I. INTRODUCTION. 



Although more than a century and a half ago Reaumur (1734) recorded some 

 of the earliest observations on the reactions to light of butterflies and moths, it was 

 possible for Loeb ('90, p. 46) to declare recently that a satisfactory explanation had 

 had not yet been found for the paradox that moths, which avoid daylight, fly into 

 a flame at night, while butterflies, which fly by day, do not possess this fatal instinct. 

 From his own observations Loeb maintained that butterflies as well as moths are 

 positively phototropic, a conclusion supported in a measure by the previous obser- 

 vations of Graber ('84, p. 208) and of Plateau ('89, p. 76) and subsequently con- 

 firmed by Seitz ('91, p. 337). He further believed that butterflies and moths show 

 in their capacity to be stimulated by light a rhythmic change corresponding in peri- 

 odicity to day and night, and that butterflies are positively phototropic in the day- 

 time but not at night, and moths the reverse. Thus a butterfly would fly towards 

 the light in the daytime but not at night, and a moth would fly toward a lamp at 

 night, but remain unaffected by daylight. Davenport ('97, p. 197) questioned the 

 accuracy of this explanation because even during the daytime moths will fly toward 

 the light, and he expressed the opinion that the two sets of animals were attuned 

 to different light intensities. According to Davenport moths fly toward a weak light, 

 such as that of a candle, but away from a strong light, like that of the sun, while butter- 

 flies respond only to strong light, such as bright sunlight, into which they fly. Since 

 Davenport has shown good reason for doubting Loeb's explanation, but has left his 

 own hypothesis untested, it follows that there is still no satisfactory explanation for 

 the condition of affairs first pointed out by Loeb. 



The following observations on the phototropism of the mourning-cloak butterfly, 

 Vanessa antiopa Linnaeus, will show, I believe, that this problem, at least so far as 

 butterflies are concerned, is much more complex than was suspected by either Loeb 

 or Davenport. The reactions of V. antiopa to light cannot be satisfactorily con- 

 sidered without dealing with the influence of heat, food, and gravity; and, though it 

 is not intended in this paper to discuss at length the effects of these stimuli, their 

 relations to phototropism will of necessity be taken into account. 



In New England V. antiopa ordinarily produces two broods a year; the imagos 

 of one of these hatch in midsummer, those of the other emerge in the late autumn 



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