TROPISMS AND RELATED PHENOMENA 157 



It had been known that a number of animals hide in crevices. 

 This phenomenon was generally ascribed to a supposed timidity or 

 photophobia of these animals, which were believed in this way to pro- 

 tect themselves from their enemies. I showed that in this case the ani- 

 mals are forced to bring their bodies as much as possible in contact 

 with solid bodies. Amphipyra is an outspokenly positively heliotropic 

 butterfly which has a tendency to creep into crevices. If a number 

 of these animals are kept in a box and a plate of glass is put on the 

 bottom of the box so that it rests upon pieces of glass just high enough 

 to allow the Amphipyra to creep under the glass plate, all of the butter- 

 flies will be found after a time collected under the plate. This happens 

 as well when they are in the dark as when the plate of glass is exposed 

 to full sunlight. As long as they cannot creep into crevices they run 

 around restlessly, while they become quiet as soon as their bodies come 

 in contact on all sides with other solid bodies. 



The crevices thus act like a trap where such animals are gradually 

 caught until metabolic changes (need of food) again make them rest- 

 less, and compel them to move about.* 



A similar form of irritability exists in ants. When sexually mature 

 ants are kept in boxes containing pieces of folded paper or cloth, all of 

 these animals will be found after some time in the folds, even if the 

 box is absolutely dark. This form of reaction leads to the foundation 

 of a nest, inasmuch as the female, after pairing, creeps into a crevice, 

 where it lays its eggs. 



This form of irritability is also found in worms. If, e.g., earth- 

 worms are kept in a glass vessel with a horizontal bottom and vertical 

 walls, they collect and crawl in the angle between the vertical and 

 horizontal side. Experiments which S. S. Maxwell made on Nereis, 

 a marine Annelid, show how great the force is which keeps such animals 

 in contact with solid bodies. These animals burrow in the sand. If 

 they are kept in a porcelain dish, into which a number of glass tubes 

 have been put which are just large enough to allow a Nereis to enter, 

 it will be found that in about twenty-four hours each tube will contain 

 a Nereis. The animals cannot even be induced to leave the tube if 

 the latter is exposed to direct sunlight, which kills them, although by 

 crawling out they might save their lives. f There are other forms 

 which avoid contact with solid bodies as persistently as the animals 

 thus far mentioned seek it. This form of irritability, negative 

 stereotropism, is found in many swimming forms, e.g. the nauplii of 

 Balanus. 



* Loeb, Der Heliotropismus der Thiere, 1889. 

 t S. S. Maxwell, Ffiiiger's Archiv, Vol. 67, 1897. 



