SARCOPHAGA AND ALLIES 11 



North American Dii^tera, seem to render a lengthy 

 discussion unnecessary here. But a few special terms, 

 or special limitations of terms, are explained. 



Head. — The width of the front is a character of 

 some importance, though subject to a moderate 

 amount of variation. The width is expressed in a 

 decimal, obtained by dividing the number of mi- 

 crometer units in the narrowest width of the front by 

 the number in the entire width of the head. Both of 

 these distances are susceptible of rather exact meas- 

 urement, hence the decimal is significent at least to 

 two places, and has regularly been carried to three, 

 though the variations in different specimens make the 

 third figure of little importance. Where there were 

 specimens enough, five males and five females were 

 regularly measured, and all the separate measure- 

 ments are given for each sex, as well as their average. 



The term hucca is applied in Dr. Hough's sense 

 (Proc. Acad. Nat. Sci. Phil., 1898, 168) to the lat- 

 eral edge of the mouth cavity, except that I extend 

 the region up to the lower edge of the eye, including 

 the narrow and in this part generally very indistinct 

 transverse impression, which was excluded by Hough. 

 Thus the height of the bucca can be roughly expressed 

 as a fraction of the height of the compound eye, al- 

 though on account of its slanting position it is im- 

 practicable to measure this relation with any pre- 

 cision. 



On the back of the head, a very distinct suture 

 extends outward and downward from the central 

 foramen, marking off the lowest part, which is the 

 metacephalon; the suture continues as a perceptible 

 groove or line to the edge of the mouth, cutting off 

 the back part of the bucca. This line often separates 

 the black and pale hairs of the bucca, the pale being 

 behind the metacephalic suture, A careful study of 

 this detail often aids in distinguishing the females of 

 species. An even more useful aid for the purpose is 

 the number of rows of black hairs behind the eye on 

 the outer part of the back of the head. The meta- 



