Nerve-Centres 



33 



(fig. 36). The hindmost of these is larger than 

 the rest and is regarded as formed of the fused 

 nerve-centres of the last six segments. For a nerve- 

 centre to each segment is the typical arrangement 

 in arthropods. 



Fusion of the nerve-centres of several segments, 

 which in the 

 Cockroach has 

 taken place to the 

 comparatively 

 slight extent just 

 mentioned, is 

 carried much 

 farther in more 

 highly developed 

 insects. In the 

 Stag-beetles the 

 three nerve- 

 centres of the 

 fore-body are dis- 

 tinct though the 

 second and third 

 are nearly united, 

 but the number of 

 nerve-centres in 

 the hind-body is 

 reduced to three. 

 In Bees there are 

 only two nerve- 

 centres in the 

 thorax and five in the hind-body (fig. 47 ggg). In 

 Gadflies the three thoracic nerve-centres are fused 

 into a single mass ; those of the abdominal chain, 

 though still distinct, are moved close together and 

 far forward. In Chafers and in House-flies and 

 their allies all the nerve-centres behind the sub- 



c 



Fig. 25. — Brain of Cockroach from side. oe. gullet : 

 o/. optic nerve ; sd. sub-oesophageal ganglion ; 

 «/«. mx. i/i.t'. nerves to jaws ; /. tentorium. 

 Magnified 25 times. From Miall & Denny 

 (after Newton). 



