LIGHT REACTIONS OF TERRESTRIAL AMPHIPODS 339 



General reactions. — The following points may be taken to 

 distinguish their general behavior: First: The smaller individ- 

 uals usually become positive to light in a markedly shorter 

 time than the larger ones. Second: Cold, moisture and quiet 

 retards positive phototaxis, while heat, dryness and activity 

 hastens it. Third: Contact stimuli, except in the form of mois- 

 ture, do not inhibit nor reverse the phototaxis of positive speci- 

 mens. Fourth: Positive specimens will travel tow rd the light 

 while lying on their sides pressed between plates of glass. 

 Fifth: Positive specimens with one eye blackened over, are 

 just as likely to perform circus movements away from the 

 normal eye as towards it. 



0. traskiana is ordinarily negative to light when it is first 

 exposed to it. The length of time during which it is negative 

 is determined by the amount of light, warmth, moisture and 

 activity it has had previous to being exposed to strong light. 

 Specimens taken from moist, cold sand or seaweed become 

 positively phototactic in from fifteen to twenty minutes. In 

 giving the time length of reactions, I set the criterion at the 

 point when all or nearly all specimens have experienced the 

 change of response. In many lots of specimens, there might 

 be one or two out of a dozen or one out of six that would still 

 remain negative or positive as the case might be, after all the 

 others had reversed the response. On the other hand, several 

 small specimens might be positive very much earlier than the 

 rest. I have endeavored to give the figures which represent 

 the reaction time of the majority of the specimens of a given lot. 



If there are a number of individuals in a dish, they form 

 aggregations at the negative end. They remain close together 

 with their heads turned from the light and held down. If they 

 are not occasionally shaken up and made to run about, they 

 may show no signs of becoming positive to the light for an hour 

 or more. This bunching together does not happen when there 

 are but a few individuals in a dish, and when therefore, there 

 is greater activity. 



The negative reaction is characterized by a hurried running 

 away from the light. After a period of going back and forth 

 along the sides of the dish (weakly negative or indifferent speci- 

 mens generally keep to the sides), they begin to run across the 

 dish at right angles to the light. Then there maybe a crossing 



