Factors Altering Spiracle Control in Adult Dragonflies: Hypoxia and Temperature

Abstract

1. The steady output of impulses in the two motor axons to spiracle 2 of mature individuals of the large African gomphid dragonfly, Ictinogompkus ferox Rambur, is reduced in frequency with hypoxia and abolished in about 2% oxygen. After perfusion of the insect with hypertonic saline the threshold of this response is raised, whereas, following perfusion with hypotonic solutions, it is lowered. 2. Teneral dragonflies give only a weak response to hypoxia--comparable to that of the mature insect perfused with hypertonic saline. This may help to conserve water before feeding commences. 3. Little change in the frequency of motor impulses follows gassing with carbon dioxide mixtures in air up to 20%; this is in contrast to the reactions reported in some other insects. 4. A raised temperature causes an increased frequency and the Q10 for the reaction is 2.7 (15-25° C). Above 30-35° C. the frequency declines rapidly. A rise in the threshold of the hypoxic response does not accompany the increased frequency brought about by a raised temperature. A similar, increase in frequency occurs when either the head or the thorax alone is heated. 5. Pterothoracic temperatures of about 30-35° C. are probably maintained in this species while perching in the sun. In this temperature range it is able to take off instantly when potential prey is sighted. High pterothoracic temperatures are considered in relation to spiracle control.