How they become aware of their surroundings 



Each facet is in fact the end of a tubular structure called an ommati- 

 dium, so that a section through the eye, shown diagrammatically, looks 

 like Fig. 36. A single ommatidium has something of the same structure 

 as a human eye, but its working is quite different. The human eye works 

 Hke a camera. The cornea and lens together throw an image on the 

 retina, as if on the film in a camera, and objects at varying distances are 

 brought into sharp focus by muscular control of the lens. In the in- 

 sea's ommatidium the facet and the crystalline cone beneath it are like 

 the cornea and lens of the human eye, and produce an image which falls 

 on a retina at the further end of the tube ; but there the resemblance ends. 

 The insect's eye has no means of focusing, and the retina of one 

 ommatidium cannot register anything more than an impression of a 

 spot of light. 



The effect of all the ommatidia together is to produce a mosaic of 

 spots of Ught, each of a brightness corresponding with that of the part 

 of the field of view that is in line with that particular tube. In this way 



Object 



Object 



Spot of light 



Lenses 

 (facets) 



long "" Pigment " short \ \ 



Cells 

 sensitive 

 to light 

 (retinulae) Spot of light 



Fig. 36. Diagram of part of a compound eye of an insect: A, 'day-adapted', 

 with each component (ommatidium) separated from the others by pig- 

 ment, so that the point on the retina, marked by the arrow, receives light 

 from one facet only; b, 'night-adapted', with the pigment drawn back, so 

 that each point on the retina receives light from a number of adjacent 



facets 



59 



