270 INSECTA. 



heart, but does not appear to be received by any vessels, and 

 therefore becomes again diffused through the body. The diffused 

 character of the circulation met with in insects may easily be made 

 a matter of observation in many of the transparent aquatic larvae 

 that are readily to be met with. When any of the limbs of these 

 larvae are examined under a powerful microscope, continual cur- 

 rents of minute globules are everywhere distinguishable, moving 

 slowly in little streams ; some passing in one direction, others in 

 the opposite : but that these streams are not contained in vascular 

 canals is quite obvious, from the continual changes which occur in 

 the course of the globules ; their movements, indeed, rather re- 

 semble those of the sap in chara, and other transparent vegetables, 

 in which the circulation of that fluid is visible under a microscope. 



The organs appropriated to furnish the different secretions met 

 with in the economy of insects, are modified in their structure to 

 correspond 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 en- 

 velope of the caterpillar, are all derived from the same source, and 

 in some mysterious manner elaborated 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. 



(312.) In the nervous system of the INSECTA, we have many 

 interesting illustrations of that gradual concentration of the parts 

 composing it, and consequently of increased proportionate deve- 

 lopement 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 su- 

 pra-O2Sophageal 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 addi- 

 tional nervous apparatus, apparently representing the sympathetic 

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

 propriated to digestion : each of these divisions will therefore re- 

 quire a separate notice. 



