130 MIMICRY AND VARIATION [ch. 



In this connection a few words on another source 

 of variation may not be out of place. The patterns of 

 butterflies are often very sensitive to changes in the 

 conditions to which they are exposed during later 

 larval and pupal life. Many moths and butterflies in 

 temperate climates are double brooded. The eggs laid 

 by the late summer brood hatch out, hibernate in the 

 larval or pupal state, and emerge in the foUowing 

 spring. This spring brood produces the summer brood 

 during the same year. In these cases it often happens 

 that the two broods differ in appearance from one 

 another, a phenomenon to which the term "Seasonal 

 Dimorphism ' ' has been applied. A well-marked instance 

 is that of the little European Vanessid, Araschnia 

 levana. The so-called levana form which emerges in 

 the spring is a small black and orange-brown butterfly 

 (PI. VI, fig. 10). From the eggs laid by this brood 

 is produced another brood which emerges later on in 

 the summer, and is, from its very different appearance, 

 distinguished as the prorsa form (PI. VI, fig. 9). 

 It is very much darker than the spring form and is 

 characterised by white bands across the wings. The 

 eggs laid by the prorsa form give rise to the levana 

 form which emerges in the following spring. It has 

 been shewn by various workers, and more especially 

 by the extensive experiments of Merrifield^, that the 

 appearance of the levana or the prorsa form from any 

 batch of eggs, whether laid by prorsa or levana, is 

 dependent upon the conditions of temperature under 



^ /"■ Congr. Internal. d'Entom., Bruxelles 1911. 



