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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 sub-atmospheric tracheal pressure during the down stroke, 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, 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 (below 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 preceding flight itself. During long spontaneous continuous flight phases (up to two hours) the valves were only slightly open (below 5%), widening shortly after transient increases of wing stroke intensity. It is an amazing paradox that the spiracles were only slightly open most of the time during sustained flight. The advantage of generating sub-atmospheric pressure, supporting a unidirectional airflow with a PO2 increase above the resting level, is discussed.

Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowfliesCalliphora vicina