3o6 Journal of Comparative Neurology and Psychology. 



to observe the precise movements concerned in orientation, and 

 in many larger forms the rapidity, irregularity, or indefiniteness 

 of their light reactions renders the same difficulty almost equal- 

 ly great. In studying the reactions of animals to light we are 

 naturally confronted with the question as to how far the move- 

 ments involved are the result of choice, or something analogous 

 thereto, and how far they may be explained as the result of 

 reflex responses to photic stimuli. If they mainly fall into the 

 latter category we are led to inquire just what these reflexes 

 are and how they produce the particular kind of behavior 

 observed. 



It is a quite commonly accepted hypothesis that the pho- 

 totactic reactions of organisms are effected by the action of light 

 directly or indirectly upon the tension of muscles concerned in 

 locomotion. In nearly all insects and in a large proportion of 

 other arthropods this tension, if it exists, must be brought 

 about through the central nervous system, since the opacity of 

 the integument prevents any appreciable direct effect of light 

 upon the musculature. In most arthropods phototactic impulses 

 are set up by means of light entering the eyes, and not as in 

 many lower forms through the stimulation of the integumental 

 nerves ; this is shown by the fact that when the eyes are black- 

 ened over or destroyed responses to light no longer occur. In 

 most animals it is not possible to observe any effect of light 

 upon muscular tension, although there is considerable indirect 

 evidence that such an effect is produced. As Radl^ has remark- 

 ed, it is difficult to explain the fact that an insect with one eye 

 blackened over moves about in a circle except on the assump- 

 tion that light affects unequally the tension of the muscles on 

 the two sides of the body. Such circus movements are com- 

 parable to those which take place in a vertebrate animal upon 

 the destruction of the semicircular canals in one side of the 

 head. After this operation there is produced a marked differ- 

 ence in the muscular tonus of the two sides of the body and, as 

 a consequence, the animal, instead of going in a normal manner 



'Untersuchungen iiber den Phototropismus der Tiere, 1903. 



