The Bionomics of South African Insects. 455 



Concerning the specimens which emerged on May 7 and 

 14, and April 28, 1899, Mr. Marshall wrote as follows : — 



" Salisbury, Aug. 29, 1899. — The case of pelasgis and 

 archesia was very puzzling, as the results were just the 

 opposite of what one would expect — the forced pupa 

 emerging as the dry-form archesia, and the normrd ones 

 as the wet-form ^;(;/«s^?'s, though this latter has disappeared 

 for some time, being re]:)laced by archesia." 



The negative results from these fairly-numerous experi- 

 ments tempt us to believe that the change from sesamus 

 to natalcnsis and natalensis to sesamus may be fixed in the 

 constitution of tiie species, and may form an alternating 

 series contemporaneous with the alternating seasons but 

 not causally connected with them. Such a view is how- 

 ever rendered improbable, as Dr. Dixey has pointed out to 

 me, because there would be nothing to prevent a gradual 

 shifting and finally an entire want of parallelism between 

 the two series. That, however, the change is essentially 

 constitutional in the species and merely requires some 

 external stimulus to set it going may be taken as certain. 

 Furthermore, it is not necessary to suppose that a stimulus 

 is required lor both changes, the return to one of them, 

 and presumably the more ancestral, may be in the nature 

 of a rebound. The slight but distinct difference between 

 the succession of the forms of sesamus in British East 

 Africa and in Mashonaland also probably indicates a 

 causal relation with the inorganic environment, and the 

 same conclusion is supported by the fact that artaxia has 

 been observed without its wet-season phase in a forest 

 region (see pp. 422-3). 



After Mr. Marshall's experiments it is difficult to believe 

 that the application of heat or moisture or the two com- 

 bined to the pupal stage can determine the production of 

 natalensis or 'pelasgis in place of sesamus or archesia, 

 respectively, at the period when the latter forms are 

 becoming abundant in nature. It is possible that here 

 we are merely witnessing the return to a more ancestral 

 phase due to purely internal causes. The reverse experi- 

 ment, viz. the application of cold, or dryness, or both com- 

 bined, to pupa:! of the earlier generations oi natalensis, might 

 produce more positive results and cause the appearance of 

 sesamus at a time of the year when it is very rarely seen, 

 although the occurrence of occasional individuals of 

 sesamus in nature in the depth of the wet season seems to 



