XI 



PHYLUM ARTHROPODA 



595 



ganglion or brain, a sub-cesophageal ganglion, also double, and a series 

 of thoracic and abdominal joairs of ganglia, which are closely united 

 together in the middle line. The brain is relatively large in the 

 higher Insects, and is divided into several lobes. It gives off nerves 

 to the antennae and ocelli and the labrum.and on each side it gives 

 off a large lobe the optic ganglion on which the compound eye 

 rests. A pair of cesophageal connect ires pass backwards on either 

 side of the mouth from the brain to the sub-cesophageal ganglia. 

 These connectives are very short, and, as a consequence, the brain 

 and sub-cesophageal ganglia are closely approximated. From the 



FIG. 485. Nervous systems of four species of Diptera to illustrate various degrees of concentration. 

 A, non-concentrated nervous system of Chir cmomvis plumosus with three thoracic, and 

 six abdominal ganglia ; B, nervous system of Enipis stercorea with two thoracic and five 

 abdominal ; C, nervous system of Tabanus bovinus. with one thoracic ganglion and with 

 the abdominal ganglia closely approximated ; D, nervous system of Sarcophaga carnaria, 

 with all the ganglia of the ventral chain united together with the exception of the sub- 

 oesophageal. (From Lang's Comparative Anatomy.) 



latter there originate nerves to the appendages of the mouth the 

 mandibles and two pairs of maxillae. There are sometimes three 

 pairs of thoracic, and as many as eight of abdominal ganglia in 

 the adult insect ; but in many cases there is a greater or less 

 degree of concentration of the ventral ganglionic chain (Fig. 485), 

 and in some of the Diptera this reaches such an extreme that all 

 the ventral ganglia, with the exception of the sub-cesophageal, are 

 united into one continuous elongate mass. The Insects, like the 

 higher Crustacea, possess a visceral or sympathetic nervous system, 

 connected with the oesophageal connectives and passing backwards 

 on the oesophagus and crop. 



The most highly-developed organs of special sense are the 



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