250 



CAUSES OF VARIATION 



In the very highest animals little but the eye is sensitive to 

 light, most parts being protected by a heavy epidermis or other 

 covering, so that heliotropism in its strictest sense is in them 

 limited to the visual parts. In lower animals, however, with 

 bodies less protected, light exerts a controlling influence upon 

 movements. 



Influence of light upon the direction of locomotion. 1 It has 

 already been explained that some animals exhibit heliotropism 

 only at certain periods of their lives, or only in certain condi- 

 tions, as when hungry. Others, however, are constantly and 

 uniformly sensitive. The common house fly is positively helio- 

 tropic, while the larvae of the same, hatched in the dark, soon 

 become strongly negative, and so continue while in the larval 

 condition. 2 



This difference between the larval and adult stage is common, 

 and led Loeb at first to suppose it to be a general principle, 

 a conclusion invalidated by the fact that caterpillars and their 

 imagoes behave alike. 2 



Both moths and butterflies are positively heliotropic, but 

 moths are " attuned " to a lower intensity. This, with their 

 more rapid flight, is responsible for their wholesale destruction 

 by the naked flame, which the slower-flying butterflies avoid as 

 a source of heat. 



The tendency of many small animals to creep into crevices as 

 if to hide must not be understood as evidence of negative helio- 

 tropism, much less as evidence of timidity. It is often due simply 

 to contact irritability (stereotropism), for it is a well-established 

 fact that living matter is sensitive to contact with other solid 

 substances. This is the principle discussed in Section IV and 

 the one that generally lies at the basis of the huddling together 

 of individuals, or of their crowding into corners or crevices. 

 Loeb brought out this principle very nicely with some negatively 

 heliotropic butterflies which wedged themselves closely between 

 two plates of glass in the presence of light, showing how one 

 tropic influence in this case contact irritability is competent 



1 C. B. Davenport, Experimental Morphology, Part I, pp. 180-210; Loeb, 

 Studies in General Physiology (1905), pp. 1-114, from which most of the examples 

 are cited. 2 Loeb, Studies in General Physiology, p. 20. 



