4] PHOTOTAXTS AND PHOTOPATHY 197 



different species, --they are closely correlated with the condi- 

 tions of light in which the organism has been reared. As a 

 result of these conditions, we may say that each organism is 

 attuned to its peculiar range and intensity of light. 



Upon the ground of this difference in attunement may be 

 explained the remarkable difference in behavior of butterflies 

 and moths to light. It is well known that butterflies fly 

 towards even the strongest sunlight, whilst moths are secluded 

 during the daytime, but at night fly towards the candle-light. 

 LOEB ('90, p. 46) has performed some experiments with these 

 insects, which I will cite in detail. 



EXPERIMENT 1. (a) Sphinx, Bombyx, and other moths were kept in a 

 large glass cage in a room illuminated only by daylight. As darkness came 

 on, the moths began to fly towards that side of the cage which was next the 

 window. Again (&), pupae of nocturnal moths, left in a room, emerged during 

 the night, and were always found in the morning at the closed window of 

 the room. Finally (c), a nocturnal moth, made to fly in the daytime, directed 

 its way to the window. Thus, nocturnal moths are positively phototactic 

 to diffuse daylight as well as candle-light. 



EXPERIMENT 2. Hawk-moths were brought into a room with the single 

 window at one end, and a petroleum lamp at the opposite end. It was 

 found that, as twilight came on, the moth flew to the window, or to the light, 

 according to the relative intensity of the one or the other at the point where 

 the moth was liberated. Thus, there is no preference for artificial light. 



The conclusion at which LOEB arrived was that these moths 

 undergo a diurnal variation in responsiveness to light, which 

 corresponds to the change from day to night. But the fact 

 that, in experiment 1 c, nocturnal moths flew, in the daytime, 

 towards the diffusely lighted window, throws a doubt upon this 

 interpretation. All the facts are equally well explained upon 

 the following ground: Butterflies are attuned to a high intensity 

 of light, moths to a low intensity ; so that bright sunlight, 

 which calls forth the one, causes the other to retreat. On the 

 other hand, a light like that of a candle, so weak as not to 

 stimulate a butterfly, produces a marked response in the moth. 

 We shall consider, in a moment, the cause of these differences 

 in light attunement. 



We have seen that one internal condition modifying response 

 is the racial quality of attunement. A second is that of period 

 of life. Thus, LOEB ('90, p. 56) has found that, at the intensities 



