The Lungs or _Air-Vessels_ of Insects, are yet exceedingly more different in Structure, Distribution, and Situation from those of perfect Animals, than those of Fishes are, and yet in their Use and Action agree perfectly with both; that is, _receiving_ and _expelling_ the Air, and _varying_ the _Dimensions_ and _Capacities_ of the _Blood-Vessels_. These having no _Thorax_, or separate Cavity for the Heart and Air-Vessels, have the latter distributed through the whole Trunk of their Bodies, by which they communicate with the _External_ Air through several _Spiracula_ or _Vent-holes_, to which are fasten'd so many little _Tracheæ_, or Wind-pipes, which thence send their Branches to all the Muscles and _Viscera_, and seem to accompany the Blood-Vessels all over the Body, as they do in the Lungs only of perfect Animals. By this disposition in every _Inspiration_, the whole Body of these little Animals is inflated, and in every _Expiration_ compress'd; and consequently the Blood-Vessels must suffer a _Vicissitude_ of Extension and Contraction, and a greater motion must thereby be impress'd upon the Fluids contain'd in them, than the Heart, which does not in those Creatures appear to be Muscular, seems capable of giving. The only Animal that is exempted from this necessary condition of _Breathing_, or _receiving_ and _expelling alternately some Fluid_ into and out of the Body, is a _Fœtus_. But this, while included in the Womb, has little more than a _vegetative_ Life, and ought scarce to be reckon'd among the number of _Animals_. For, were it not for that small share of _Muscular_ Motion, which it exercises in the Womb, it might without absurdity be accounted for as a Graft upon, or Branch of the Mother. [Sidenote: Boyle _of the Elasticity of Air. Pechlinus de Aeris & Alimenti defectu._]