The Mechanisms of the Trapping Effect of Artificial Light Sources Upon Animals

In: Archives Néerlandaises de Zoologie
Author: F.J. Verheijen1
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  • 1 1Laboratory of Comparative Physiology, University of Utrecht
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Attempts were made to find out why insects and fishes can be captured with the help of lamps, why birds fly against lighthouse lanterns, and why in the laboratory phototaxis is preponderantly positive phototaxis. An extensive review of the literature revealed that none of the numerous old and new theories on photic orientation can account for either of these phenomena. Analysis of the abundance of data on the trapping effect of an artificial light source upon insects, fishes and birds has led to the working hypothesis according to which the low illumination intensity of the environment around such a light source interferes with normal photic orientation resulting in a drift of the animal towards the light source. The observed concentration of animals in the vicinity of a lamp is thought to be the statistical result of this drift. Experiments with insects (bees) demonstrated that an adequate screening of the light scattered from the sky, together with the elimination of the reflection of light by the environment really result in a disorientated drift towards the light source, even when this is the natural light source (the sun). Fishes and birds were forced to move towards a lamp under similar illumination conditions. Photic orientation is assumed to be accomplished by the goal-directed functioning of a number of hierarchically coordinated centres. The animal's movements are controlled by optic feedback based upon the normal differences in the intensities of the light stimuli acting upon the respective photosensitive surfaces. During more detailed orientation, fixation mechanisms are put in circuit by higher coordinating centres in response to sign stimuli. The normal values of these stimuli are determined by the normal angular light distribution in the animal's habitat, which is caused by: i. the nature of the light sources (sun, moon, stars); 2. the scattering and absorbing capacities of the media (the atmosphere and the water) ; and 3. the reflecting capacity of the environment. The abnormal feedback resulting from the abnormal angular light distribution around a lamp-brought about by the elimination of the factors 2 and 3-makes the animal deviate from the intended position or direction of locomotion. Moreover, the servomechanisms of lower coordination levels controlling the fixation movements of the eyes become a play-thing of the stimuli from the lamp that are quantitatively supernormal as compared with the adequate sign stimuli which normally activate the higher coordination centres of the fixation mechanisms. In this way these higher centres are more or less eliminated from the orientation process. Under extreme laboratory illumination conditions this results in a forced drift of the animal towards the lamp irrespective of factors which are incompatible with survival. Similar phenomena in human beings suffering from disturbance of the centres mediating eye movements, and in patients with far advanced cerebral degenerations (apallic syndrome) are thought to favour this concept. The implications of the present concept of photic orientation and disorientation are discussed with regard to the current concepts of pho- totaxis and photokinesis, the light trap technique, some optical illusions, and glaring lights in traffic. The tendency among cyberneticians to overrate the performances of life-imitating-e.g. "phototropic"- machines, which trifle with the complexity of living organisms, is criticized.

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