4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



the light itself. ... If the current curves of radiating energy, e. g., light rays, 

 strike an animal on one side only, or on one side more strongly than on the sym- 

 metrical side, the velocity or the kind of chemical reactions in the symmetrical 

 photosensitive points of both sides of the body will be different. The consequence 

 will be in a positively heliotropic animal a stronger tension or tendency to con- 

 tract in the muscles connected with the photosensitive points of the one side of 

 the body than in those connected with the opposite side. 



Mast (56, pp. 57, 58) points out several problems in the study of 

 light reactions which entomologists should carefully consider. They 

 are discussed with others as follows : 



(b) Are light reactions adaptive f — No, according to Loeb's defini- 

 tion. He says that animals go toward light neither because it is useful 

 for them to do so nor because they enjoy it, but because they are 

 photopositive. Mast strongly refutes this explanation by saying (56, 

 pp. 298, 237) : 



Reactions to light are in general adaptive. There are, however, certain reac- 

 tions which are clearly injurious and often fatal; as, for example, the flying of 

 insects into a flame and the positive reactions of organisms which live in dark- 

 ness. But the positive reactions of insects are ordinarily advantageous. It is 

 only under artificial conditions that they prove fatal, and the ancestors of many 

 animals which now live in darkness lived in the light. Positive reactions were 

 probably advantageous to them, and the power to respond thus was probably 

 inherited by the offspring, in which it is useless. . . . Negative response to light 

 tends to keep these creatures [blowfly larvae] buried in cadavers where they find 

 food. It is ordinarily only under artificial conditions that the reactions of organ- 

 isms to light prove fatal. Positive reactions to candle, lamp and lighthouse de- 

 stroy untold numbers of moths and flies and bees and beetles and birds, but who 

 has seen such fatalities under natural conditions? Under such conditions the 

 responses to light direct these animals to the advantage of their well-being. 



Loeb's (43, p. 160) explanation of the origin of adaptive light 

 reactions follows : 



The fact that cases of tropism occur even where they are of no use, shows 

 how the play of the blind forces of nature can result in purposeful mechanisms. 

 There is only one way by which such purposeful mechanisms can originate in 

 nature ; namely, by the existence in excess of the elements that must meet in 

 order to bring them about. 



Mast (56, p. 368) adds that light reactions are variable, modifiable, 

 and in general adaptive, and that regulation constitutes perhaps the 

 greatest problem of life. Loeb (43, p. 125) believes there is a photo- 

 tactic difiference between the sexes of Lepidoptera, for male moths are 

 more apt to fly into candle flames than are the females. It is well 

 known, however, that both sexes are attracted to strong electric lights. 

 It was assumed long ago that moths fly into flames because they are 

 fond of light, but Loeb assures us that this is a purely mechanical 



