SLIME SLUGS, MYRIAPODS AND INSECTS 131 



ocelli. The external surface of the compound eyes is revealed 

 by the microscope to be divided into many small hexagonal 

 facets. Each of these is a minute, rigid, flat lens behind which 

 exists, arranged as a slender, rod-like organ, 

 a perceptive unit of the compound eye. 

 These units are called ommatidia, and each 

 one is composed of a crystalline part just 

 behind the external lens, surrounded by pig- 

 ment, and behind this a sensitive nerve end- 

 ing, called a rhabdome, also surrounded by 

 pigment. From the rhabdome a fine nerve 

 runs backward to join with the similar fine 

 nerves from the other ommatidia to form 

 the large optic nerve going to the brain. 

 Each ommatidium is thus a complete little 

 eye with light gathering and transmitting 

 and perceiving apparatus, but without means 

 of change of focus, and only slight means of 

 adjustment to varying intensity of light. 



This slight means of adjustment depends 

 on the power of the insect to move the pig- 

 ment surrounding the ommatidia back and 

 forth, and thus to arrange it in a way to 

 allow more or less light to reach the rhab- 

 domes. 



Most of the light rays that enter each om- 

 matidium must be those that fall nearly ver- 

 tically on the external lens, and pass verti- 

 cally in to the sensitive rhabdome. Thus 

 each ommatidium "sees" only any object or 

 part of any object directly in the line of its ^; pigment; >-., 

 long axis, and as the surface of the compound retinal parts; O.H., 

 eyes of insects is usually strongly curved, (fter "j^ner; 

 separate ommatidia usually see only separate greatly magnified.) 

 points in objects of the environment. These 

 points put together side by side form a mosaic correspond- 

 ing to the object or objects in front and at the sides of the 

 eyes. Such seeing is called mosaic or apposed vision. 



-on 



FIG. 53. Lon- 

 gitudinal section 

 through a few 

 facets and eye-ele- 

 ments (ommati- 

 dia) of the com- 

 pound eye of a 

 moth. /., cor- 

 facets; cc., 



