 parent butterfly. But strange to say, instead of the brightly- 

 mottled Arasclinia levana, what does come out of these chrysalises 

 is the dark, pale-banded Araschnia prorsa. The caterpillars hatched 

 from the eggs laid by prorsa feed up and pupate during the summer. 

 Now these pupte resulting from the eggs laid by prorsa may behave 

 in either of two ways. They may either produce the perfect insect 

 before the end of the warm weather, or they may last on as pupje 

 throughout the winter, not giving rise to the perfect butterfly until 

 the succeeding spring. In the former case, the butterflies when 

 they appear are of the prorsa form, like their parents; in the latter 

 case they emerge, not as prorsa, but as the brightly-variegated form 

 levana. We thus see that one and the same kind of butterfly may 

 exist in two forms so difierent in appearance from one another that 

 they may be, and indeed have been by good authorities, considered 



