Eyes -i,"] 



microscope shows us that it presents the appearance 

 of" a honejxomb, being divided into a vast number 

 of small six-sided figures (fig. 28 a), each of these 

 is a corneal facet ; there are about 1 800 of them 

 in the eye of the cockroach. In many insects, whose 

 eyes are globular in form and much larger relatively 

 to the size of the head than those of the cockroach, 

 the number of corneal facets is greatly increased ; 

 some Dragonflies have 20,000 and some Hawk-moths 

 as many as 27,000. On the other hand, the number 

 of facets is sometimes very small ; the eye of the 

 well-known little Springtail, the " silver-fish " insect, 

 has only twelve. In some water-insects the facets 

 are relatively small in 

 number and large in 

 size, being also circular 

 in shape instead of hex- 

 agonal (fig. 28 h). Eyes 

 like this containing a 

 number of corneal 

 facets are known as 



7 / ^^ o-r \ Fig. 28. — a. Hexagonal corneal facets from 



compound eyes (20, 21;. the eye of a Fly (^w;//i). Magnified 



The compound eyes of 4°° times. ^^ Eye of female Midge 



r J (C limio) with few circular facets. 



male insects are often 



larger relatively than those of their females (fig. 



31). 



By cutting sections vertically through the eye, 



we are able to trace the connection between the 

 facets and the nerve-fibres of the optic lobe of the 

 brain. Each facet is seen to be composed of two 

 transparent layers — an outer layer convex on both 

 surfaces, and an inner layer concave outwardly and 

 convex inwardly. With its base on this inner sur- 

 face rests a short blunt crystalline cone surrounded 

 by pigment ; the apex of the cone is supported by 

 a nerve-rod (rhabdom). The rod when cut across 



