Prostigma
In flies of the order Diptera, the prostigma is the anterior of the two pairs of spiracles opening on the pleura. The mesothoracic pair is located between the pro- and mesothorax and the metathoracic pair between the meso- and metathothorax. The following illustration shows the prostigma as number 13 on the right side of the image. The function of the prostigma is to provide an airway into the insect's thorax to facilitate respiration.