Why is the mesothorax of a true fly so large and muscular?

The mesothorax of a true fly (order Diptera) is large and muscular because it serves as the primary powerhouse for flight. In insects, the thorax is composed of three segments—the prothorax, mesothorax, and metathorax—with the mesothorax and metathorax typically bearing wings.1

In true flies, the mesothorax is highly developed to house the massive flight muscles required to power the wings. These muscles operate indirectly by deforming the shape of the thoracic wall; longitudinal muscles depress the wings, while vertical muscles elevate them. Because true flies rely on these specialized thoracic structures to achieve high-frequency wing beats—which can exceed 600 beats per second in some species—the mesothorax must be robust and muscular to support the necessary aerodynamic forces and the rapid contraction-relaxation cycles of flight.1