REACTIONS OF LAND ISOPODS TO LIGHT 



237 



TABLE 11 



When Porcellio is compared with Asellus, the contrast is not 

 so striking, because Porcellio is somewhat like Asellus in not 

 being always negative. However, the fact that Porcellio, like 

 Oniscus, responds to intensities as low as 0.0119 CM. suggests 

 that it, too, is more discriminating in its reaction to light than 

 Asellus. Whether or not negative phototaxis was intensified as 

 an adjustment to land life, it at least appears to be a more im- 

 portant factor in the activities of land isopods than of aquatic 

 isopods. 



In this connection a few observations will be given concern- 

 ing one of the most primitive of land isopods, so regarded because 

 its structure is less specialized for life on land. The large active 

 isopod, known as the sea slater (genus Ligyda), lives in crevices 

 of rocky shores just above the high-tide line among the San 

 Juan Islands of the northern part of Puget Sound. At low tide 

 the.se isopods emerge from their hiding-places and migrate down 

 the rocks nearly to the low-tide line. During this period they 

 can often be seen moving about with apparent disregard of the 

 bright sunlight. . At other times, however, they are found in 

 crevices, and in the laboratory they give a negative reaction to 

 sunlight. In their case the reversal of reaction keeps them in 

 the damp region close to the water's edge, so perhaps it is advan- 

 tageous for them not to have so consistent a negative reaction 

 to light as that of Oniscus. 



From this ecological study of the phototaxis of land isopods 

 it may be concluded that the usual negative phototaxis is advan- 



THE JOURNAL OF EXPERIMENTAL ZO6LOGY. VOL. 27, NO. 2 



