The interaction between visual edge fixation and skototaxis in the mealworm beetle Tenebrio molitor

Summary

(1) It has been shown in earlier experiments that during the visually guided orientation response of the mealworm beetle,Tenebrio molitor, edge fixation and skototaxis interact. Under certain stimulus conditions these two effects act against each other and relative movement between the retina and the surroundings shifts the balance in favour of edge fixation. In this paper, three further parameters are described which change the contribution of the two mechanisms to the turning tendency of the animals. (2) When the centre of a broad black stripe lies on one side and one of the edges on the other side of the animal (Fig. 1B, C) then motion of an edge from front to back (progressive movement) more effectively increases the relative weight of edge fixation than do regressive movements (motion of an edge from back to front). (3) In the same situation temporal modulation of the overall illumination weakens skototaxis, dimming more than brightening (Fig. 2 A). From this it was predicted — and confirmed — that the dependence on the direction of motion will be reversed if both the centre and the edge closest to the midline of a broad black stripe lie on the same side of the animal (Fig. 2 B). (4) Animals with one eye blinded can fixate an edge (Fig. 3). Furthermore, edge fixation is mediated mainly by the ventral part of the eyes and skototaxis by the dorsal (Fig. 5). (5) The possible significance of the results for the animal's natural behaviour and for the underlying neural circuitry is discussed.