NATURE 



[September 12, 1912 



Polymorphism in a Group of Mimetic Butterflies of the 

 Ethiopian Nymphaiine Genus Pseudacrxa. 



A LITTLE moic than two years ago Dr. Karl Jordan 

 informed me that he had been studyiny the male 

 jjonital armature of the Pseudacrieas, and that he 

 could not find any difference between the "species" 

 of a large group made up of Linn^'s euryins and its 

 numerous allies on the west coast, of Neave's 

 lioblcyi, terra, and obscura in Uganda, of Trimen's 

 rogcrsi of the Mombasa district and his imitator ot 

 Natal. All these forms possess patterns mimetic of 

 species of the .Acraeine genus Planema. The con- 

 clusion was a very startling one. If each mimetic 

 Pseudacraea had been confined to a single area and 

 had interbred on its margin with the Pseudacraeas of 

 surrounding areas with different mimetic patterns, 

 we should have been confronted with a more remark- 

 able and complex example than any as yet known 

 (except perhaps Papilio Jarclanus), but one that raised 

 no special difficulty. Dr. Jordan's discovery, however, 

 involved far more than this : it led to the remark- 

 able conclusion that the sexually dimorphic P. hobleyi, 

 mimicking the sexually dimorphic Planema macarista 

 in the Entebbe district, was the same species as the 

 two monomorphic Pseudacraeas flying in the same 

 forests with it, viz. P. terra and P. obscura, 

 mimicking respectively the sexually monomorphic 

 Planema telhis and P. paragea. 



Dr. Jordan communicated his discovery to the First 

 International Entomological Congress, meeting at 

 Brussels in igio, and at the same meeting I brought 

 forward the results of researches in igog bv Mr. C. A. 

 Wiggins, D.P.M.O. of the Uganda Protectorate, 

 upon the Pseudacrjeas and other mimetic butterflies 

 of the forests near Entebbe. The preparation of this 

 latter paper afforded the opportunity of testing Dr. 

 Jordan's conclusions by the careful study of a splendid 

 mass of material. 



Out of the long series of Pseudacraeas, two speci- 

 mens yielded strong support : (i) a male P. terra 

 with a pattern approaching the male of P. hobleyi ; 

 (2) a female P. hobleyi bearing the mimetic colours of 

 its own male. Nevertheless, I felt, and stated in the 

 paper published in the Proceedings of the Congress, 

 that conclusions so far-reaching ought not to be finally 

 accepted until they had been tested by breeding. 



Mr. Wiggins has continued his fruitful study of 

 mimicry in the forest butterflies of Uganda from the 

 point reached in the Brussels paper up to the present 

 time. 



There was, however, a break in igii, when he 

 came home and worked with me upon his material 

 in the Oxford museum, preparing a tabular statement 

 for the Second International Congress which has just 

 met at Oxford. The results which have been gained 

 from his enthusiasm and energy will throw far more 

 light on the proportions of mimics and models at 

 different seasons and in different years than has been 

 shed by any other naturalist in any part of the world. 

 A few more intermediate Pseudacraeas, and one or 

 two more male-coloured females of P. hobleyi, ap- 

 peared in the wonderful series obtained by him, but 

 the collection as a whole shows that in the forests 

 within a few miles of Entebbe two out of these three 

 mimetic Pseudacraeas, viz. hobleyi and terra, are 

 w^onderfully constant and sharply marked off, and that 

 the se.xual dimorphism of one of them is nearly always 

 pronounced. In the Entebbe district the third forni, 

 P. obscura, is so much rarer than the others that it 

 is at present impossible to speak with any certainty 

 of its constancy. 



While Mr. Wiggins was continuing his researches 

 on the mainland. Dr. G. D. H. Carpenter, a member 

 of the Royal .Society's .Sleeping .Sickness Commission, 

 NO. 2237, VOL. 90] 



began to study Pseudacraeas in the intervals of his 

 work on the tsetse-flies of the islands in the north- 

 west of the Victoria Nyanza. During a large part of 



191 1 Dr. Carpenter was on Damba Island, on the 

 Equator, about twenty miles south-east of Entebbe. 

 Early in the present year he moved to Bugalla, one of 

 the Sesse islands, to the south-west of Damba. In 

 both these islands he found the Planema models rare 

 as compared with their mimics. The Planemas are 

 apparently more exclusively forest butterflies than the 

 Pseudacraeas, and the forested areas on the islands 

 may not be extensive enough for them to establish 

 themselves freely. Furthermore, the proportions of 

 the three forms of Pseudacr^as are very different. 



On the adjacent mainland hobleyi is by far the 

 coinmonest form, and obscura much the rarest. 

 Although on the islands the exact proportions have not 

 been ascertained, it is clear that terra is by far the 

 commonest, and obscura quite abundant. Male- 

 coloured females of hobleyi are rare, although ap- 

 parently less rare than in the Entebbe district ; but 

 the chief interest of the island Pseudacraeas lies in the 

 extraordinary number of transitional forms — between 

 terra and obscura, between terra and the female 

 hobleyi, between terra and the male hobleyi, between 

 obscura and the female hobleyi. Dr. Carpenter has 

 also observed on Bugalla the male of hobleyi pursuing 

 the female of terra, the male of terra the female of 

 hobleyi, and the male of obscura the female of terra. 



All these facts offer the most convincing support to 

 Dr. Jordan's conclusions, as well as to an interpreta- 

 tion of mimicry based on natural selection. Where 

 the models — which are different species sharply cut off 

 from one another — are predominant the mimetic. 

 forms of an interbreeding community are also sharply 

 cut off, and intermediates are rare ; where the models 

 -^although all of them exist — are in a small minority, 

 the forms of the mimetic community tend to run into 

 one another, The results here summed up were com- 

 municated to the Entomological Society, and will be 

 found in the Proceedings for 191 1 (pp. xci.-xcv.) and 



1912 (pp. xix.-xxiii., and later pages as yet unpub- 

 lished). 



Finally, my friends Mr. Guy A. K. Marshall, 

 scientific secretary of the Entomological Research 

 Committee of the Colonial Office, and Mr. S. A. 

 Neave have given me ■ the, opportunity of study- 

 ing the collections made in 191 1-12 by the 

 latter over a wide area in the Uganda Protectorate 

 — from the Mount Elgon district fai; to the north- 

 east of Entebbe, to Buddu in the south, and Ankole, 

 Unyoro, and Toro, as far as Ruwenzori and the 

 Semliki Valley, to the west. As yet- only a few of the 

 specimens have been "set," so that it has been im- 

 possible to study the patterns in detail, but one 

 important conclusion has emerged, viz. that male- 

 coloured females of P. hobleyi were relatively far 

 commoner outside the Entebbe district than we know 

 them to be within it. The most remarkable mani- 

 festation of this tendency was encountered (."August 

 13, 1911) on the Siroko River (3600 ft.), near the 

 western foot of Mount Elgon, when Mr. Neave col- 

 lected ten males of P. hobleyi, eight male-coloured 

 females, and four normal femaleSi This change in 

 the proportion of the females corresponds with a 

 change in that of the models, Planema poggei, with 

 both sexes resembling the male of P. macarista, being 

 commoner and P. macarista rarer outside than they 

 are within the Entebbe district, so well investigated, 

 by Mr. C. A. Wiggins. Mr. Neave never saw the 

 latter species near Mount Elgon, in Kavirondo, or 

 indeed anywhere to the east of the Nile at Jinja. 



We now come to the evidence furnished by breeding, 

 which indeed is the object of the present letter. Ever 

 since the Brussels Congress I have tried to induce 



