164 THE SENSES OF ANIMALS 



number to one looking upwards and a pair looking side- 

 ways. In some insects such as dragonflies their structure leads 

 us to suppose that they give a sharp image and are used for 

 sight, but in others their function is probably different. In 

 higher insects the ocelli may be concerned with perception 

 of the plane in which light is polarized — in the honey-bee 

 orientation and direction-finding are certainly connected 

 with this ability. The exact function of the ocelli in most 

 insects is, however, not fully understood. 



Most insects and most of the higher Crustacea are charac- 

 terized by the possession of a pair of compound eyes each 

 of which consists of a very large number of minute tubular 

 cameras stuck tightly together and arranged so that lenses 

 form part of the surface of a hemisphere. The details of the 

 structure of the tubular cameras, or ommatidia, are com- 

 plicated, and differ widely in different insects, but essentially 

 each consists of a lens at the outer end and light-sensitive 

 cells connected to nerve fibres at the inner end. Each tube 

 is surrounded by pigment cells so that light cannot pass 

 sideways from one to another. In some insects a fine net- 

 work of air tubes also surrounds each tube and acts as a 



Fig, 8. A section of part of the compound eye of a Honey Bee showing the 

 closely packed ommatidia (highly magnified). 



reflector, much as does the tapetum in the eyes of some 

 mammals ; it is the reflection from this network that makes 

 the eyes of moths glow when a light is shone upon them at 

 night. 



