472 GENERAL BIOLOGY 



psychic mechanism, the more lasting appears to be the 

 result of getting used to a stimulus. 



Study 61. Observations on certain reactions of caterpillars. 

 Materials needed: Living caterpillars in normal and 

 healthy condition, and a supply of their appropriate food. 

 Silk worms (and mulberry leaves) will do for this, or almost 

 any of the larva? of the larger butterflies or moths. The 

 larva? of the common milkweed butterfly have been used 

 successfully. 



1. Study the creeping reactions of an active caterpillar. 

 Observe first its method of locomotion. Then place the 

 specimen on a reversible cord (as, for example, a bow cord 

 and note the persistence of its almost unvarying reaction to 

 the pull of gravity. It crawls upward. Reverse the cord 

 betimes, and observe the result. 



2. Then test the variability of reactions to stimuli such 

 as will stop its crawling; a puff of breath, or a sharp rap on a 

 table on which its support is resting. Most caterpillars 

 have a characteristic habit when stopped of turning the 

 head either up or down. Determine by trial (using a watch 

 for timing) how often it is necessary to repeat the puffs of 

 air or the raps on the table in order that they may fail to 

 elicit the reaction. 



3. Test the time it takes to inhibit the feeding reaction 

 by substitution of unsuitable food. Get a hungry cater- 

 pillar to feeding on the margin of the leaf of its proper food 

 stuff, and slip up beside the leaf a sheet of some thin sub- 

 stance unsuitable for food (such as a leaf of some other plant 

 which the larva of itself will not eat, or a sheet of paper or of 

 tinfoil) . Note any evidence of dissatisfaction with the sub- 

 stitute food. Then, allowing the caterpillar to resume eat- 

 ing the right leaf, determine by trial what interval must 

 intervene before it will again bite of the substitute. Ob- 



