522 



THE EYE IN EVOLUTION 



Figs. 687 to 691. — The Nervous System of Insects. 





Fig. 687. 



Fig. 688. 



Fig. 687. — The nervous system of the larval stage of Lepidoptera (caterpillar). 

 Note the cerebral (supra-oesophageal) ganglia connected with the sub-oesophageal 

 ganglion by the circum-cesophageal nerve ring and the chain of ganglia of the 

 ventral cord. 



Fig. 688. — The nervous system of Hemiptera (water-bug). The ganglia of the 

 ventral cord are fused into one. 



Fig. 689. 



Fig. 690. 



Fig. 691. 



Figs. 689 to 691.^The nervous system of diptera, showing the general arrange- 

 ment of the cerebral and sub-oesophageal ganglia closely approximated and the 

 thoracic and abdominal ganglia of the ventral chain. On either side of the cerebral 

 ganglion the enormous optic lobes and compound eyes project laterally, each larger 

 than the ganglion itself (modified from Lang). 



Fig. 689. — Chironornus, with three thoracic and six small abdominal ganglia. 



Fig. 690. — Tabanus, with one (fused) thoracic ganglion and seven abdominal 

 ganglia closely approximated. 



Fig. 691. — Sarcophaga, with all the thoracic and abdominal ganglia of the ventral 

 chain united in one mass. 



On either side of the protocerebrum there emerge the relatively enormous 

 OPTIC LOBES contained in eye-stalks which bear the compound eyes. The 

 reconstructed eye-stalk of the fresh-water crayfish, Cambarus, is seen in 

 Fig. 692 (Bernhards, 1916 ; Welsh, 1941). Herein several neuropile masses 

 form opti ianglia ; of these, as a general rule in Crustaceans and Insects, 



