The role of vision in distance, position and size discrimination in prey capture has been investigated in normal adult water stick insects (Ranatra linearis L.: Heteroptera) and in ones with one eye covered.

Both monocular and intact Ranatra were able to discriminate between two targets subtending the same angle but presented at different distances. They usually chose the target nearer to their foreleg claws. Although monocular subjects undershot more often than controls, they could still estimate distance correctly.

When presented with two different-sized targets at the same distance, both monocular and intact subjects usually preferred the larger target within a 1°-10° range, even though monocular animals chose the larger object less consistently. They were able to distinguish between two targets differing in size by only 1°.

Asymmetrical presentations of two identical targets stressed the importance of the central position. Intact animals always preferred the target nearer their midline.

These data also revealed the unexpected ability of Ranatra to strike accurately at two targets or prey items simultaneously. When two identical targets were presented simultaneously and symmetrically, aims were directed at both targets, and one was grasped by each raptorial foreleg, thus indicating an absence of confusion.

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