244 CHARLES HARLAN ABBOTT 



from the light, and by calculation of the average 'angle of 

 negativeness.' 



3. Porcellio is in general negative to the same stimuli as 

 Oniscus, but is, of the two genera, the less consistently negative. 



4. Oniscus and Porcellio were found to respond to a range of 

 intensities from 100 CM. to 0.01 CM. The response is the 

 same to all intensities. On account of the extreme sensitive- 

 ness to low intensities, the threshold of stimulation was not 

 determined. 



5. No consistent change in the phototaxis of Oniscus was 

 caused by repetition of stimuli. Under these conditions, the 

 animal usually became less active, but accuracy of orientation 

 was not interfered with. 



6. The reaction of Oniscus was essentially the same whether 

 previously exposed to strong light or to dark and whether kept 

 in a maximum or minimum of moisture. Porcellio was some- 

 what less negative after living in a dry habitat, and some indi- 

 viduals of this genus were found to be positive after exposure to 

 strong light. 



7. When the isopods were immersed in water, they did not 

 respond to light. 



8. Land isopods are not greatly different structurally from 

 their aquatic relatives. They are confined to a limited habitat 

 on the land, where humidity is comparatively high and the 

 evaporating power of the air is relatively low. 



9. Negative phototaxis appears to be a factor in keeping these 

 animals in a suitable habitat, and is thus important in fitting 

 them for life on land. 



10. Oniscus, with a more restricted habitat than Porcellio, 

 has a more consistent negative reaction to light. 



11. Land isopods are more definitely negative to light than 

 the fresh-water isopod, Asellus communis. Negative photo- 

 taxis appears to be a more important ecological factor for land 

 isopods than for aquatic isopods. 



12. Oniscus appears to be oriented directly by light, and to 

 respond, at least under some circumstances, to a continuous 

 light stimulus acting at a constant intensity. 



