The ocellar component of flight equilibrium control in dragonflies

SummaryThis paper describes the dynamics of light-evoked head reflexes in the dragonflyHemicordulia tau under light conditions which were selected to optimally address the ocelli.1.The responses occur only during flight.2.Stimulation by a light positioned to address the median ocellus evokes a head movement around the pitch axis. The threshold is in the order of 107 photons · cm−2 · s−1. With increasing intensity, the responses become progressively faster but do not increase in amplitude.3.Stimulation by lights positioned to address the lateral ocelli evokes head movements around the roll axis with a similar threshold and similar dynamics as in the pitch responses. The responses are strongest when two sources at either side of the animal are switched in alternation.4.No evidence is found for interactions between the lateral and the median inputs.5.During sustained illumination from the median source, the head is tilted towards it indefinitely, and increasing the intensity causes only a small additional change of head position. Decreasing the intensity causes a large movement of the head away from the source, and then the system readapts rapidly and the head returns to the on-position (high pass filtering). If increment pulses are superimposed on a steady background, the magnitude of their effect is a function of both their duration and amplitude.6.If the median source is modulated by a square wave of a frequency above the high pass cut-off, the amplitudes of the responses are proportional to modulation depths and independent of average intensity over 4 log units.7.At intensities below 1011 photons cm−2s−1, the spectral sensitivity has a maximum in the green, exceeding the UV-sensitivity by a factor of 5; at higher intensities the responses become more sensitive to UV than to green (reverse Purkinje shift). It is suggested that the reverse Purkinje shift is a functional adaptation to optimize the detectability of the contrast between sky and ground both in dim light and in direct sunlight.8.The dynamics of the behavioural responses can be largely accounted for by known properties of the neuronal elements of ocellar systems.