An Ocellar Dorsal Light Response in a Dragonfly

One of the functional roles suggested for the insect ocelli is their involvement in flight behaviour such as the maintenance of stability by monitoring movements of the animal relative to the horizon (Hesse, 1908). In a recent study on second order ocellar neurones in locusts, Wilson (1978) concluded from physiological and anatomical considerations that this system is ideally suited 'to detect rapid movements of the whole visual field which represent instability in flight'. In fact, indirect behavioural evidence for this concept was provided by Mittelstaedt (1950), who observed that dragonflies with occluded ocelli go through a sequence of unstable flight attitudes and stalls when released in the presence of a single light source of small angular dimensions. We report here simple experiments which give direct evidence that the ocelli do function as equilibrium organs during flight. Experiments were performed on imagines of Hemicordulia tau (Anisoptera, Cor-duliidae) mounted in a wind tunnel and flying at wind speeds between 2 and 4 ms-1. Light sources for stimulation were arranged as in Fig. 1, and additional infrared illumination was provided where necessary to allow direct observations and video recordings. All stimulus sources had small angular dimensions in order to provide a minimum amount of stimulation to the compound eye system as a whole. Because the ocelli have wide visual fields and defocused optics, point sources are equivalent to extended sources (Wilson, 1978). The sources A, B and C were positioned to address the ocelli separately, and switching them evoked changes in head posture as shown in Fig. z(ad). When the source B was turned on, the head moved downward from the position 2(0) to the position 2(b). Similarly, activating source A or C evoked a head roll to the right or left (Fig. zc, d). The rolling response was much more marked (rotation of up to 90 0), when the sources A and C were activated in alternation. When the median ocellus was reversibly blinded with plasticine the movement (a-b) no longer occurred while the response (c-d) was not affected. Single frame analysis of video recordings showed latencies in the order of 40 ms, and the movements were completed within 100 ms. To further identify the sensory organs which mediate the light evoked head movements , the head was continually illuminated with white light from one of the alternative positions D and E (Fig. 1). Under these conditions, passing a sharp 1 mm …