. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Photoreception 393 It has already been stated that jxiired intensity receptors may result in a type of orientation called tr()]X)ta.\is, involving simultaneous comparison of two intensities incident on different end-organs. Among the arthropods there are instances of a type of orientation called telotaxis, in which orienta- tion in the direction of one source of stimulation occurs without compari- ;*^ An organism exhibiting telotaxis orients to one source of stimulation, even though other sources may be
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Photoreception 393 It has already been stated that jxiired intensity receptors may result in a type of orientation called tr()]X)ta.\is, involving simultaneous comparison of two intensities incident on different end-organs. Among the arthropods there are instances of a type of orientation called telotaxis, in which orienta- tion in the direction of one source of stimulation occurs without compari- ;*^ An organism exhibiting telotaxis orients to one source of stimulation, even though other sources may be present. This implies that, out of a variety of stimuli, one dominates and controls the resultant Fig. 108. The itinerary of an ant subjected to sunlight from difFerent directions (After Santschi). The ant was returning to its nest with the sun on its left. At 1, 2, 3, and 4, the ant was shaded from direct sunlight and subjected to the sun's image from a mirror. The animal preserved its position relative to the source of light. From Fraenke] and Gunn.*' A second type of orientation among animals possessing compound eyes is the light compass reaction. This reaction is of considerable importance in the "homing" insects, the ants and the bees, and forms the basis of the dom- inant sensory clue which guides them back to the nest or hi\'e. The struc- tural basis for the light compass reaction is the compound eve, in which a small source of light can stimulate only a few or, perhaps, only one ommati- dium at a time. Once a certain angular position relative to the sun is assumed, the arthropod has merely to retain the sun's image in the same ommatidium in order to remain oriented. The dominant nature of this orientation is shown in Figure ;*^ A third type of behavior pattern exhibited bv certain aquatic arthropods, although not limited to them, is the dorsal light reaction. The orientation of the bodv is determined bv the direction of illumination and
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