206 PROCEEDINGS OF THE AMERICAN ACADEMY. 



III. Phototaxis. 



The influence of light on the direction of locomotion has heen very 

 generally noticed among organisms, even the mostly widely separated. 

 The swarm spores of many algae, desmids, and other lowly organized 

 plants, are as truly responsive to light stimuli as are crustaceans or verte- 

 brates. According to the character and direction of the stimulating 

 light rays, two kinds of light responses have been distinguished. Photo- 

 taxis is the response with reference to the direction of the rays of light. 

 The organism moves in the path of the ray, either positively (toward) 

 or negatively (away from it). The response to different intensities of light 

 from which the directive force of the rays has been eliminated is known 

 as photopathy. A photopathic animal is one that selects, out of a series 

 of uniformly increasing intensities of light, a limited field of a certain 

 intensity. 



Some animals, like butterflies and fresh-water Entomostraca, are 

 strikingly positively phototactic to diffuse daylight ; others, such as the 

 earthworm and the leech, are as pronouncedly negative. The kind of re- 

 sponse (positive or negative) may be different in closely allied forms and 

 in different stages of development of the same species. For example, 

 butterflies are attracted by strong sunlight, while moths are repelled 

 by it. The adult house fly is positively phototactic to daylight ; its 

 larva, negatively (Loeb, '90, pp. 69-77, 81-83). 



The phototactic sense has been shown in some forms to change with 

 different intensities of light. Thus, Famintzin ('67) found that swarm 

 spores positively phototactic to a certain intensity of light became 

 negative to a light of greater intensity. The same phenomenon has 

 been found true of various flagellates, desmids, diatoms, oscillariae, etc. 

 Wilson ('91, p. 414) found that Hydra fusca was attracted by diffuse 

 daylight and repelled by strong sunlight. Finally, the moth's liking- 

 for candlelight and aversion to daylight is well known. The fact 

 that many organisms are photopathic — that is, have a preference 

 for light of a certain intensity — makes it probable, in connection with 

 these observed variations in phototactic responses, that, for most organ- 

 isms, there is an optimum intensity to which they will respond posi- 

 tively. This optimum will vary widely in different species, probably 

 according to the habits and the usual environment of the species. In- 

 habitants of sunny pools or the open air will have an optimum of rela- 

 tively high intensity ; those which dwell in the ground or in shady places 



