The operation of the thoracic spiracular valves was analysed using anatomical and physiological techniques. Dense spiracular filter trichomes impede a diffusive gas exchange. However, the hinged posterior filter flap of the metathoracic spiracle (Sp2) opens passively during upstroke of the wings and closes by the suction of the subatmospheric tracheal pressure during the downstroke, which supports a unidirectional respiratory airflow. The action of the interior spiracular valve lids was recorded by photocell sensors oriented above the enlarged spiracles and projected onto the screen of a video camera. The thoracic spiracles opened much quicker (approximately 0.1 s) than they closed (1 s), suggesting that the spiracular muscles are openers, as confirmed by experimental induction of muscle contraction. Simultaneous photocell measurement revealed that the first and second thoracic spiracles act concordantly. At rest, the spiracles were mostly closed or only slightly open (<1%). During intermittent short flights, the valves opened wide at the start of the flight for a short time, and in many cases opened again after the flight ended. Often, the opening was wider after the flight ended than during the flight itself. During long spontaneous continuous flight phases (up to 2 h), the valves were only slightly open (<5%), widening shortly after transient increases of wing stroke intensity. It is an amazing paradox that the spiracles were only slightly open when the requirement for O 2 was high during sustained flight. The advantage of generating sub-atmospheric pressure, supporting a unidirectional airflow with a P O2 increase above the resting level, is discussed.