The housefly Musca domestica, is a fly

Fly

True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

 of the suborder Cyclorrhapha

Cyclorrhapha

Cyclorrhapha is an unranked taxon within the infraorder Muscomorpha. They are called "Cyclorrhapha" with reference to the circular aperture through which the adult escapes the puparium...

. It is the most common of all domestic flies, accounting for about 91% of all flies in human habitations, and indeed one of the most widely distributed insects, found all over the world; it is considered a pest

Pest (animal)

A pest is an animal which is detrimental to humans or human concerns. It is a loosely defined term, often overlapping with the related terms vermin, weeds, parasites and pathogens...

 that can carry serious diseases.
The adults are 8–12 mm long. Their thorax

Thorax

The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

 is gray, with four longitudinal dark lines on the back.