6 DIPTERA OF NORTH AMERICA. [PART II. 



a small and imbedded hypopygium. In some genera the one 01 

 the other of the paired organs are so small as to escape notice. A 

 further and more thorough investigation has as yet to show whe- 

 ther in some genera, and this seems, for instance, to be the case 

 with Hypophyllus, a still larger number of appendages does not 

 occur, and whether in general the organization of all the genera 

 can be reduced to the common type, described above. 



Of the first abdominal segment often only the upper half is 

 perceptible in the male and the female ; this is especially the case 

 in those genera in which the metathoracic epimera cover a con- 

 siderable breadth of the basis of the abdomen ; in other genera 

 the inferior half of the segment is much shortened, and in a few 

 cases only equal in length to the upper half. In several genera 

 the lateral margin of the second and of the following segments 

 shows deep punctures, arranged in a longitudinal row. 



The fore coxae, which are somewhat distant from the middle 

 ones and placed much higher than those, are also longer, and reach 

 as far as about the middle of the latter. The feet are in general 

 slender, the hind pair generally longer and stouter than the others ; 

 the femora are often rather strong. Besides short hairs, the feet 

 bear usually a number of stiff bristles, especially the tibiae. The 

 pulvilli are only of moderate size ; the empodium is linear and 

 always distinct ; the claws (ungues) are plain and small. 



We have already explained above what is necessary for the 

 understanding of the very characteristic neuration of the wings. 

 The surface of the latter is microscopically hairy upon its whole 

 extent. The tegula? are ciliated with bristle-like hairs, and some 

 smaller ones besides ; they are simple, as the inferior duplication 

 is wanting or only apparent as a narrow membranous stripe ex- 

 tending towards the corner of the scutellum. 



The sexes of the same species show, besides the difference in 

 structure of the genital organs, other important differences in their 

 organization ; the coloring of the same parts of the body is often 

 different. Here, as in many other families, it is in the male sex 

 that these peculiarities assume the character of variously modified 

 ornaments, and it seems impossible to trace any relation between 

 them and the sexual functions or the sustenance of the male. The 

 only exception in this respect is afforded by the considerable dif- 

 ference in the structure of the parts of the mouth, which is much 

 more developed in the females, in accordance with their greater 



