THE SELECTION OF RANDOM MOVEMENTS AS A 

 FACTOR IN PHOTOTAXIS.^ 



By S. J. Holmes. 



Contributions from the Zoological Laboratory of the University of jMichigan, No. gi. 



It is customary to explain tiie phototactic movements of 

 organisms as dependent upon a sort of forced orientation which 

 is brought about by the fact that when the two sides of the 

 body are stimulated unequally they give unequal muscular re- 

 sponses which result in turning the animal until both sides are 

 stimulated to the same degree, when the creature moves either 

 towards or away from the light in the direction of the rays. 

 But however satisfactory this explanation may be for the photo- 

 tactic movements of most organisms, there are several cases 

 where locomotion is directed by the rays of light which cannot 

 be accounted for in this way. It is well known that earth- 

 worms, leeches, and the larvae of house-flies, blow-flies and 

 many other insects have a strong tendency to shun the light 

 and collect in the darkest regions they can reach. These forms 

 have been cited as affording typical illustrations of negative pho- 

 totaxis. The often precise orientation of these organisms to 

 the direction of the rays very naturally disposes one to explain 

 their phototactic movements as taking place according to the 

 scheme just mentioned. No one has attempted to work out in 

 detail the exact mode of response in any of these forms, al- 

 though the fact of their orientation to the direction of the rays 

 of light has been described by several different observers. A 

 variety of explanations may be offered according to the gener- 



' The descriptive part of this paper with the exception of a few minor changes 

 was read before the section of Animal Morphology of the International Congress 

 of Arts and Sciences at St. Louis, Mo., Sept. 21, 1904. 



