40 ANIMAL ACTIVITIES. 
movement of the eyelids when a sudden blow is 
threatened. Very simple and familiar illustrations of 
this power are common even among plants. In higher 
animals the response to external stimuli is often so 
complicated with voluntary movements that the two 
can hardly be distinguished, but in every animal there 
doubtless exists the power to respond in some way to 
movements of the world outside its own body. These 
movements may affect the animal through simple 
touch, or through any or all of the other senses. 
Every animal has at least one of the five senses. 
Fic. 42.—The Nervous Chain of a Cockroach. 
Sight. Although the eyes of the grasshopper are 
large and composed of many parts, his power of vision 
is doubtless far inferior to ours. Many experiments 
have been made on the sight of insects, and all seem 
to show that they can neither see far nor clearly. The 
compound eyes of insects, as we have seen, are made 
up of many hexagonal facets. Each of these facets 
has a tiny lens for focussing the rays of light on a 
nerve which transmits vibrations caused by these rays 
to the nerve-centres within. 
Hearing. The hearing of the grasshopper is prob- 
ably more acute than his sense of sight. Vibrations 
of the air set in motion the ear-drum on the first seg- 
ment of the abdomen, and these are conveyed to 
nerves which connect with nerve-centres in the thorax. 
The fact that grasshoppers and their relatives are able 
to make noises which doubtless are understood by 
their friends is a reason for believing that they can 
hear. These noises do not issue from organs of speech 
