Peering insects

Over 100 years ago, Exner (1891), pondering the eyestalk movements of crabs, speculated that invertebrates might use image motion to estimate object range. However, the first clear evidence to support this conjecture did not arrive until the middle of the twentieth century, when Wallace (1959) made the astute observation that a locust sways its head from side to side before jumping onto a nearby object (Fig. 80.1A). Wallace hypothesised that this peering motion, typically 5 to 10 mm in amplitude, was a strategy for measuring object range. To test this hypothesis, he presented a locust with two objects subtending the same visual angle. One object was relatively small and was placed close to the locust, while the other was larger and situated farther away. He found that the locust, after peering, jumped almost invariably to the nearer object. In a further series of elegant experiments, recently confirmed more quantitatively by Sobel (1990), a target was oscillated from side to side, in synchrony with the insect's peering movements. When the target was oscillated out of phase with the movement of the head, thereby increasing the speed and amplitude of the object's image on the retina, the locust consistently underestimated the range of the target (Fig. 80.1C). On the other hand, when the target was oscillated in phase with the head, it consistently overestimated the range (Fig. 80.1B). Thus, reduced image motion of the target caused the insect to overestimate the target's range, while increased motion had the opposite effect. These findings demonstrated convincingly that the peering locust was estimating the range of the target in terms of the speed of the image on the retina. It is now known that certain other insects, such as grasshoppers (Eriksson, 1980) and mantids (Horridge, 1986; Kral, 1998; Kral and Poteser, 1997; Poteser et al., 1998), also use peering to measure object range.

Figure 80.1..  

Experiments investigating how locusts measure the range of a target by peering, that is, moving the head from side to side. Range is estimated correctly when the target is stationary (A), overestimated when the target is moved in the same direction as the head (B), and underestimated when it is moved in the opposite direction (C). Thus, the range of the target is estimated in terms the motion of the target's image during the peer. (Adapted from Sobel, 1990.)