634 



CLASS IV. INSECTA. 



by the shifting backwards of the mouth ; this supplies the 

 antennae ; (iii) the tritocerebrum which theoretically supplies the 

 lost intercalary or premandibular segment and which sends 

 branches to the labrum (Fig. 389). This part of the brain probably 

 takes a large share in the formation of the circumoesophageal 

 commissures which end in the infraoesophageal ganglion. This, 

 like the supra- oesophageal nerve-mass, is composed of three Jiised 

 pairs of ganglia, which correspond with the maiidibular and 



1st and 2nd maxillary seg- 

 ments respectively. The in- 

 fraoesophageal ganglion sup- 

 plies these appendages and 

 contains a maxillula neuro- 

 mere (Fig. 390). It also sends 

 nerves to the salivary glands. 

 In some genera, e.g. Nepa, 

 Acanthia and a few others, the 

 infraoesophageal ganglion is 

 fused with the first thoracic, 

 but the more usual arrange- 

 ment is that this ganglion is 

 followed by a series of three 

 others which belong respec- 

 tively to the pro- meso- and 

 meta-thorax, and supply 

 nerves to the appendages, 

 muscles and other structures in 

 these regions. The nervous 



FIG. 391.-Cephalic and ventral chain of gan- S y stem haS typically a pair of 



fOT 



segment, but a greater or less 

 fusion nearly always takes place. The posterior ganglion is 

 always complex, and the entire abdominal chain may be concen- 

 trated into one ganglion, as in Gyrinus and in many Diptera 

 (Fig. 391 B), or even drawn up into the thorax and fused with 

 the meso- and meta- thoracic ganglia as in Lachnosterna fusca. 

 This concentration reaches its height in the parasitic Diptera 

 Pupipara where infraoesophageal, thoracic and abdominal 

 ganglia are fused into one common mass. 



A system of " sympathetic " or " stomatogastric " nerves 



