ON SEXUAL DIMORPHISM IN BUTTKKFLIKS. 90 



Comments on Dr. F. A. Dixey^ s paper on Seasonal Dimorphism 

 in Butterfiies and an Attempt to explain the Jrregnlarities 

 referred to *. By C. N. Barker, F.E.S. 



Seasonal Dimorphism amon^- butterflies, in those countries 

 that })ossess well-defined wet and dry seasons, is a fact which 

 has been proved again and again by the actual breeding ot* 

 one form from eggs laid by another form ; in many cases of 

 a strikingly different appearance. Perhaps the most extra- 

 ordinary example yet known to us is the breeding, by 

 Mr. G. A. K. Marshall in the year 1898, of Precis sesamus. 

 Trim., from eggs laid by P. o eta via natal en sis, Stand.; 

 forms so unlike as to bear no resemblance to one another. 

 The influences that produce seasonal dimorphism in butter- 

 flies are not, however, the same in all parts of the world. In 

 Europe and other cold temperate regions, the experiments 

 and researches of Standfuss^ Weissmann, Merrifield and 

 others^ have proved tliat humidity has little or no influence 

 in producing variation, temperature alone, or chiefly at any 

 rate, being responsible. In semi-tropical or tropical countries, 

 in which regular wet and dry seasons recur, humidity, or the 

 lack of it, appears to be the principal exciting cause. It is, 

 to my mind, however, exceedingly probable that the great 

 differences of temperature between day and night, with 

 accompanying dews, may have considerable influence in 

 determining results by reaction upon the pupa. The greater 

 or lesser degree of the?e factors, during the critical pupal 

 period, may account for the appearance of the many 

 transitional forms intermediate between the extreme wet and 

 dry phases. 



In some tropical countries rain prevails more or less the 

 year through, as for instance in N.E. Sumatra, in some parts 

 of West and Central Africa, in Singapore, and in some 

 districts of Ceylon which have poorly defined seasons. 

 In such countries it is, as might be expected, highly 

 exceptional to meet with butterflies wearing the dry season 



* Transactions of the Entomological Society, 1902, p. 189. 



7* 



