NEBVOUS SYSTEM OF INSECTS. 327 



with in the economy of insects are modified in their structure to cor- 

 respond with the character of the circulation, and are invariably simple 

 tubes or vesicles of various forms immersed in the fluids of the body, 

 from which they separate their peculiar products. The poisonous saliva 

 of bugs, and the innoxious salivary fluid of other insects the bile and 

 auxiliary secretions subservient to digestion the venom which arms the 

 sting of the wasp, and the silky envelope of the caterpillar, are all 

 derived from the same source, and in some mysterious manner elabo- 

 rated from the blood by variously-formed vessels : but of this we have 

 already given many examples, and others will present themselves in the 

 following pages. 



(855.) In the nervous system of the INSECTA, we have many interest- 

 ing illustrations of that gradual concentration of the parts composing it, 

 and consequently of increased proportionate development of the nervous 

 centres, corresponding with the more active movements and higher 

 faculties by which the class before us is so remarkably distinguished 

 from those forms of articulated animals that we have hitherto had an 

 opportunity of examining. The supra-cesophageal ganglion, or brain, 

 assumes a preponderance of size in relation to more perfect organs of 

 sense and to instincts of more exalted character ; the chain of ganglia 

 placed along the floor of the abdomen is composed of a few large masses 

 of sufficient power to animate the strong and energetic muscles of the 

 limbs ; and, moreover, anatomists have detected the existence of an 

 additional nervous apparatus, apparently representing the sympathetic 

 system of vertebrate animals, which is distributed to the viscera appro- 

 priated to digestion. Each of these divisions will therefore require a 

 separate notice. 



(856.) The brain, or encephalic ganglion (fig. 164, l), is a nervous 

 mass of considerable size placed above the gullet : it consists essentially 

 of two ganglia united into one mass ; and from it all the nerves appro- 

 priated to the special instruments of the senses are derived ; so that it 

 may naturally be regarded as the chief seat of sensation and intelligence. 

 The nerves originating from this common sensorium are seen upon an 

 enlarged scale in fig. 165 : they are the optic (fig. 165, a), supplying the 

 eyes, and the antennal (fig. 165, e\ which run to the special instruments 

 of touch, or antennce organs of a very singular character, that we shall 

 examine more minutely hereafter. Two other cords of variable length 

 (fig. 165,# g) are given off from the inferior aspect of the brain, and 

 serve to connect it with the anterior ganglion of the ventral chain 

 (fig. 165, Ti), to which some writers have thought proper to give the name 

 of cerebellum, though upon what grounds it is difficult to conjecture : the 

 mass last mentioned gives off various nerves to supply the parts con- 

 nected with the mandibles, maxillce, and other organs of the mouth. 



(857.) The rest of the ventral chain of ganglia forms a continuous 

 series (fig. 164, 2, 3, 4, 5, 6, 7, 8) of nervous centres arranged in pairs and 



