MISCELLANEOUS PAPERS. 389 



The movement is heliotropic. The assumption is that chemical 

 substances are acted upon by the light in such a way as to produce 

 changes of tension in the contractile tissue. Symmetry of form is 

 necessary. 



Again, a galvanic current is passed through a medium in which 

 are placed a number of Palpemonetes. The movement of these ani- 

 mals is toward the annode. Hence, galvanotropism is the term used 

 to explain the orientation. 



Another illustration is that of contact instinct. The crawling of 

 animals into crevices is shown not to be for the purpose of self-preser- 

 vation, but rather to get the body in solid contact as extensively as 

 possible. No center theory is necessary. A peculiar species of 

 butterfly, amphipyra, will run about until it finds a corner or crack 

 into which it can creep. These animals were placed in a box, one- 

 half of which was covered with glass, the other half with a non-trans- 

 parent body. Small glass plates were placed in the bottom of the 

 box, raised just enough to allow the butteriiies to creep under. They 

 collected without any choice of light effects. As soon as their bodies 

 were brought in contact with solid bodies they became quiet, even 

 when exposed to direct sunlight. It is not necessary to look for a 

 center of self-concealment in these animals. The reaction is common 

 to plants. Loeb has given this kind of irritability the name of 

 stereotropism. 



It is a wonderful arrangement in nature that in some species the 

 female lays her eggs in places where the larva? will find just the kind 

 of food they require, decaying meat, cheese, and refuse of animals. 

 The fly lays the eggs on lean meat and not on fat. The larvae will die 

 if fed on fat meat. Further study of larvjB showed that they are ori- 

 ented by certain substances. which radiate from a center — the center 

 of diffusion. The chemical effects of the diffusing molecules on cer- 

 tain elements of the skin influence tension of muscles. Such orienta- 

 tion is termed chemotropism, and if the organism shows the positive 

 kind it is led to those substances which are capable of furnishing 

 appropriate stimuli. Such substances are the volatile nitrogenous 

 compounds. The eggs are deposited in response to a "tropism" and 

 not in response to experience or volition. The presence in the skin 

 of a substance which is altered by the above-mentioned volatile com- 

 pounds, together with bilateral symmetry of the animal, explains the 

 reaction. 



Caterpillars of Corthesia (butterflies) are oriented by the light. 

 Until they have taken food they are positively heliotropic, and this 

 leads them to the tips of the branches, where they find their food ; 

 and as soon as they have eaten they crawl downward. The taking of 

 food destroys the susbtances in the skin which are sensitive to light. 



