4 DIPTERA OF NORTH AMERICA. [PART III. 



the presence of a row of bristles on each side near the orbit, be- 

 sides the bristles common to all the Ortalidse. These bristles 

 are a character so exclusively peculiar to the Trypetidse that 

 we cannot but consider those two species as belonging to that 

 family (as I have already proved it elsewhere). They cannot, 

 therefore, be further considered here. 



Ortalis vibrans, the female of which has only four segments 

 on the abdomen, approaches, in the absence of bristles upon the 

 first longitudinal vein, Palloptera and the related genera so 

 much, that one might be tempted to place it among the Pal- 

 lopteridee; but there are other genera having the first longitu- 

 dinal vein bare, to which Ortalis vibrans is still more closely 

 allied, and which, as I will have occasion to show hereafter, 

 cannot possibly be separated from the Ortalidse. Such being 

 the case, 0. vibrans has to remain in this family, and this is 

 also justified by the large size of the two posterior cells in this 

 species, which is a proof of its relationship to the other Orta- 

 lidse. There is no doubt, at the same time, that this species is 

 a stranger in Meigen's genus Ortalis. 



Ortalis syngenesise is also distinguished from the other species 

 of Ortalis by its abdomen, which has only four segments in the 

 female ; in other respects it is more related to them than O. 

 vibrans; it is more closely allied to the species of the genus 

 Platystoma than to the other species of Meigen's Ortalis. 



The other European Ortalidse. 



After having thus disposed of those species placed by Meigen 

 in the genus Ortalis, which have either to be entirely withdrawn 

 from the family of Ortalidse, or which can only conditionally be 

 received in it, the next step to be taken, in order to chalk out 

 the whole extent of the family Ortalidse, is to discover such 

 other genera as may likewise possess the characters common 

 to the species of Ortalis. After this, we will have to point 

 out such genera as possess not all, but most of those charac- 

 ters only, and especially the principal ones ; and thus we will 

 reach a limit beyond which only such genera will be found, 

 as, on account of important differences from the species of 

 Ortalis, cannot any more be united in one family with them. 

 This research has also to show us which among the characters 

 common to the above enumerated species 01 Ortalis have to be 



