28 ECONOMIC ENTOMOLOGY 



3. The Proctodeum, embracing the portion posterior to the stomach. 

 The stomodoeum and proctodoeum are ectodermal in origin while the 

 mesenteron is probably entodermal. 



(e) Nervous System.— The nervous system of insects consists 

 essentially of a series of ganglia joined by a double nerve-cord lying 

 along the ventral surface of the body. Each primitive segment had a 

 double ganglion, but in most insects fusion of ganglia occurs in the head, 

 the thorax, the anterior and the posterior portions of the abdomen. 

 Great variations occur even in the same order. The largest ganglion 

 is in the head, and is the result of the fusion of three pairs. It forms 

 the brain or supra- (esophageal ganglion, lying above the oesophagus. 

 There is also another large ganglion, the suh-cesophageal, lying below the 

 oesophagus, and connected with the brain by a double nerve-cord about 

 the oesophagus, the oesophageal nerve collar or commissure. It also repre- 

 sents the fusion of three pairs of ganglia. 



From the brain nerves are given off to the antennae, eyes, and 

 labrum. The sub- oesophageal ganglion controls the mouth-parts. 

 From the ganglia in the thorax and abdomen nerves supply the various 

 segments and control their movements and activities (Fig. 26). 



In addition there is the Sympathetic nerve system which mainly lies 

 along the dorsal hne of the alimentary canal. It consists of a recurrent 

 nerve arising from a, frontal ganglion and ending in a stomachic ganglion. 

 Two pairs of lateral ganglia are connected with the recurrent nerve and 

 supply nerves to the dorsal vessel and the tracheae of the head. A 

 ventral system lies in the main nerve cord and activates the spiracles. 



Nerve-cord and Ganglia. — The nerve-cord consists of an axis-cylinder 

 of fibrillae and a sheath. It is concerned with the transmission of 

 impulses and stimuli. The ganglion is a centre for the regulation of 

 nutrition. It consists of a dense cortical layer of gangUonic cells with 

 large nuclei, a clear medulla from which nerve fibrillae originate, and a 

 nerve sheath. 



(/) Reproductive System. — In all insects the sexes are distinct. 

 The sexual organs are situated in the abdomen and consist in the 

 female of a pair of ovaries and a pair of oviducts opening into the vagina 

 and frequently externally by an ovipositor, and in the male of a 

 pair of testes and a pair of seminal ducts {vasa deferentia) opening into 

 the ejaculatory duct and externally by an intromittent organ. The 

 external opening lies between the eighth and ninth segments of the ab- 



