PHYLUM ARTHROPODA 



247 



thcr. 1 



thor.Z 



plan as in the Crustacea. There is a double supra- 

 (Ksophageal ganglion or brain (br.\ a sub-cesophageal ganglion 

 (inf.\ also double, and a series of pairs of thoracic and 

 abdominal ganglia, which are 

 closely united together in the 

 middle line. The brain is re- 

 latively large in the higher In- 

 sects, and is divided into several 

 lobes. It gives off nerves to the 

 antennae and ocelli (see p. 248) 

 and to the labrum, and on each 

 side arises a large lobe, the optic 

 ganglion, on which the com- 

 pound eye rests. A pair of ceso- 

 phageal connectives (conn.) pass 

 backwards on either side of the 

 mouth from the brain to the sub- 

 cesophageal ganglia. These con- 

 nectives are very short, and, as 

 a consequence, the brain and sub- 

 cesophageal ganglia are closely 

 approximated. There are some- 

 times three pairs of thoracic and 

 as many as eight of abdominal 

 ganglia in the adult Insect, but 

 in many there is a greater or less 

 degree of concentration of the 

 ventral ganglionic chain. 



The most highly developed 

 organs of special sense are the 



large compound eyes, which are situated on the sides of 

 the head. The surface of the compound eye is marked out, 

 as in the case of the Crayfish, into a great number of minute 



FIG. 138. Cockroach (Peripla- 

 neta). General view of the 

 nervous system, abd. 6. sixth 

 abdominal ganglion ; ant. an ten- 

 nary nerve ; br. brain ; conn. 

 cesophageal connective ; inf. 

 sub-cesophageal ganglion ; opt. 

 optic nerve ; thor^, thor*, thor*, 

 first, second, and third thoracic 

 ganglia. (After Miall and 

 Denny.) 



