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THE POPULAR SCIENCE MONTHLY. 



increased size of the cerebral ganglia, and also to further development 

 of some of the ganglia pertaining to the ventral cord, with concentra- 

 tion or even suppression of others. 



In such insects as butterflies, bees, and dragon-flies, in which the 

 visual organs are enormously developed, and in which the power of 

 vigorous and sustained flight is correspondingly increased, the nervous 

 system attains its maximum of development among the Arthropoda. 

 The brain of these creatures differs from that existing in all other 

 members of the class by. reason of the great development of those 

 portions of it in relation with the visual organs. A ganglionic swell- 

 ing is frequently found where the nerve joins the brain (Fig. 6 ? _Z?,)and 

 in some insects there are also small ganglionic swellings at the cor- 

 responding parts of the antennal nerves. 



As in spiders, the oesophageal ring is very narrow, owing to the 

 greatly-diminished size of the cesoj)hagus in the imago forms of higher 

 insects. The double upper or cerebral ganglion is, however, con- 

 nected in all insects with a separate sub-cesophageal ganglion, from 

 which nerves are given off to the mandibles, the maxilla?, and the 

 labium, though in spiders, crustaceans, and myriapods, as I have before 

 stated, this part has no existence separate from the thoracic ganglia. 



In insects the three thoracic ganglia also often preserve a separate 

 xistence (Fig. 6), though in such higher types as I have named above 



% 



Fig. 7. Brain and Adjacent Parts op Nervous System of the Privet Moth in the 



Pdpa State. 



these ganglia are more frequently fused into a single, lobed mass. 

 The eight abdominal ganglia, which are always much smaller than 

 the thoracic, continue to have a separate existence among some of the 

 less developed types of insects, though it is more frequent for some, 

 or even all, of them to disappear. 



The stomato-gastric system also attains considerable complexity 



