RECENT STUDIES UPON THE LOCOMOTOR RESPONSES OF 

 ANIMALS TO WHITE LIGHT. 



BY 



E, D. CONGDON. 



During the last few years attention has been given to the Hght reactions of nearly 

 all the large groups of invertebrates. The sudden appearance of data upon the 

 photic responses of animals differing greatly in habits and in mechanism of locomo- 

 tion has naturally resulted in a variety of opinions as to the proper classification of 

 their orientations. The wide latitude as to precision of light control, amount of 

 quantitative experiments, emphasis laid upon the mechanism of locomotion, and the 

 like, exhibited by various investigators, has increased this diversity. Nevertheless 

 recent discussions make good the claims of triaF and phototaxis^ as two mutually 

 exclusive but closely associated categories within which most features of animal 

 light response may find a place. Papers not concerned with these subjects may in 

 most cases be best considered in relation to the animal group to which they refer. 

 The period to be given attention extends from the year igoo to 1907 inclusive. 



PHOTOTAXIS. 



Although some of the postulates of the mechanical phototactic theory of a few 

 years ago have not survived, there can be no doubt that most of the animals to which 

 it was applied have one characteristic in common. They align with the light by a 

 movement whose direction has a definite relation to a localized photic stimulus. 

 Some recent papers may help us to determine the accuracy and speed with which 

 they align and the relation of bilateral symmetry to the procedure. 



Harper's accounts ('05, '07) of the behavior both of the earthworm Perichaeta 

 and the larva of the insect Corethra have an important bearing upon the questions 

 just suggested. The earthworm is found to react by the trial method if the light 

 be of low intensity. Under greater illumination exploring movements in the direc- 

 tion not aiding orientation gradually disappear. The animal then aligns itself 

 with the light by a few quick turns. This procedure illustrates the fact that the 

 turning provoked by localized stimulus may consume an appreciable amount of 

 time and may consist of a series of movements. It might be mentioned here that 

 Parker found Planaria to orient phototactically by a curved course of some length. 



The larval Corethra has a discontinuous jerky locomotion. The successive 

 advances are invariably in a path bending towards the source of light. Neverthe- 



' The expression " trial and error " may be shortened to " trial " because the second term is implied 

 in the first. 



^The view of Radl is here adopted that the older term phototropism should be applied to all motor 

 responses of animals which are produced by light as does geotropism to those produced by gravity, 

 chemotropism to those produced by chemical stimuli, etc. 



