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. All 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 



FIG. 151. A dragon-fly, showing the large com- 

 pound eyes on the head. 



FIG. 152. Some of the facets 

 of the compound eye of a 

 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. 



