370 C. R. OSTEN SACKEN'S PRODROME 



beyond the usual crossband, whether this infuscation is connected with the crossband or 

 se2:)arated from it. The hA'ahne interval between the crossband and the apical spot, I have 

 usually called the hynllne triangle. 



In identifying species of Chrysops, the principal attention should be 2)aid to the design of 

 the wings. It is cha met eristic of the species and, at the same time, remarkably constant 

 within the same species. It also helps to refer the male specimens to their females, however 

 different in coloring the sexes may be. The wings of the male are usually somewhat darker 

 on their proximal half; the infuscation of the two basal cells reaches farther towards the 

 distal end of these cells, the anal cell and anal angle often have a brownish tinge, which 

 may not exist in the female. But in the distal half of the wing, esj^ecially in the sliajoe of 

 the apical spot and of the hyaline triangle between it and the crossband, the resemblance 

 between the sexes is, in most cases, much greater. The coloring of the antenna? and of 

 the legs is very variable within the same species and in the same sex, and not to l^e relied 

 upon. The relative extent of black and reddish on the abdomen is also very variable in 

 the same species ; often the whole appearance of a specimen is changed by an extensive 

 jirevalonce of the one or the other color. The coloring of the wings is the only safe guide 

 through all these difficulties. 



The insufficiency of most of the descriptions by Walker and Macquart has compelled me, 

 in many cases, to quote their descriptions as doubtful synonyms of my supposed new spe- 

 cies. As I give detailed and I hope siifficiently clear, descriptions of all the species, I trust 

 that in most cases there will be but little doubt as to the species I mean to describe. The 

 other question, that of the names to be given to these species, is comparatively of a sec- 

 ondary importance, and can be settled only through the comparison of the original types of 

 the earlier descriptions in London and Paris. 



I describe in this paper twenty-three species of Chrysops distriliuted over the area con- 

 tained between the District of Columbia, Wisconsin and Canada (Ijesides C. atropos, from 

 Florida) ; no dou))t many more species will be discovered. Tlie whole of Europe north of 

 the Alps possesses only twelve species ; the southern peninsulas have four sjiecies more ; 

 total for Europe sixteen species. Among the twenty-four North American species here 

 described seven have no apical spot, that is the whole apical portion of the wing is hyaline. 

 Of the sixteen European species not a single one belongs to this category. Four Evu'opean 

 species have a hyaline or subhyaline spot in the middle of the crossband ; no such species 

 have been discovered yet in the Atlantic basin of North America ; but I have seen one or 

 two from California. 8uch are the results of a comparison of both faunas, based on the de- 

 scriptions of the European species. A comparison of sj^ecimens would probably disclose 

 still other curious analogies or differences ; unfortunately I have specimens of only two Eu- 

 ropean species before me. 



ANALYTICAL TABLE OF THE SPECIES (CONSTRUCTED FOR FEMALE SFECIJIENS ONLY). 



^ jThe apex of the wing, liej-oml the crossbaiul, is hyaline 2. 



" 1 The apex of the wing, beyond the crossband, is more or less infuscated 8. 



[The whole wing, except the apex, is black, fading into davk-gray posteriorly . 1. atropos u. sp. ^7 

 "■ 1 The whole wing, besides the apex, is nut black 3. 



