148 COMPOUND EYES. 



once seen that the pigment is differently placed, being 

 in front of the rods, while in the vertebrate eye it is 

 behind them. Again, the position of the rods them- 

 selves is reversed in the two cases. 



Passing on to the compound eye. Fig. 100 gives a 

 section of the eye of a cockchafer (Melolontha), after 

 Strauss-Diirckheim. The separate facets of such an 



Fig. 100.— Section through the eye of a cockchafer (Melolontha) ; after Strauss- 

 DUrckheim. 



eye act themselves as lenses, and give a very perfect 

 image. 



As regards the number of facets, Leeuwenhoek calcu- 

 lated that there were 3180 facets in the compound eye 

 of a beetle which, however, he does not name. In the 

 house-fly (Musca) there are about 4,000 ; in the gadfly 

 (CEstrus), 7,000; in the goat moth (Cossus), 11,000; 

 in the death's-head moth (Sphinx atropos), 12,000 ; 

 in a butterfly (Papilio), 17,000; in a dragon-fly 

 (iEschna), 20,000; in a small beetle (Mordella), as 

 many as 25,000. 



