INSECT STRUCTURES 13 



14. Nervous System. The central nervous system 

 consists of two nerve cords running the length of the body 

 and resting on the ventral or lower wall. Typically, there 

 is an enlargement of these cords in each segment. These 

 enlargements are called ganglia. Each ganglion gives off 

 nerves which supply the motor impulses and receive the 

 sensations for that segment. In the head the two cords 

 separate, one running on each side of the oesophagus. 

 Above the oesophagus they unite again and form what is 

 usually the largest ganglion of the body and is called the 

 brain. From this ganglion nerve fibers go to the com- 

 pound eyes, the ocelli and the antennae and the labrum. 

 There is a separate fiber for each facet of the compound 

 eyes which, as we have seen, might require as high as 

 fifty thousand for each compound eye and this will ex- 

 plain the larger size of the ganglion. There is, in the 

 head, a second ganglion below the oesophagus which ener- 

 vates the mouth-parts. This is the sub-cesophageal gan- 

 glion while the so-called brain is the supra-oesophageal 

 ganglion. From the latter arises also the sympathetic 

 nervous system which consists of a few fibers running pos- 

 teriorly along the top of the alimentary canal and carry- 

 ing the impulses incident to digestion. 



15. Reproductive System. The reproductive system is 

 similar in its organization to that of the vertebrate ani- 

 mals. It consists primarily of two sets of fine tubes com- 

 municating with the outside of the body through passages 

 at first distinct but later united. In the females each one 

 of these sets of tubes forms what is called an ovary. Eggs 

 develop in each tube and, when fully grown, pass out into 

 the oviduct, a passageway into which all the egg-tubes 

 open. From the oviduct, the egg is carried to the vagina, 

 which is formed by the union of the oviducts from both 



