

JOURNAL OF VARIATION. 



No. 5. Vol. II. July 15th, 1891. 



MELANISM AND MELANOCHROISM IN BRITISH 

 LEPIDOPTERA. 



By J. W. TUTT, F.E.S. 

 {Continued from page 8o.) 



AVING now considered this subject from many points, 

 I think we may safely assume that the general cause 

 of melanism is to be sought for in meteorological in- 

 fluences, and that this cause must act in some way 

 on the larva. It is during the larval existence that the insect is 

 constitutionally strengthened or weakened, and that it is during 

 this period that some active exciting cause acts, is evident, other- 

 wise we should not get a sudden development of dark forms in 

 certain seasons. Mr. J. Jenner Weir writes : — " You are 

 quite correct about the colour of the wings being determined 

 by the condition under which the larva feeds. This is quite 

 true of the horeomorphic species of Rhopalocera in India, 

 Java and other Eastern districts " {m litt.), vide also Entom. 

 Record, etc., vol. i., pp. 30 and 31. Mr. Merrifield — as I have 

 pointed out before — also found the larval, the stage most 

 affected by external conditions. I think we may safely 

 assume that the larval, is the stage in which the melanism- 

 producing factor exerts the greatest influence. 



To account for this, no theories have as yet been advanced 

 worthy of the name. I have two, however, one written by 

 Mr. T. D. A. Cockerell, the other by Dr. Chapman, two of our 

 most original and thoughtful lepidopterists. Mr. Cockerell 

 assumes what I have proven, that the change is accompanied 

 by a change in humidity. He then supposes that moist air 

 may absorb some melanism-producing product of decomposi- 

 tion, and then he adds : — " A moderately moist air in a town 

 might produce the same effect as very moist air — say, in 



