434 Mr. G. A. K. Mnrshall on 



nohara, it is always compensated for by an exceptional 

 dulness on the part of their respective females. I fear I 

 do not feel sufficiently competent to attempt an explan- 

 ation of the above facts, but I think you will agree that 

 as a whole they hardly bear out the suggestion that 

 distasteful species are compelled to adopt protective 

 colouring in winter through the keener struggle for exist- 

 ence ; and for the present I am tempted to incline to the 

 view that the less marked cases of dimorphism may be 

 attributable to purely climatic causes. The colouring of the 

 other open veldt Acr/iw, viz. halali, cixina, and asema, is 

 somewhat puzzling ; for in the two latter it is far fi'om 

 being very brilliant or conspicuous ; in halali, the male in 

 summer is very brilliant, but the blackish or brownish 

 grey of the female is certainly protective, and the insect 

 when alarmed is very hard to follow with the eye in its 

 low dodging flight over the herbage. In the winter the 

 colouring of both sexes of all tliree species is certainly not 

 very conspicuous among the withered grass. Either their 

 unpalatabiiity must be of a low order, or else they must 

 be more subject to attack by some particular enemies than 

 the woodland species. I should not be surprised if the 

 rollers, of which we have five species, or cuckoos (also 

 five) were to eat Acra^as, as they are all far from particular 

 as to their diet." 



This hypothesis concerning certain of the smaller 

 Acrseas had been in Mr. Marshall's mind for a long time. 

 Thus he wrote in 1896 from Natal : — 



" Estcourt, Oct. 15, 1896.— I have an idea that all the 

 species of the genus Aer/va. are not protected equally by 

 nauseous taste, etc., and some of them perhaps not at all ; 

 for in many of the smaller species there is a marked 

 seasonal dimorphism which has clearly a protective value. 

 Now such a change seems hardly in keeping with warning 

 coloration, which must be constant to impress itself on the 

 minds of enemies, and moreover a species which requires 

 protection by seasonal dimorphism cannot be very much 

 protected in other ways, not to mention the fact that its 

 colouring cannot be both warning and protective at the 

 same time. There is, of course, nothing to show how 

 much of the seasonal change we can attribute to climate 

 alone. For instance, in comparing the slight alteration in 

 an A. acnra with the marked change in female A. petrma, 

 are we to suppose that the dark-grey female of the latter 



