REACTIONS OF LAND ISOPODS TO LIGHT 195 



versity and of the laboratory for the opportunities and facihties 

 for research which were furnished me. My greatest debt is to 

 Prof. H. E. Walter, under whose supervision the work was done, 

 for stimulating my first interest in behavior studies, and for helpful 

 suggestions and criticisms at every stage of the investigation. 



II. HISTORICAL 



While naturalists have commonly concluded that land isopods 

 are negatively phototactic, very little experimental work has 

 been done on their reactions. The papers of Cole ('07) and 

 Torrey and Hays ('14) appear to form the entire literature on 

 the subject. 



Cole ('07, pp. 371 to 375) in "an experimental study of the 

 image-forming powers of various types of eyes" found Oniscus 

 asellus negative to light, but not so uniformly as some other 

 animals, for example the mealworm (larva of Tenebrio molitor 

 Linn.). "Only 45 per cent to 51 per cent of the reactions were 

 away from the light, in excess of those toward the light." Cole 

 exposed Oniscus to illumination from the side, and recorded the 

 place where the animal crossed the circumference of a circle 

 which had a radius of 10 cm. The intensity of the light used 

 varied from 1.5 to 5 candle meters. Cole further found that 

 Oniscus shows a slight discrimination between two sources of 

 light of different diameters which deliver the same intensity on 

 the animal. 



Torrey and Hays ('14), working on the orientation of Por- 

 cellio scaber, recorded that the animal has a negative response 

 to hght, and may be oriented very accurately, 1) by a constant 

 light from behind, 2) by a sudden exposure to lateral light, or 

 3) by a "sudden exposure to light from the front at angles be- 

 tween 90° and 15°." "When exposed suddenly to hght coming 

 from the front at angles less than 15°, Porcellio moved with less 

 consistency away from the light, but the reactions were, on the 

 whole, markedly negative." These authors concluded that the 

 orientation of this species is direct, and is not brought about by 

 selection of random movements. 



