The Bionomics of South. Afriain Insects. 443 



yielded negative results. Before describing his experiments 

 and discussing the results, it is desirable to show the 

 mode of succession of the phases in the wild state. By 

 far the most complete evidence I have been able to obtain 

 relates to a single species, P. sesanius. 



G. The succession of the two Seasonal Phases of Precis 

 sesamus in Natttre. 



The following extracts from Mr. Marshall's letters from 

 1897-1900 give an account of his experience of the 

 succession of wet and dry forms of this species in the wild 

 state, and also show how the conviction was gradually 

 forced upon him that the early appearance of occasional 

 specimens of the dry phase in the heart of the wet 

 season is not due, as he thought at first, to exceptional 

 climatic conditions (see also his paper in Ann. and Mag, 

 Nat. Hist., ser. 7, vol. viii, Nov. 1901, p. 402). 



" 2falvern, Natal ; March 12, 1H97. — You will notice that 

 the dry forms of several species made their appearance at 

 the Karkloof in the middle of February : this is most 

 unusually early. I do not know whether it is a feature 

 and characteristic of that locality or whether it is due to 

 the abnormally dry weather during that month, which is 

 usually one of the wettest in the year. The average rain- 

 fall for this February was considerably lower than it has 

 been for twenty years. It is true that here the insects 

 are still all of the true summer form, but the proximity of 

 the sea may account for that." 



" Salisbury, March Q, 1898. — Scsamns was unusually early 

 here this year, appearing at the beginning of February, 

 full six weeks before its usual time. This I am inclined 

 to attribute to the exceptionally dry January and February 

 we have had — normally our wettest months— though I 

 am aware that Weismann considers that exceptional 

 seasons have little or no effect on seasonal forms, which 

 certainly does not accord with my experience in S. 

 Africa." 



'' Salislmrij, Feb. 12, 1899. — I send a specimen of P. 

 sesamus captured on Jan. 27, 1899, on which day I also 

 saw another. These two examples are of considerable 

 interest as bearing on the problem concerning the stimulus 

 which actually induces seasonal change in this sjDecies. 



