NERVOUS SYSTEM. 



537 



The ventral chain, which with its lateral nerves may be compared 

 to the spinal cord and the spinal nerves, preserves the primitive 

 uniform segmentation in most larva?, and is the least modified in 

 insects with a free prethorax and long abdomen. In such insects, 

 not only do the three large thoracic ganglia, which supply the wings 

 and legs with nerves, remain separate, though certainly they are 

 often strengthened by the anterior abdominal ganglia, but also a 

 larger number of abdominal ganglia. Of the latter, the last, which is 

 formed by the fusion of several ganglia and gives off numerous nerves 

 to the ducts of the generative apparatus and to the rectum, is always 

 distinguished by its considerable size. The gradually progressing 

 concentration of the ventral cord, which may lie followed out in the 

 larval and pupal development,* is ex- 

 plained by the crowding together of 

 the abdominal ganglia, as well as by 

 the fusion of the thoracic ganglia. Of 

 the latter, those of the meso- and 

 meta-thorax first fuse to a large pos- 

 terior thoracic mass, which then fuses 

 with that of the prothorax to form a 

 common thoracic mass. When the 

 latter is finally united to the fused 

 mass of the abdominal ganglia, the 

 highest grade of concentration, which 

 is found in the Dipt era and Hertiip- 

 tera, is reached. 



The visceral nervous system is di vie 1 < ( 1 

 into the system of the cesophageal 

 nerves and the true sympathetic. In 

 the former we can distinguish unpaired 

 and paired cesophageal nerves. The 

 unpaired system springs from the anterior surface of the brain by two 

 roots, which unite in front to form the so-called frontal ganglion (fig. 

 445 Gfr.) In its fm-ther course on the dorsal surface of the oesophagus 

 it forms a number of fine plexuses in the muscular layer of that organ 

 (tig. 445). The paired <esophageal nerves spring on either side from 

 the posterior surface of the brain, and swell out at the sides of the 

 ii'sophagus to form larger ganglia, which also supply nerves to the wall 

 of the oesophagus. A system of pale nerves, first described by Newport 



* Compare especially the numerous papers of Ed. Brandt, ' Ueber die meta- 

 morphose des Nerrensystems." 



'IG. 41.5. Cerebial ganglion and ceso- 

 phageal nerve gauirlia nt' SjJiiii.r 

 ligustri (after Newport). G/V., Frontal 

 ganglion ; g' , g', ganglia, of the 

 paired cesophageal nerves. 



