DIPTERA. 411 



for which we have no common names, but all composing the 

 tribe of Muscans, or Muscad^e. Some of these flies do not 

 strictly conform to the foregoing characters of the tribe, in all 

 respects ; but the exceptions are few in number, and the most 

 remarkable of them will be noticed in the following pages. 



Many flies of this tribe are parasitic in their larva state, their 

 young living and undergoing their transformations within the bod- 

 ies of other insects, particularly in caterpillars, which they there- 

 by destroy. These flies belong chiefly to the family of Ta- 

 chinad^e, a name applied to them on account of the swiftness of 

 their flight. In form they somewhat resemble house-flies ; like 

 them they have very large winglets, and their wings spread apart 

 when they are at rest. They are easily distinguished, however, 

 by the stiff hairs wherewith they are more or less covered, and 

 by the bristles on their antennae, which are not usually feathered. 

 A large fly of this kind, the Tachina vivida of my "Catalogue," 

 is often seen on fences, and on plants, and sometimes in houses, 

 towards the end of June and during the month of July. Its 

 large, oval hind-body is of a clear and light red color, with two 

 or three black spots, in a row, on the top of it, and a thick row 

 of black bristles across each ring. The face is grayish white, 

 like satin, and the eyes are copper-colored. The thorax is gray, 

 with brownish lines upon it. The antennae, proboscis, and legs 

 are light red. Its body is short and thick, and is about half an 

 inch long, and its wings expand rather more than nine tenths of 

 an inch. 



Most insects are hatched from eggs which are laid by the 

 mother on the substances that are to serve for the food of her 

 young. Some flesh-flies produce their young alive, or already 

 hatched, and drop them on the dead and putrefying animal matter, 

 which they are obliged to consume and remove in the shortest 

 possible time. An exception from the usual course among insects 

 appears therefore to have been made in favor of these viviparous 

 flesh-flies, to enable their young promptly to perform their ap- 

 pointed tasks. These insects produce an immense number of 

 young, as many as twenty thousand having been observed by 

 Reaumur in a single fly.* Our largest viviparous flesh-fly is the 



* "Memoires." Vol. IV. p. 417. 



