310 yoiirnal of Comparative Neurology aud Psychology. 



less they do not produce alignment because the animal always curves too far. In 

 spite of its zig-zag path the larva always responds to the greater illumination of one 

 side by turning in that direction. Alignment is defeated by a peculiarity in the 

 method of locomotion. 



Harper points out the inconsistency of applying to Corethra the mechanical 

 theory of phototaxis as it was stated by Davenport for the earthworm. It was the 

 suggestion of that author that orientation would result in a simple and mechanical 

 way if we suppose, first, that light directly modifies the tonus of the muscle and, sec- 

 ond, that optimal illumination gives the highest tonus. Under these conditions the 

 side of the animal towards the more nearly optimal light would contract the most 

 and the animal thus turn towards optimum. In spite of the fact the Corethra is a 

 worm-like larva, the theory cannot apply to it because it contracts on the side away 

 from the optimum. 



Radl ('03) is the author of the most complete account of phototaxis that has 

 yet appeared. A considerable part of his monograph is occupied by his own study 

 upon insects and other arthropods. A variety of interesting facts have been brought 

 to light. Butterflies of various families may be found toward sunset perching upon 

 flowers with body pointed away from the sun, wings outspread and head raised or 

 depressed so as to bring the back of the wing as nearly perpendicular as possible 

 to the sun's rays. In the middle of the day certain species close their wings and 

 align with the light. In general Radl says: "Some butterflies so orient with the 

 sun's rays that in weak sunlight they expose the greatest possible surface, in strong 

 sunlight the smallest surface of the wings." He leaves the explanation for later 

 investigators. Bohn has described similar orientation in butterflies. Certain 

 dragon flies persistently orient with the right side to the sun. Midges have a curi- 

 ous way, little understood, of flying in a circle or spiral within a small area at some 

 pomt near a light. This place may be forsaken and a new position taken up, only 

 to repeat the previous behavior. Actively moving Cladocera are found in dense 

 swarms within the free spaces among the algal clumps in fresh water ponds. There 

 IS a clear band several centimeters wide between them and the shore. The face of 

 the moving mass is clean cut and follows every irregularity in the lateral surface 

 of the algal mass. A similar condition of things is not obtained in the laboratory 

 and an explanation has not yet been discovered. 



Radl also found that many aquatic arthropods show an orientation to the light 

 divorced from their locomotor response. Daphnia, for example, regularly orients 

 with its back to diffuse or direct sunlight, while at the same time moving about in 

 a non-directive way. It will turn its back upward if the light be made to come from 

 above or downward if the direction be reversed. Animals were kept in an inverted 

 position for two weeks in this way with no diminution in the precision of the 

 response. He also obtained that locomotor reaction described bv Davenport, 

 Yerkes and others, for Crustacea, which is characterized by rapid orientation to 

 light and a symmetrical arrangement of the body in relation to the direction of the 

 rays. The conditions which determine whether orientation with back towards the 

 light or with orientation with long axis in line with the light and accompanied by 

 locomotion, shall control the animal have not been determined. 



Other peculiarities regarding the relation of body to locomotion have been 

 recently described for arthropods. Pycnogonids (Cole '01) go towards the light 

 with the head leading provided they are crawling, but swim toward it with the abdo- 



