170 TROPISMS 



is familiar to many readers and the writer may be par- 

 doned for quoting from a formerly published observation. 



Ammophila, a solitary wasp, makes a small hole in the ground and 

 then goes out to hunt for a caterpillar, which, when found, it paralyses 

 by one or several stings. The wasp carries the caterpillar back to the 

 nest, puts it into the hole, and covers the latter with sand. Before this 

 is done, it deposits its eggs on the caterpillar which serves the young 

 larva as food. 



An Ammophila had made a hole in a flower bed and left the flower 

 bed flying. A little later I saw an Ammophila running on the sidewalk 

 of the street in front of the garden, dragging a caterpillar which it held 

 in its mouth. The weight of the caterpillar prevented the wasp from 

 flying. The garden was higher than the sidewalk and separated from it 

 by a stone wall. The wasp repeatedly made an attempt to climb upon 

 the stone wall, but kept falling down. Suspecting that it might have 

 a hole prepared in the garden, I was curious to see whether and how 

 it would find the hole. It followed the wall until it reached the neigh- 

 boring yard, which had no wall. It now left the street and crawled 

 into this yard, dragging the caterpillar along. Then crawling through 

 the fence which separated the two yards, it dropped the caterpillar near 

 the foot of a tree, and flew away. After a short zigzag flight it alighted 

 on a flower bed in which I noticed two small holes. It soon left the bed 

 and flew back to the tree, not in a straight line but in three stages, 

 stopping twice on its way. At the third stop it landed at the place where 

 the caterpillar lay. The caterpillar was then dragged to the hole, pulled 

 into it, and the hole was covered with tiny stones in the usual way. 293 



It is not enough to say that the animal possesses 

 associative memory and returns to the hole; we must 

 add that the brain image of the region of the hole becomes 

 the source of a forced orientation of the animal — of an 

 added special tropism — compelling the animal to return to 

 the region corresponding to the image. And the same 

 may be said in regard to the return of the wasp to the 

 caterpillar which had been temporarily deposited at the 

 foot of the tree. 



This example, which might be easily multiplied, will 



