308 VICTOR E. SHELFORD. 



V. REACTIONS TO LIGHT. 



I. SPECIFIC PECULIARITIES General Results of preliminary 



Experiments performed. 



A large number of experiments was performed with light. 

 Diffuse day light, tungsten lamps, Nernst lamps, with and 

 without the cylindrical lens were used so as to obtain variations 

 in direction and intensity. Since some of the animals react to 

 intensity, some to direction and not to intensity, and since some 

 readily move out of strong light while others tend to stay out 

 of it, special methods were demanded. 



The general characters of the reactions of different species 

 are given below. 



Etheostoma appears indefinite to light of the intensities used. 

 Individuals make no recognizable response to either direction or 

 intensity. 



Cambarus does not react sharply to ordinary differences of 

 intensity or to direction. In general they appear slightly negative 

 to strong daylight often resting with the anterior end in the lighter 

 parts. In an intensity gradient they back into the dark when 

 the water or the apparatus are jarred which may account for 

 their apparent negativeness, under experimental conditions in 

 which the surrounding medium is disturbed. Finley found that 

 they turn back from white paper used in making records. 



Goniobasis appears positive to direction and to all intensities 

 of room light when intensity accompanies direction and in Fig. 

 38 is believed to be less strongly positive than the animals usually 

 are due to slowness of movement. 



Hydropsyche is apparently indifferent to intensity; reacts posi- 

 tively to direction. 



Argia does not react to intensity, orients negatively to direction 

 reversing soon in some cases. 



Perla starts into greater intensity and turns back; orients neg- 

 atively to direction when a light is turned on. 



Heptageninae orient negatively to direction at first but quickly 

 reverse in high intensity. Do not react clearly to intensity. 

 Turn back on encountering strong light. 



Psephenus is negative to direction, less so to intensity. 



