114 



The first of these forms I named Syrphus torvus, the second 

 S. rectus. Between the males of these forms there is hardly 

 any difference at all, except, as just shown, in the condition of 

 the eyes, pubescent in the one, glabrous in the other. Be- 

 tween the females, the difference consists, besides the condition 

 of the eyes, in the color of the hind femora. Minor differences 

 existing" between the two forms, are almost unappreciable, and 

 would have been neglected, without the help of those leading 

 characters. 



As both forms occur in large numbers in the same localities 

 (my specimens were principally from the White Mountains), 

 the question naturally arises whether they occur promiscuously, 

 or at different seasons ? The answer would have been easy, if 

 the dates of the capture of each of those 300 specimens had 

 been noted. Although this was not the case for a considerable 

 majority among them, still, from the data in my possession, it 

 seems probable that S. torvus is the form commonly occurring in 

 the early snmmer, S. rectus that of the late summer and au- 

 tumn. If such is the case, the next question would be whether 

 these two forms are distinct species, or whether they represent 

 a case of seasonal dimorphism of the same species? The latter 

 alternative seems to me the more probable, although of course 

 it would require further proof, by way of observation and ex- 

 periment, before being accepted. 



About ten years ago, Mr. Malm in Goteborg, Sweden, ex- 

 pi'essed the opinion that the representatives of my S. rectus and 

 S. torvus in Europe, S. ribesii, S. vitripennis and S. topiariuh 

 are not species, but only varieties, each occurring more abun- 

 dantly in its own season: topiarius (torvus), in the spring; 

 ribesii (rectus), in autumn ; and vitripennis between the two, 

 in midsummer. There seems to be a difference, however, be- 

 tween the case, as it stands in Europe, and as I find it here. 

 The European S. vitripennis is said to be an intermediate form 

 between the two other species ; it has glabrous eyes, but at the 

 same time the femora of the female are dark at the base. 

 Among the 300 American specimens which I have examined, 

 there was not a single case of this kind : as shown above, all the 

 females with glabrous eyes have the femora yellow from the 



