Fig. 1. Geometry of prey catching in archer fish. (A) Immediately as their dislodged prey starts falling (arrow), archer fish can dart right towards the later point of impact P. To do so the fish must correctly predict the bearing of this point. In principle, however, the fish's `internal estimate' of this point could be anywhere within the green area defined by the bearing error the fish makes: erroneous estimates P' or P'' would still lead to a correct bearing. The present study excludes this view and shows that the fish can also predict their distance from the point of impact reasonably well. (B,C) Predicting the bearing requires three independent pieces of information to be taken into account. The prey insect is shown at start of falling after being dislodged by a successful shot, and the turn a bystander fish (initially oriented along the dotted black line) must make is illustrated. (B) At a given position and orientation of the responding fish, turn size must be matched to the initial height (H) and speed (s) of the falling prey. For small values of H or s (H<, s<, later point of impact marked blue) the fish must make a smaller turn than for large values of H or s (H>, s>, later point of impact marked red). (C) At given H and s values the responding bystander fish must also take the initial direction of the prey's motion into account in selecting which bearing to choose. This direction is not cued by the shot itself because it scatters widely around the shot's direction.