198 LIGHT AND PROTOPLASM [Ch. VII 



employed, the wingless plant lice were hardly responsive to 

 light. The winged form was markedly positively phototactic. 

 So, likewise, in the case of ants (Lobb, '90, p. 63), during the 

 period of the marriage flight the males and females (but not 

 the workers) are strongly positively phototactic, but after that 

 period they show themselves neutral. The case of the house- 

 fly, Musca, is interesting, since the larva and adult are photo- 

 tactic in opposite senses (see table). In most of these cases, 

 the difference in responsiveness is associated with a difference 

 in habit. 



The sense of response depends, also, as we have seen, upon 

 external conditions. In this regard, the immediately preceding 

 conditions of light, the temperature, the concentration, and the 

 supply of oxygen have important effects. We shall consider, 

 in order, the action of these conditions. 



Light can modify the response to light ; thus, Gkoom and 

 LoEB ('90) have shown that the nauplii of Balanus, as well as 

 other pelagic animals, come to the surface of the sea during 

 the night, but descend before the strong sunlight. This does 

 not indicate merely a low light-attunement of the race ; for 

 nauplii exposed to sunlight in the early afternoon are all posi- 

 tively phototactic, and only gradually, as the day progresses, 

 move from the sunny window, until, finally, even as dusk 

 approaches, all are found on the side away from the window. 

 Nor have we here to do with a diurnal change in the sense of 

 the response. For if a culture is kept in the dark, it is found 

 to be at first positively phototactic at whatever time of day it 

 is exposed ; only later acquiring the negative phototaxis. In 

 the same way, when the young Balanus larvae leave the interior 

 of the shell of the parent, they are at first positively phototactic ; 

 but after being in the light for from -^ to 2 hours, they become 

 negatively phototactic. The more intense the light, the quicker 

 its effect. 



Another observation upon the nauplii is representative of a 

 new class of light effects. When nauplii which have become 

 negatively phototactic through exposure are covered for a few 

 minutes, and then suddenly again exposed to light, they move 

 momentarily towards the light, and then begin their negative 

 movement again. Somewhat similar are the results obtained 



