— 114 — 



On the Classificaticn of North American Diptera. 



(Third Paper.)* 

 By Dr. S. W. Williston. 



"The families Stratiomvidae, Tabanida?, Xylopl:iagida,-, Acanthome- 

 ridx* and Leptidce form a natural group, which has several characters in 

 common: three pulvilli, total absence of macrochaetse, and smooth leg.^, 

 deprived of those bristles and spines that distinguish the Asilidse, and, in 

 a lesser degree, most of the Bombylidce 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. ' — 

 Osttu Sachfi.** 



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

 North America. These are: Arthropeas Loew, Glutops Burgess, and 

 ''Nov. Gai." Osten-Sacken (to include Arthropeas /ep/isO.-S. and an un- 

 described species fiom Washington Territory). 0[ Bolboniyia 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 Xylophagidse and Stratiomyidae, would place them under the 

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

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

 genus; its general habitus is so markedly Leplid, 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 Stratiomyldae; 

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

 tiomyid^, 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. lo for second paper. 

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



*** Denkschr. d. matli.-naturw. Clas?e d. kais. Akad. d. Wissenschaften, Band 

 XLIV, pp. 59 — 110. 



t Denkschr. etc. Band XLVII, p. 23. 



