1S15.J On the Uses of the Dorsal Vessel. S/j 



same time that the small circulation experiences the action of the 

 air in the lungs. Some insects hiive even peculiar reservoirs of air 

 in their liodies, reservoirs so numerous, that those which are pro- 

 vided witli them ought to have a respiration more than double. 

 The.-<e reservoirs oi' air, called vesicular trachece; but which may 

 likewise l)e called pneumatic pockets, exist only in those insects 

 which have to exert a great muscular force, and which, having 

 great spaces to traverse, ought to have their body specifically light. 

 From what has been said, it is evident that the air ought to have 

 a great influence on the general economy of insects. We shall 

 see hereafter, that it is the only fluid wiiich has a real circulation 

 in them. Now the greater this influence is, the more ought it to 

 exert itself on the organ of circulation, provided such an organ 

 exist. As the dorsal vessel has been considered by ditferent ana- 

 tomists as coming in place of the heart in insects, let us see whether 

 the influence of the respiratory organ is sensible on this vessel. 

 But to lay open that influence in a more certain manner, let us 

 examine in the first place the organs of respiration themselves, and 

 see whether, when these organs undergo modifications, the dorsal 

 vessel feels their effects. 



All the organs of respiration in insects may be reduced to simple 

 trachccc, The*e organs indeed have not all the same composition ', 

 we must therefore divide them into two orders: the first, which we 

 shall call tubular trachece, on account of their disposition, are com- 

 posed of three membranes, one external, one internal, and one 

 Intermediate. The two first are formed of a cellular membrane, 

 pretty thick, and very extensible; while the intermediate one con- 

 sists of a cartilaginous string, rolled up into a spiral, which may 

 be unrolled with great facility. It is the convolutions of this 

 elastic string which forms those brilliant and silvery looking con- 

 duits, destined to bold air, and to transmit it to all parts of the body. 

 These traebejB are always kept stretched by means of this carti- 

 laginous string, 'i'hey form tul)es, and have a great elasticity, 

 which enables them to dilate, when the air, continually circulating 

 in them, expands. These tracheic are likewise the only ones which 

 are ramified, tbe multiplied branches of wiiich go to all parts of 

 tlie body, to communicate the impression of air, the aliment of 

 life as well as of tlame. 



The second order of tracheae, or the I'esicular trachece, do not 

 form tubes like the prece<ling ; they present, on the' contrary, 

 pouches more or less extended, wbich communicate with one 

 andtlier by means of rainifications, always single, and never 

 branched like those proceeding from the spiral tracheae. These 

 vesicular trachea,' are com])o«ed of two very white cellular mem- 

 branes, very supple au'i very e.xtenblblc. As these trachea; have not 

 the spiral string dbservalilc? in the first order, they never communi- 

 cate iinmediateiy with the air, but always by means of spiral 

 traclicee. Accordingly, in liie species which have neec] uf a great 

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