— 114 — 



On the Classification of North American Diptera 



(Third Paper.)* 



By Dr. S. \V. Williston. 



"The families Stratiomyidae, Tabanidae, Xylophagidae, Acanthome- 

 ridae and Leptidse form a natural group, which has several characters in 

 common: three pulvilli, total absence of macrochajtae, and smooth legs, 

 deprived of those bristles and spines that distinguish the Asilida. 1 , and, in 

 a lesser degree, most of the Bombylidre and Therevidae. " 



"The principal families in this group are well marked enough, but 

 there are, along side of them, many forms of transition, so-called synthetic 

 types, which render the exact difinition of the families very difficult." — 

 Osttn Sat ken** 



Three or four of these synthetic genera are peculiar to, or occur in, 

 North America. These are: Arlhropeas Loew, Ghttops Burgess, and 

 "Nov. Gen." Osten-Sacken (to include Arlhropeas leptis O.-S, and an un- 

 described species fiom Washington Territory). Of Bolbomyia Loew, not 

 enough is known to base any definite opinion — its author was in doubt 

 about it himself. Dr. Brauer, in his recent extensive review*** of the 

 genera of Xylophagidae and Stratiomyidce, would place them under the 

 Xylophagidae. Baron Osten-Sacken, on the other hand, would locate 

 them under the Leptida). I know but one species, pertaining to the new 

 genus; its general habitus is so markedly Leptid, that I would agree 

 with Osten-Sacken that the structure of the antennae should hold a sub- 

 ordinate value. The larvae possibly, when found, may give more decisive 

 evidence. Subula, another disputed form, Brauer retained in the Xylo- 

 phagidae, but pointed out larval relationships with the Stratiomyidx'; 

 Osten-Sacken gave reasons (I.e.) for including this genus under the Stra- 

 tiomyidae, a view which Brauer afterwards accepted, f 



With these changes, however, I am utterly at loss to find characters 

 that will distinguish these families, and I am here compelled to give an 

 artificial analysis to separate the disputed forms. 



The veins separating the posterior cells are here spoken of simply as 

 posterior veins. In the distribution of the groups I follow Brauer chiefly. 



* See ante p. 10 for second paper. 

 ** Berlin. Ent. Zeit., Band XXVI, 1SS2. p. 363. 



*** Denksclir. d. math.-natnrw. Classed. kaisi Akad. d. Wissenschaften, Band 

 XLIV, pp. 59 — 110. 



t 1 lenkschr. etc. Band XLVII, p. 23. 



