EXPLANATION OF PLATE III. 



Fig. 1. Diagram of a frontal section through the head of a nor- 

 mal wild fly. 



Fig. 2. Diagram of a frontal section through the head of a fly of 

 of eyeless stock, having ommatidia present on both sides of 

 the liead. 



Fig. 3. Diagram of a frontal section through the head of a fly of 

 eyeless stock, totally eyeless on one side of the head but hav.ng 

 ommatidia present on the other side of the head. 



Fig. 4. Diagram of a frontal section through the head of a fly of 

 eyeless stock, totally eyeless on both sides of the head. 



Fig. 5. Diagram of a longitudinal section of a single ommatidium. 



9. external pigment cell 



10. sheath of retinulae cells 



11. rods of the rhabdome 



12. internal pigment cell 



13. nerves 



14. retinular nucleus 



15. basilar membrane 



