Spiracle Control in Adult Dragonflies (Odonata)

Abstract

1. Spiracle activity is described in four species of adult dragonfly (Anisoptera) at rest, in flight and during recovery from flight. The closer muscle of spiracle 2 reacts directly to locally applied carbon dioxide. In one species this results in maintained opening of the valve, and in the other three in movements synchronized with abdominal ventilation. Neither reaction involves a change in the patterns of nerve impulses in the motor nerve to the closer muscle. 2. Conditions of partial anoxia have no direct action on the spiracle but they reduce the frequency of impulses in the motor nerve, thereby increasing the sensitivity of the closer muscle to carbon dioxide. 3. A central inhibitory reflex accounts for spiracle opening at the start of flight. Various stimuli are described which excite this reflex. 4. Rhythmically-firing ventilation centres are confined to the abdomen, but the brain and thoracic ganglia exert a considerable influence over ventilation. In species whose thoracic spiracles are at times synchronized with ventilation, volleys of impulses ascend the cord and regularly superimpose a burst at high frequency (and in one species a subsequent brief inhibition) on the otherwise constant firing of the spiracle motor neurones. In the presence of carbon dioxide these bring about synchronized movements. 5. Synchronized spiracle movements probably give rise to a one-way airstream in tracheae which pass axially up through the flight muscles and then posteriorly along the abdomen. In some species the stream may pass in the reverse direction. 6. Among thirty-five species of dragonfly examined in Uganda synchronized movements of the thoracic spiracles are found in all species active during the day; in five species which are active only during crepuscular periods they are absent. The possibility is discussed that these movements are important for cooling as well as for respiration.