How ladybirds approach nearby stalks: a study of visual selectivity and attention

Summary

• 1. An open- and closed-loop study of the way ladybirds,Coccinella septempunctata, approach vertical posts emphasizes two features of this reflex. First, ladybirds turn preferentially towards close objects, obtaining the necessary distance information from optic flow. Secondly, once an object has been fixated, movement of its image over frontal retina tends to suppress any response to other laterally viewed targets.

• 2. Recordings were made of the trajectories followed by ladybirds in an open arena which contained a single, vertical post on the floor. Ladybirds often approached and climbed up the post. Many approaches consisted of straight-line segments interrupted by abrupt turns. These turns are either appropriately sized so that the ladybird faces the post after one turn or, more often, they are too small in which case the ladybird's trajectory is a spiral. The gain of the turn (i.e. the ratio of the size of the turn to the size which is needed to fixate the post) increases slightly as the insect approaches the post.

• 3. The ladybirds' preference for close objects was examined further on a Y-maze. Insects placed on the trunk of the maze ran towards the fork and down one arm. Each arm of the maze led to a rectangle. Rectangles were placed at various distances from the fork. Their size was adjusted so that viewed from the fork they all subtended 18° horizontally and 63° vertically. Ladybirds chose predominantly the arm which led to the closer rectangle.

• 4. Open-loop tests indicate that this preference for close objects is caused by the pattern of optic flow resulting from the ladybirds' normal forward locomotion. Insects were fixed in front of a computer screen and carried a small ring which they rotated beneath them. Any attempted turn was manifest in a turn of the ring in the opposite direction. Turns were regularly elicited by small backward movements of a vertical stripe across the retina, as would occur during forward walking. Forward motion of the stripe over the retina rarely evoked turns. Turns increased in frequency and size as the speed of backward image motion was raised from 3°/s to 70°/s. The largest turns were evoked with the stripe placed at an eccentricity of about 90 degs from the midline. Amplitude dropped as the stripe was positioned further frontally or posteriorly.

• 5. Approaches to a target were modelled using a saccadic system in which gain varied with distance from the target. This simulation generated spiral trajectories. Thus, the spirals described by a ladybird when it walks towards a post may be a consequence of the way the insect uses motion parallax to restrict its attention to nearby objects.

• 6. Open-loop turns to a stripe moving over peripheral retina are prevented by the concurrent motion of a stripe viewed by frontal retina. This longrange inhibition means that, once a stripe has been fixated, an insect's attention is less likely to be distracted by objects seen peripherally.