TACHINID^. 359 



ginal and lateral, and usually with discal bristles; some- 

 times nearly covered with strong, erect spines. Legs 

 usually rather stout, seldom moderately elongate; always 

 with bristles. All the veins of the wings simple; basal 

 cells large; three posterior cells present, the first of which 

 is always narrowed or closed (save in those rare cases in 

 which the distal section of fourth vein is obliterated); aux- 

 iliary vein distinct in its whole course. Squamae large. 



No other group of flies has presented so many difficul- 

 ties to the student of systematic dipterology as the pres- 

 ent one. In its entirety, with few exceptions, it is easily 

 differentiated. Flies that have the arista bare (or at the 

 most, pubescent), the squamae well developed, and the 

 first posterior cell narrowed or closed, may be unhesita- 

 tingly referred to this family. But the family is not 

 always easily distinguished from the Dexiidae. The 

 presence, however, of aristal plumosity, with the other 

 characters of the more typical tachinids, that is those of 

 which the dorsum of the abdomen is distinctly bristly, will 

 remove doubt of the correct location of any species pos- 

 sessing such characters, even when the legs do not have 

 the elongation characteristic of the more typical dexiids. 

 Unfortunately the differentiation of the genera and species 

 is vastly more difficult, and will require much patient 

 study even yet before a satisfactory stability is reached. 

 The student should not be discouraged if he is not always 

 sure of his results, — the most of us are often in doubt 

 about Tachinidae. About one hundred and eighty-five 

 genera have already been accredited to North America^ o* 

 Some of these will doubtless be reduced to syuonyc^s, <y" 

 but not many, since the characters used for generkAtim->' J** 

 itation are often exceedingly slight, such as u^m^ny ^ 

 other families are considered of specific value jeily^ So 

 inconspicuous are the distinguishing character^ j^pmany 

 species that onlv the experienced eye wiUR^let^'t,-them. 



