270 NORTH AMERICAN DIPTERA. 



so well known as 'cheese-mites'. These larvae live in 

 cheese, in ham or bacon, or in general in any fatty ma- 

 terial, and often do much damage, being especially 

 troublesome in pork-packing establishments. From the 

 peculiar power of leaping possessed by the maggots they 

 are often called 'skippers'; the act is performed by the lar- 

 va seizing with its extended mouth-hooklets the edge of 

 the posterior truncature of the bod}' and then suddenly 

 releasing it while pulling hard. The larvae are some- 

 what conical, pointed anteriorly, truncated behind, and 

 about five millimeters in length. They are shining and 

 smooth; the antennae composed of two equally long joints; 

 the mouth hooklets separated, short and divergent; the 

 anterior spiracles whitish, the abdominal legs roughened, 

 the anal segment with four fleshy protuberances. Pupa- 

 rium elliptic, rugose. 



TABLE OF OENERA. 

 i. Head depressed; antennae elongate. ..... 2 



Head not depressed; antenna? not reaching beyond the oral mar- 

 gin 5 



2. Second joint of antenna; elongate, the third oval (4). Prochyliza, 

 Second joint of antennae short, the third elongate. Tylomyia 



3. Auxiliary vein distinct; palpi vestigial. ..... 4 



Auxiliary vein indistinct, or wanting. . . . . . 6 



4. Abdomen curved, narrowed at base, the second segment swollen. 5 

 Abdomen straight or but slightly curved, but little narrowed at 



base, the second segment not swollen. . . Themira 



5. Front femora of male with tubercles on under side (1, 2, 3) . Sepsis. 

 Front femora of male not with tubercles on under side. 



Nemopoda. 



6. Wings with blackish spot at tip; antenna.- reaching nearly to the 



oral margin Mycetaulus. 



Wings wholly hyaline: antenna.* shorter ^5, 6 1. . Piophila. 



