Osten Sacken.] 136 [October 6, 



remarkable, and there are not many groups of Diptera in which a 

 similar difference exists. America possesses, it is true, in the allied 

 genus Mesograpta, a peculiar form of its own, which seems to flourish 

 especially in the tropics; still the number of species of this genus is 

 by far not sufficient to balance the large number of European Syrphi. 

 The most troublesome problem I had to deal with in preparing the 

 present paper, consisted in the discrimination of the species repre- 

 senting in America the European S. ribesil and S. topiarius. After 

 carefully examining about three hundred specimens (most of which 

 recently taken, and therefore in good condition for a delicate exami- 

 nation of this kind), I have succeeded in distinguishing two forms, 

 which may be defined as follows: — 



d", ?. Eyes pubescent ; hind femora black, except at the tip. 



S. torvus (Syn. topiarias Zett.) 

 cf, ?. Eyes glabrous; 



c?, all the femora black at the base; hind femora black, 



except the tip. 



2, all the femora yellow from the very base (the coxae 



being black) ; hind femora usually with a brown ring 



before the tip. . . . S. rectus (Syn. ribesii Lin. ?) 



S. rectus is very variable in size, in both sexes, while S. torvus 



varies much less. The number of minor differences, taken from all 



parts of the body, sufficiently establish the distinctness of these two 



forms. In all other respects these forms are most remarkably alike, 



and an unpractised eye would probably fail to detect any difference. 



(For the details, see below, the description of S. rectus.) 



As S. torvus and S. rectus occur in the same localities, for instance, 

 in the White Mountains, from the early summer till late in autumn, 

 the question arises whether they oceur promiscuously, or at different 

 seasons? Unfortunately, the specimens which I examined were not 

 all dated, 1 but, from the dates in my possession, it seems to result 



1 Mr. B. P. Mann collected about sixty specimens on the 7th of July, 1874, in 

 the subalpine region of Mt. Washington; they were all S. torvus, except two or 

 three females of the other form. Mr. Morrison, who collected in the White Mts. 

 for two months, brought home about fifty torvus and one hundred and sixty rec- 

 tus; his specimens were not dated; he remembers, however, that flies of this kind 

 after having been very abundant, became scarce for a time, after which they be- 

 came abundant again. The specimens of S. rectus of my own collecting are 

 mostly dated from August and September. 



