1. K. Hausen Photoreception and Vision in Invertebrates M. A. Ali Ed. (Plenum New York 1984) p. 523.
2. Preparation and setup: Female blowflies ( C. erythrocephala ) were prepared as described in (17). Visual stimuli were produced by a grating drum illuminated from the inside by an arc lamp the image of which was projected onto a screen (10 × 8 cm) positioned 10 cm below the fly (Fig. 1A). The fly was mounted to look down on the stimulus and the objective was above the back of its opened head capsule. The square wave grating had a spatial wavelength of 26° a mean luminance of 17.7 cd/m 2 and a contrast of 92%. The velocity of the moving pattern depended on the type of experiment and is indicated in the figure legends. Electrophysiological recording: Electrodes were pulled on a Brown-Flaming micropipette puller (P-97 Sutter Instruments) using thin-wall glass capillaries with a diameter of 1 mm (GC100TF-10 Clark). When filled with 2 M KAc 0.5 M KCl and 8.8 mM calcium green they had resistances of about 30 to 40 MΩ. A SEC-10L amplifier (npi-electronics) was used throughout the experiments and was operated in the Bridge or discontinuous current clamp mode. For data analysis the output signal of the amplifier was fed to a PC 486 via a 12-bit A/D converter (CIO-DAS16 ComputerBoards) at a sampling rate of 3 kHz and stored to hard disc. The motor control unit for the rotating cylinder was controlled by a PC 468. Optical recording: We used an upright epifluorescent microscope (Axiotech Vario Zeiss) with the fluorescein isothiocyanate filter set 9 from Zeiss (excitation filter 450 to 490 nm; beam splitter 510 nm; barrier filter 516 to 565 nm) an Epiplan ×10/0.20 objective and a charge-coupled device (CCD) camera (PXL Photometrics) connected to a Power-Mac (Apple). Images were taken at 1 Hz (Fig. 1) or at 4 Hz (Figs. 3 and 4) at 128 × 128 pixel resolution and were evaluated with the IPLab software (Signal Analytics). The first frame of each image series was taken as the reference frame which was subtracted from each following image. This resulted in a series of difference images (Δ f ) which were subsequently divided by the reference frame (Δ f / f ). The Δ f / f time courses shown in Figs. 1 3 and 4 were obtained by averaging the pixel values in different areas in the Δ f / f image series.
3. Borst A., Egelhaaf M., Proc. Natl. Acad. Sci. U.S.A. 89, 4139 (1992).
4. Egelhaaf M., Borst A., J. Neurophysiol. 73, 2540 (1995).
5. Hengstenberg R., Hausen K., Hengstenberg B., J. Comp. Physiol. A 149, 163 (1982);