Stimulation of the large tangential horizontal system (HS) cells expressing Crei\ChR2^{H.C128T.Scer\UAS.T:Avic\GFP-YFP} (under the control of Scer\GAL4^GMR27B03) with a one second 472nm light pulse leads to a depolarisation of 4.4mV on average, as measured 3 to 5 seconds after the offset of the optogenetic stimulus, well within the range of control visually evoked motion responses.

Depolarization of horizontal system (HS) cells in the right hemisphere of Crei\ChR2^{H.C128T.Scer\UAS.T:Avic\GFP-YFP}-expressing flies (under the control of Scer\GAL4^GMR27B03), by means of a 1 second pulse of blue light, results in a subtle head movement in a clockwise direction, starting at the onset of blue light. This movement is not observed in controls. These flies also show a consistent body turning towards a stimulation (in a wing beat analysis), while control flies so no consistent directional bias.