True flies are insects of the order Diptera.
Insects of this order use only a single pair of wings to fly.
The hindwings having evolved into advanced mechanosensory organs known as halteres, which act as high-speed sensors of rotational movement.
They allow dipterans to perform advanced aerobatics.
Diptera is a large order containing an estimated 1,000,000 species.
These include horse-flies, crane flies, hoverflies and others, although only about 125,000 species have been described. (source)
Can guinea pigs eat flies?
They can eat flies if they happen to catch one, but they have no nutritional value for them and shouldn’t be purposely fed to them
For more foods that guinea pigs can and can’t eat check out our guinea pig food list.