The Bionomics of Soitth African Insects. 855 



flight ; it' liovvever they be thoroughly frightened by 

 continued strokes of the net, they will dart rapidly on for 

 a short distance, then vanish — or, in other words, they settle 

 with extreme suddenness, and their under-side colouring 

 harmonizes so well with the sandy soil tliey love that they 

 are very difficult to detect. It seems to me that such a 

 habit can only have been developed for the purpose of 

 escaping from birds, and must be very effectual in most 

 cases. I have noticed that the summer form,^, which have 

 not the sandy-coloured under-side do not adopt these 

 tactics, but rely on their flight alone — probably because 

 food is more plentiful for insectivorous birds at that 

 season. 



[After this, Mr. Marshall kept a careful record of obser- 

 vations. His results, including one observation made at 

 an earlier date, are shown on pp. 357-9 in the form of 

 a diary. The two following letters bear on the same 

 subject.— E. B. P.] 



Scdishurif, March 6, 1898. — I was much interested in your 

 arguments * for Common Warning Colours in butterflies 

 and your remarks on their probable enemies, but I must 

 candidly confess that I am not altogether convinced. The 

 difference in our views lies in your fundamental proposi- 

 tion that butterflies are an easy prey for birds to capture 

 from a general point of view. If this proposition be 

 correct, then I quite agree that your theory offers the most 

 natural and probable explanation of the predominance 

 of bright colours among butterflies. But from what I 

 have seen of the South African species I could not truth- 

 fully say that I consider that they would be likely to fall 

 an easy prey to birds, indeed I should say that the average 

 insectivorous bird would not have a chance against most 

 of the swift-flying species when on the wing, and would 

 only be able to catch them under exceptionally favourable 

 circumstances when the insects were off their guard. If 

 this supposition be correct it would go a good way to 

 explain how so many butterflies have been able to acquire 

 such brilliant colours, and particularly in the case of 

 those species which have protectively-coloured under-sides, 

 which is the rule rather than the exception. Birds would 

 soon learn the futility of attempting to 'pursue such species, 

 and Avould only capture them by stealth, and in a more or less 



*■ Some of tlie arguments here referred to are set forth on pages 

 500 to 502 of the present memoir. — E. B. P. 



