Fig. 5. How take-off speed appears to be determined by distance plus remaining time of falling. (A-D) Plots of take-off speed vs `virtual speed', i.e. distance (d) of responding fish from the predicted catching point divided by the time () the prey has still to fall when the fish has finished its turn and is ready to take off. Virtual speed is the speed the fish would have to choose in order to arrive simultaneously with the prey after a course with constant velocity. (A) Plot that includes responses (within the grey area) in which virtual speed was above the actual speed limit of the fish (arrow), corresponding to 15-25 fish lengths s^-1. Take-off speed was determined from the first 20 ms of translation. (B-D) Plots of take-off speed vs virtual speed within the accessible range of speed values that the fish can potentially realize (N=60 responses). (B) Speed in the first 20 ms interval after take-off; (C,D) speed in the subsequent two intervals of 20 ms duration. In each case correlation coefficients are highly significant (P<0.0001; r²=0.45, 0.35 and 0.54 in B-D, respectively) and higher than the corresponding correlation coefficients in the plots of take-off speed against distance (Fig. 4A-C).