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- 

 son."*^ 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 orientation. 



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 108."*^ 



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 not bv gravitv. 

 Similar reactions are exhibited by the carp louse, Argiihis foJiacens; the 

 brine shrimp, Artemia salina, and the prawn, Processa caniciilata. In other 

 animals, such as the prawn, Leander xiphias, and the fish, Crenilahrus ro- 

 siatus, the dorsal light reaction is elicited only after removal of the stato- 

 cysts and the labyrinths.'^ 



The Vertebrate Eye: Morphology. The eye of man is rather generalized 

 in structure and may be taken as representative of the vertebrate group. The 

 structure of the right human eye is shown in horizontal section in Figure 109. 



The eye is comparable to a camera in many respects. There is a photosen- 

 sitive screen (retina) on which the cornea and lens form an inverted image 



*Recently Waterman (Science 10: 252-254 (1950)) has confirmed, by recording 

 optic nerve impulses, the earlier evidence from behavior that compound eyes of ar- 

 thropods can distinguish the plane of polarization of polarized light. 



