38 HUMBLE CEEATURES. 



The remaining three are small, simple in their struc- 

 ture, and disposed in a triangle on the top of the 

 head, between the two compound eyes. We shall 

 direct your attention chiefly to the last-named, the 

 compound organs. At the first glance, these re- 

 semble two small hemispheres, covered with a bright 

 brown varnish ; when you examine them more closely 

 with a lens, the convex surface appears to be marked 

 with a species of network (PI. IV. fig. 1), and, when 

 placed under the microscope, this network is foimd to 

 consist of a vast number of convex lenses, disposed in 

 regular rows upon the projecting surface. These 

 lenses, each of which forms the external surface ot a 

 simple, but perfect organ of vision, have been carefully 

 counted, and their number amounts to 4000; in 

 other insects they are far more numerous. 



If we make a vertical section of one of these com- 

 pound eyes, dividing it by an incision across the 

 middle of its circumference, and examiae it under the 

 microscope (PI. IV. figs. 2, 3, 4), we shall find it to be 

 constructed as follows : — Each simple lens or facet is 

 double convex and hexagonal in form; and behind 

 this is a six-sided, transparent pyramid, so attached 

 to the facet, that the latter forms, as it were, the base 

 of the pyramid directed outwards*. Each pyramid 

 is imbedded in a dark pigment ; so that the light 

 which enters one facet may not be dispersed, nor 



* Bymer Jones calls the pyramid " an hexaedral prism, 

 whose office may be compared to that of the vitreous humour 

 of the human eye." 



