32 THE TROPISMS 



contain are consequently ejected. The prawns replace 

 these by small bodies which they pick up with their chelae and 

 put in the statocyst. Kreidl placed several Palsemons after 

 moulting in a dish containing soire small bits of iron, which 

 the prawns put into their statocysts in place of the usual 

 sand. If now a strong magnet was brought near, the bits 

 of iron would be brought toward it and exercise a pull 

 analagous to the vertical pull of gravity. Kreidl found that 

 by changing the position of the magnet the Palsemons could 

 be induced to orient their bodies in any desired position. 



When in the web many kinds of orbweaving spiders hang 

 with their heads down. Mosquitoes, on the other hand, 

 when resting on a vertical wall usually have the reverse 

 orientation. Other insects when under rocks or boards 

 tend to hang down from the upper surface rather than rest 

 on the lower one. Lady beetles, according to Loeb, when 

 placed in a dark box uniformly crawl upward and come to 

 rest at the highest point. Cockroaches prefer to rest on the 

 vertical sides of a box rather than the top or bottom, but they 

 do not seem to have a marked tendency to crawl upward to 

 the highest point. A tendency to crawl upward is not un- 

 common in insect larvse, especially those which feed on 

 plants, a trait which is naturally of service in leading them 

 to their food. 



The tendency to maintain a certain orientation with 

 respect to gravity is more common than the bent toward 

 upward or downward migration. This is especially true 

 of higher animals, and among the vertebrates orientation is 

 practically the only response to this force. The inner ear 

 in vertebrates plays an important part in the maintenance 

 of the normal position, but a description of the numerous 

 experiments on this subject would carry us too far. The 

 presence of a specific sense organ for the maintenance of 



