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Data from previous experiments on predator-prey distance estimation during ontogeny are compared to theoretical interpretations. In Ranatra the relationships between performance (maximum reactive distances), effectors (length of forelegs) and receptors (eyes) do not remain constant during nymphal development, contrary to Mantids (Maldonado et al., 1973). The hypothesis of an automatic morphological adaptation occurring after each moult cannot be retained. Burkhardt et al.'s (1973) theoretical analysis of binocular vision in insects was applied to Ranatra : for the first four nymphal instars, the calculated limits of binocular vision coincide with maximum reactive distances. This could explain why these animals do not react to prey items presented at distances equal to the length of their forelegs before the 5th instar. The theoretical limits of binocular vision are further away than the maximum capture distances and the length of their forelegs for 5th instar nymphs and adults ; the length of their forelegs would then limit capture possibilities.