26o LIGHT AND THE BEHAVIOR OF ORGANISMS 



is not a reaction to light at all, but a reaction to what light 

 represents. 



The distinguishing characteristic which differentiates the 

 responses in the organisms in this and the following group 

 is however superficial. The fundamental principle involved 

 in the reactions of the organisms in both is the same, for 

 the reactions in the former as well as those in the latter are 

 responses to signs. To those in this group the stimulating 

 light condition is a sign of another condition of light either 

 more or less intense than the one to which they respond; 

 to those in the next group it is a sign of an object. And it 

 is this more or less intense light, or the object represented, 

 that is of vital importance to these organisms, not the con- 

 dition of light to which they respond. 



c. Reactions due, not to any effect of light in itself, but 

 to what a given light condition or configuration may repre- 

 sent. — (a) Reactions caused by shadows or a sudden 

 decrease -in light intensity, representing either enemies or 

 food : Examples — the sudden contraction or movement of 

 tubicolous annelids, numerous echinoderms, mollusks and 

 arthropods, and the response of Clepsine, shrimps, prawns, 

 mosquito larvae and Fundulus. In case of Clepsine the 

 shadows undoubtedly represent food; in the rest, with the 

 probable exception of shrimps and prawns, it represents 

 enemies, (b) Reactions to sudden exposure to light or 

 increase of Intensity probably representing enemies, espe- 

 cially in case of earthworms : Examples — Arenicola and fly 

 larvae, earthworms and a few mollusks. (c) Reactions 

 caused by the size of the luminous area in connection with 

 intensity: Examples — Butterflies (Vanessa), Water scor- 

 pion (Ranatra) and frogs (Rana and Acris). In some of 

 these organisms the positive reaction to a large area in 

 preference to a small one of the same intensity prevents 

 flight toward the sun, and it probably has something to do 

 with mating, (d) Reactions caused by size, form, varia- 

 tion In shadow and color of luminous area: Example — 

 The higher animals, especially man. In these organisms 



