HELIOTROPISM OF ANIMALS 387 
perature for demonstrating their heliotropic activity lies 
between 20° and 30°. 
The experiments on the caterpillars of Porthesia chry- 
sorrheea are typical. I have repeated them on some hundred 
species of insects, but I have never found a positively 
heliotropic insect whose dependence upon light was of a 
different kind from that found in Chrysorrhea. This 
fact has given me the impression that all animal proto- 
plasm, as perhaps all plant protoplasm, is heliotropically 
irritable, and that where this is apparently not the case 
the heliotropic reaction is inhibited, either temporarily or 
permanently, by other causes. For this reason it would 
be useless to publish here every single experiment I have 
made. This would result in repeating each time the same 
phenomena, only under the name of a different insect. 
Since there are only negatively and positively heliotropic 
animals, it would be of secondary interest to know to which 
of the two classes the individual animals belong. But I 
believe it necessary to show by concrete examples what part 
heliotropism plays in the habits and ecology of animals. 
VI. THE POSITIVE HELIOTROPISM AND THE SLEEP OF 
BUTTERFLIES 
Our knowledge of the behavior of butterflies toward 
light has, on the whole, remained at that point which is 
marked by the statement of Réaumur that “‘it is a singular 
fact that those butterflies which shun the daylight are pre- 
cisely those which fly into lighted chambers.”’ The paradox 
has not yet been explained why those butterflies which are 
not to be:seen by day fly into the flame at night, while the 
day butterflies apparently do not possess the tragic “instinct” 
of the night Lepidoptera. There is no lack of conjecture 
on this point. Romanes believes that the lamp is a “strange 
object” to the moths, and that ‘the desire to examine this 
Digitized by Microsoft® 
