THE RESPONSE OF THE HOVERING HAWK MOTH MACROGLOSSUM-STELLATARUM TO TRANSLATORY PATTERN MOTION

1. The European hawk moth Macroglossum stellatarum, while collecting n ectar in hovering flight in front of flowers, follows moving stripe pa tterns in the lateral visual field. This response counteracts a second one, that is the animals' effort to stabilize their distance from dum my flowers. We investigated the response to motion stimuli in the late ral visual field using sinusoidally oscillating stripe patterns (Fig. 1), as well as its interaction with the distance stabilizing response. 2. In both responses moths attempt to compensate for image speed. The balance between the two depends on the number of elementary motion de tectors stimulated by the dummy flower and the stripe pattern, respect ively. Increasing the diameter of the dummy flower (Figs. 2 to 4) or t he spatial Frequency of the stripe pattern (Fig. 7) shifts the balance in favour of distance stabilization. The reverse is true when the len gth of the stripes in the pattern (Fig. 5) or their number is increase d (Fig. 6). It does not matter whether the stripe pattern is presented in the lateral (Fig. 4A) or in the dorsal and ventral visual field (F ig. 4B). 3. The gain-frequency relations of the response,lo the latera l stripe pattern obtained with dummies in two different positions with in the drum have their maxima around 3 Hz and decline rapidly towards lower and higher frequencies like the response of a bandpass filter. T he distance stabilizing response also has bandpass properties, but wit h a broad plateau between 0.15 and 5 Hz (Fig. 8). The most likely expl anation for this difference is that there is a regional or direction-d ependent variation of motion detector properties. 4. The responses to ramp-like stimuli are phasic in accordance with the amplitude frequenc y characteristics, but the responses to progressive (front to back) an d regressive motion of the pattern differ (Figs 9, 10). 5. The respons e appears to depend on the azimuthal position of the stripe pattern wi thin the visual field (Fig. 11). II is strongest when the pattern cove rs equally large parts of the frontal and caudal visual fields. The op tomotor sensitivity to translational pattern motion is higher in the f rontal than in the caudal visual field (Fig. 12, Table 1). 6. When the stripe pattern on one side is removed, the response amplitude is halv ed. There is no detectable turning response around the vertical axis t o the oscillation of the stripe pattern (Fig. 13, Table 2). 7. The pos sible role of the response to pattern movements parallel to the longit udinal body axis under natural conditions is discussed.