This respiratory system is called open tracheal system, and it is common among insects.
The spiracles can be opened and closed, which allows the fly to control the flow of air in and out of the body.
This is important for maintaining a proper oxygen balance and for preventing water loss.
Flies have a high metabolic rate, and they require a lot of oxygen to support their energy needs. The open tracheal system allows them to breathe efficiently and maintain a high level of activity.