THE SPECIAL SENSES 239 
eye an organ of excessively complicated structure and of 
remarkably perfect seeing capacity. Our own eyes are 
organs of extreme structural complexity and of high de- 
velopment, although some of the other vertebrates have 
undoubtedly a keener and more nearly perfected sight. 
The crustaceans and insects have eyes of a peculiar 
character called compound eyes. In addition most insects 
have smaller simple eyes. Each of the compound eyes is 
composed of many (from a few, as in certain ants, to as 
many as twenty-five thousand, as in certain beetles) eye ele- 
ments, each eye element seeing independently of the other 
eye elements and seeing only a very small part of any ob- 
ject in front of the whole eye. Al] these small parts of 
the external object seen by the many distinct eye elements 
are combined so as to form an image in mosaic—that is, 
made up of separate small parts—of the external object. 
If the head of a dragon-fly be exam- 
ined, it will be seen that 
two thirds or more of the 
: : Fra. 152.—Some of the facets 
Fia. 151.—A dragon-fly, showing the large com- of the compound eye of a 
pound eyes on the head. dragon-fly. 
whole head is made up of the two large compound eyes 
(Fig. 151), and with a lens it may be seen that the outer 
surface of each of these eyes is composed of many small 
spaces or facets (Fig. 152) which are the outer lenses of 
the many eye elements composing the whole eye. 
