1875.] 137 [Osten Sacken. 



that S. torvus principally occurs earlier, and S. rectus later, in the 

 season. Thus the idea naturally suggests itself to my mind that we 

 have here a case of so-called seasonal dimorphism, and that S. torvus 

 and rectus are but two forms of the same species. 



More than ten years ago, Mr. A. W. Malm (in his Anteckningar 

 bfver Syrphici, Goteborg, 1863) expressed the opinion that the three 

 European species, S. topiarius Zett., ribesii and vitripennis, are but 

 varieties of the same species, each occurring more abundantly at a 

 particular season, topiarius in the spring, ribesii in midsummer, vitri- 

 pennis in autumn. But Mr. Malm finds passages between these 

 European forms, which prevent their separation as species (for in- 

 stance, an occasional presence of hairs on the eyes of both S. ribesii 

 and vitripennis), while my researches have resulted in the definition 

 of two absolutely distinct forms, which, but for the hypothesis of 

 seasonal dimorphism, might be considered as separate species. 



The intermediate form, S. vitripennis, which exists in Europe, has, 

 in most cases at least, glabrous eyes, but, at the same time, in the 

 female, dark hind femora, yellow only at the tip. I have not met 

 with a corresponding form in America. In a careful scrutiny of more 

 than one hundred and fifty North American female specimens, I have 

 not found a single one combining glabrous eyes with dark hind fe- 

 mora. My material, however, was principally derived from New 

 England, and especially the White Mountains, ft remains to be seen 

 whether collections made in more southern or western localities will 

 not modify in certain respects the results thus far reached by me. 



The European species of S. ribesii, which I have been able to com- 

 pare with specimens of S. rectus, are indistinguishable from them. 

 But whether the smaller varieties of the latter, for instance, the 

 female specimens with a brown ring on the hind femora, also occur 

 in Europe, I do not know. 



The interest attached, in the recent developments of natural sci- 

 ence, to varieties,- in connection with the doctrine of evolution, gives 

 the further investigation of the history of S. ribesii and its North 

 American forms, an importance reaching beyond the scope of de- 

 scriptive entomology. Without pretending to have brought that in- 

 vestigation to a final conclusion, I hope that the hints thrown out by 

 me will not be lost to collectors. 



