32 THE TROPISMS 
contain are consequently ejected. The prawns replace 
these by small bodies which they pick up with their chelz and 
put in the statocyst. Kreidl placed several Palemons after 
moulting in a dish containing some 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 Palzemons 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 larve, 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 
