34 Mr. G. A. K. Marshall on Seasonal Dimorphism 



taken too strictly, for true sesamus is occasionally found in 

 company with its summer form in open hill-tops, but princi- 

 pally at the change of seasons ; but natalensis, so far as my 

 experience goes, is never to be found in the more shady 

 stations frequented by sesamus. The latter, moreover, is dis- 

 tinctly warier and more difficult of approach when not feeding, 

 and if alarmed flies off with a rapid, and often zigzag, flight, 

 settling abruptly among rocks or herbage, when its greenish- 

 black underside colouring is equally protective. Sesamus is 

 more often observed in gardens, and not unfrequently enters 

 human habitations in search of a shady resting-place. This 

 difference in the stations frequented by the two forms may 

 possibly go some way to account for Mr. Distant's experience 

 in Pretoria — at least so far as the rarity of the wet-season 

 form is concerned. The town is situated in a hollow " almost 

 surrounded by high and barren hills," and it does not seem to 

 me improbable that the natalensis form might only be found on 

 the summits of the range. A somewhat parallel instance was 

 pointed out to me by Mr. J. M. Hutchinson in the case of his 

 OAvn farm in Natal, which lies in an open plain between two 

 ranges of hills. He has found sesamus resident on the farm, 

 occurring fairly commonly along the banks of the sprints 

 during the winter, whereas natalensis is very much scarcer 

 and non-resident, the examples seen having always been 

 travelling from one range of hills to the other, on both 

 of which it is common. This I had a good opportunity of 

 observing for myself when enjoying Mr. Hutchinson's hospi- 

 tality in 1896. 



The curious congregating habits of typical sesamus (which 

 likewise constitute a marked distinction from its summer form) 

 have already been recorded, but I have only had two oppor- 

 tunities of observing them in anything like numbers : once 

 in April 1893, when I disturbed about 30 specimens from 

 under the bank of the Sundays River in the north of Natal. 

 The second occasion was at Hartley Hills in Western 

 Mashonaland, when I found a large concourse of these insects 

 going to roost under a projecting granite rock ; I managed to 

 number them up to eighty as they flew out, but then lost 

 count, there being certainly over a hundred. I have observed 

 this habit of roosting in company in species of Euralia, also 

 in Belenois, lleryamia eriphia, and Teracolus eris } but the 

 reason for it is not altogether evident to me. 



The divergences in habits between natalensis and sesamus 

 are, however, insignificant as compared with their radical 

 differences in colouring. The brilliant salmon-red upperside 

 of the wet form with its black borders and spots stands in 



