Insects and their world 



the compound eye produces a picture of the scene in front of it, very 

 much Hke a photograph as reproduced in a newspaper or book by the 

 half-tone process. 



That such a mosaic is produced in fact, and not just in theory, was 

 shown by a German, Exner, seventy years ago, who aaually took a 

 photograph through the eye of an insect; so did Eltringham in England, 

 forty years later. 



The tubular part of an ommatidium is called a lens-cylinder , and 

 normally its sides are black for at least part of their length. If the lens- 

 cylinder is entirely black (Fig. 36A), and each ommatidium is therefore 

 a dark tube cut off from the others, then the spots of light in the com- 

 pound image are clear and distinct. This gives a picture of maximum 

 definition, like a picture printed on good quality book-paper, through 

 a fine half-tone screen. This is generally what happens in day-flying 

 insects, when the light intensity is ample. 



Night-flying insects often have the lens-cylinder longer, and without 

 pigment for much of their length. This allows the Hght coming straight 

 down one cylinder to be reinforced by rays coming in through the sides 

 from adjacent facets (Fig. 36B). Thus the spots of light in the compound 

 image are made brighter, but of course correspondingly more blurred. 



Fig. 37. A Polyphemus moth, one of the silk-moths. Order Lepidoptera 



60 



