148 



COMPOUND EYES. 



once seen that the pigment is differently placed, being 

 in front of the ri>ds, while in the vertebrate eye it is 

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

 selves is reversed in the two cases. 



Pnssing on to the compound eye, Fig. 100 gives a 

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

 Strauss-Diirckheim. The separate facets of such au 



^1 



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

 DiiickheiiD. 



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 

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

 house-fly (Bliisca) 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 

 (^schna), 20,000; in a small beetle (llordella), as 

 many as 25,000. 



