Orientation and "fright" reactions of whirligig beetles (Gyrinidae).

Abstract

In the laboratory the stream-dwelling Dineutus discolor Aube and pond-dwelling species of Dineutus and Gyrinus oriented themselves in accordance with the direction of the light source, visible either to their dorsal or ventral eyes. No orientation to complex patterns was obtained. In the field, orientation with respect to the direction of light, or relative position of light and dark portions of the visual field, may partially account for the maintenance of swarms of beetles. But orientation to patterns must still be postulated for the greater part of the observed behavior. We have to suppose a certain degree of habituation to a particular pattern otherwise associated with a favorable situation, so that variations of the visual pattern beyond a certain degree are sufficient to induce a return of the insect to the region to which it is habituated. In the laboratory with the only light source submarine, diving toward it was a frequent reaction. In the dark, disorientation was complete with no gregariousness. The beetles did not show that they detected any difference between standing and running water or the direction of the current. In their natural habitat variations in the visual field congruent with the prevailing locomotor activity are accepted as routine variations, and a state of alarm results only from incongruity of visual and locomotor processes. The "fright" reaction to stimuli of minimal or nearly minimal size (a spot of about 76 sq. deg. or a circular spot subtending a plane angle of 10 deg.) was elicited only when the stimulus increased the complexity of the visual field. Decreases in complexity produced no visible reaction. Changes in brightness of small portions of the visual field were without visible effect. Reaction to the appearance of a spot seems to occur only after a period of habituation (averaging 1 min.) to the less complex visual pattern. The larvae of D. discolor were found under stones in the current of the river at depths up to two feet. Adults of this species transferred to a lake lived there at least 4 weeks but none was found a year later.