Fannia canicularis.

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Male, dorsal view. 
A few species of the genus Fannia  are commonly found breeding in animal manure and confined livestock facilities. The most common is the little house fly, Fannia canicularis. It is smaller than the house fly being about 5-6mm long. The adult is blackish-gray with three indistinct black dorsal longitudinal stripes. The fourth longitudinal vein (M1+2) is straight (not bent as in the house fly).Adults of the little house fly can be recognized by their behavior in animal facilities. The adults slowly in circles and tend to hover periodically. They are often seen circling above animal pens and walkways.
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Fannia canicularis. Male, lateral view. 
The legs are black, the halteres yellow. Fannia species rarely feed directly from animals; however, the few that do are attracted to smeared feces, sweat, and mucus. They occur near excrement, especially when it is rich in nitrogen. The common species can be abundant in poltry facilities, cattle-confinement areas, and dairies.