112 NORTH AMERICAN DIPTERA. 



and elongated, with the abdomen narrowed at the base, thick- 

 ened and curved downward toward the extremity ; with trans- 

 parent, irridescent wing, usually hyaline, but often witli a spot 

 or spots toward the end, and are usually observed about decay- 

 ing vegetables, excrement, cheese, ham, etc., often in swarms. 

 The flies for the greater part, run about actively and are quick 

 in flight. The best known are the species of Piophila, the 

 larvae of which are so well known as "cheese-mites." These 

 larvae live in cheese, in ham or bacon, or in general in any 

 fatty material, and often do much damage, being especially 

 troublesome in pork-packing establishments. From the pecu- 

 liar power of leaping possessed by the maggots they are often 

 called " skippers "; the act is performed by the larvae seizing 

 with its extended mouth-hooklets the edge of the posterior 

 truncature of the body and then suddenly releasing it while 

 pulling hard. The larvae are somewhat 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 booklets separated, 

 short and divergent; the anterior spiracles whitish, the ab- 

 dominal legs roughened, the anal segment with four fleshy 

 pr6tuberances. Puparium elliptic, rugose. 



TABLE OF GENERA. 



1. Head depressed ; antennae elongate. . . . . . . 2 



Head not depressed; antenna? not reaching beyond the oral margin. 3 



2. Second joint of antennae elongate, third oval. PROCHYI.IZA Walker. 

 Second joint short, third elongate. . . TYI.OMYIA Giglio-Tos. 



o. Auxiliary vein distinct; palpi rudimentary (S<?/>siW). ... 4 

 Auxiliary vein indistinct or wanting (Piophilince) 5 



4. Front femora of the males with tubercles on the lower side. 



SF.PSIS Fallen. 

 Front femora of male without tubercles below. NEMOPODA Desvoidy. 



o Face elongate and truncate; ant-like in appearance. CEPHALIA Meigen. 

 Face not elongate and truncate, but rounded. . . . . 6 



