Microscopical Essays, 



life. As the trachea are divided and fubdivided to a prodigious 

 degree of minutenefs, it has been conjeclured by fome writers, 

 that they may act as fo many fieves, which, by feparations pro- 

 perly contrived, filtrate the air, and fo furniih it to the body of 

 different degrees of purity and fubtilty, agreeable to the purpofes 

 and nature of the various parts. The experiments that have been 

 made with the air-pump are by no means conclu five ; the injury 

 the infe6l fuftains when the atmofpheric preffure is taken from the 

 body, does not prove that it infpired and exfpired the air that we 

 have removed ; it only {hews that an incumbent preffure is 

 neceflary to their comfortable exiffence, as it prevents the fluids 

 from difengaging themfelves in an aerial form, and as it counter- 

 balances and re-a&s on the principle of life, and by keeping the 

 action thereof in proper tone and order, confines and regulates 

 its energies. 



Though it is difficult to afcertain whether fome infects refpire, 

 at lead at certain periods of their exiffence, yet there are others 

 to whom the infpiration and exfpiration of air feems abfolutely 

 neceflary : there are many aquatic infefts who are obliged to keep 

 their tails fufpended on the furface of the water for this purpofe. 

 To prove this, keep the tail under water, and you will perceive 

 the in feci to be agitated and uneafy, and to feek for fome open- 

 ing to expofe this part to the air ; if it finds none, it foon goes 

 to the bottom and dies. Some aquatic beetles refift the trial for 

 a confiderable time, while their larva can only fupport it for a few 

 minutes. There is a circumftance which renders all experiments 

 on this fubje£l with infe&s doubtful and difficult, namely, the vaft 

 tenacioufnefs of the life principle in the lower orders of animated 

 nature, and it's diffemination through their whole frame. 



G g 2 M. Muf- 



