INSECTS. 



fumed in tliis cafe, and that the refidue of the air is ren- 

 dered impure, by the admixture of carbonic acid gas. 



An important refult of refpiration feems to be the main- 

 taining the initabiliiy of the moving powers : hence the de- 

 finitive refult of this proccfs, with refpeil to the fibre, is 

 power in executing the motions which it has to perform. 

 Thus the energy of the motive force will be in proportion 

 to the quantity of relpiration. The organization of infefts 

 correfponds to this theory : they are the only clafs among 

 the lower ranks of animated exilience, which pofTeffes the fa- 

 culty of flying, and we confequently find the tracheae car- 

 I rying air over the whole body, fo that refpiration is carried 

 j on at all points. In thofe infetls which do not fly, the force 

 ! of the nuifcles may be cllimated by the rapidity of their 

 i other motions. The running of the millepede and the jump- 

 I ing of the flea, ftiew that they belong to a very irritable 

 i clals of animals ; the cafe is analogous to that of the oftrich 

 I and caflbwary, which run with great celerity, although they 

 I are birds without wings. 



Rej'p\raUon of the Griijlacea. — Their branchix are larger in 

 ' proportion than thofe of moll of the moUufca. In the de- 

 ' capoda they are attached to the bafe of the feet, under tlie 

 ; lateral and defcending edge of the thorax, which confines 

 them within a narrow fpace. The branchiopoda have no 

 [ gills in this fituation, but under the tail, between the fins, 

 i and floating loofely in the water. 



I In the crabs the llruclure is in fome refpefts peculiar. 

 \ Each gill reprefents a triangular elongated pyramid, attacli- 

 \ ed by its bafis only, and having the point directed upwards. 

 The middle of the pyramid is divided by a plane, proceed- 

 ing from the apex to the bafis, and compofed of a double 

 membrane ; and the body of the pyramid is formed by a 

 large number of plates placed one over the other, perpendi- 

 cular to the vertical plane jull mentioned, and confiUing 

 merely of doublings of the double membrane. A large 

 veflel runs along each of the two longitudinal edges of this 

 plane, and penetrates at its bafis into the thorax of the ani- 

 mal ; one of thefe is arterial, and the other venous. If we 

 inflate them, immediately all the fmall laminx which com- 

 pofe the pyramid are dilteiided with air. In the fame way 

 the blood is expanded over all the furfaces, and is thus fa- 

 vourably difpofed for tlie action of the water. 



There are feven of thefc pyramids on each fide. As the 

 edge of the thorax, which embraces and confines them, is 

 inflexible, a particular mechanifm became necefiary for re- 

 newing the water which walhes the furface of the organs. 

 Thisetfecft isfecured by two plates of a fubilance refembling 

 parchment, articulated on the cheil near the maxillae, very 

 elongated, and paffing obliquely, the one within, between the 

 branchiie and the body, the other without, between them and 

 the edge of the cheil. By comprefling the branchiae, thefe 

 plates Iqueeze the water from the intervals of the lamina:, and 

 when the prefTure is remitted, they allow the introdudlion of 

 a new portion of fluid. 



The branchial pyramids, although placed in a finiilar 

 fituation, are more numerous and complicated in the deca- 

 poda with long tails, as the lobiler, cray-filh, and pahnurus, 

 Fabr. They have rows of cyhndrical filaments, inilcad ot 

 laminae, piled on each other on the two fides of the vertical 

 plane ; fo that their furfaces refemble the ftiag of velvet. 

 There are feveral thoufands of thefe filaments in a pyramid ; 

 each of them is formed of an artery and a vein united ; 

 each pyramid has two, its large artery and large vein ending 

 in the body. 



Thefe villous pyramids ef the long-tailed decapoda are ar- 

 ranged in groups, between vertical laminae, of which one 

 afceuds behind each group. Thefe laminae are attached to 



the firfl: articulations of the feet, which cannot' move with- 

 out moving them, and without cither comprefling or fctting 

 free the branchire. The lobiler and cray-fifli have five groups 

 of lour pyramids each, and a fiihtary one before and behind, 

 of which the anterior is very fmall. This wiil make twenty- 

 two gills on each fide. The firft group is attached to the 

 moil exterior pair of maxillae : and the folitary pyramid in 

 front to the pair concealed by the former. The fecond 

 group is connedled to the claws, and the others to the fol- 

 lowing feet, except the laft, which has only a folitary pyra- 

 mid. In each group the exterior pyramid is attached to the 

 pedicle of the parchment-like plate, and moves with it : the 

 three others adhere to the body of the animal, and have no 

 feparate motion. The fird fohtary pyramid is alfo attached 

 to its plate ; but the lad is fixed to the body, and has only 

 a rudiment of a plate behind it. Moreover, two other 

 plates are attached to two maxiilae, anterior to thofe of 

 which we have already fpoken, and fupport no gills : yet 

 they are placed obliquely again It thefe organs, and contri. 

 bute to their compreffion and relaxation. The aflion of all. 

 thefe plates occafions the expulfion at the two fides of the 

 mouth of all the water in contaft with the branchis. 



The tail of the branchiopoda, particularly of the fquill.Tr 

 bears on its under furface five pairs of fins, forming broad 

 membranous and ciliated oars, divided into two large lobes, 

 an exterior, which is rather anterior ; and an interior, which 

 is a little poRerior. The gill is attached to the root of the 

 firll at its inner edge. It confifts at firil of a conical pedicle 

 compofed of the two large veflels : from this proceeds a row 

 of cylindrical tubes, gradually diminifhed in fi/e from the 

 baiis towards the apex : each of thefe is curved, and forms 

 a long conical and flexible tail, which alfo fupports a very- 

 numerous row of long and ioofe filaments. Each gill con- 

 tains a very confiderable number of thefe, and refembles, on: 

 the firft view, a large brufli : it is only by feparating the 

 filaments that the regularity of their infcrtion and fucccffioit 

 can be perceived. It is almoft unnecelfary to add that eaclv 

 filament contains two ^-effels ; and that each tail and eaclv 

 tube has the fame vafcular arrangement, juft as in the general 

 pedicle. The branchis float in the water, are moved like- 

 the fins, and are even agitated between the two lobes of the^ 

 latter : confequently no mechanifm is required for renewing 

 the water in contaft with them. 



Trachea and Refpiration of hfeHs. — Some remarks on this 

 fubject will be found in the article Entomologv, under the- 

 divifion Stigmata. We ftiall enter here more minutely into 

 the anatomical details. 



We have already ftated that the furrounding element, the 

 air, dift.ributed by means of an infinite number of tubes,, 

 exerts its adlion on all points of the interior of the body.. 

 Thefe canals have been called trachere, on account of their 

 analogy to the trachea or large veflel which conveys air into- 

 the lungs of fuch animals as pofl'efs thofe organs.. Their 

 ftrufture is remarkalile : they are compofed of three mem- 

 branes, an internal and an external one of the common ilruc- 

 ture ; and a middle one compofed of an elaftic thread, pof- 

 fefling a fine metallic lullrc, rolled fpirally, or in a double 

 fpiral courfe round the canal, from one extremity of the 

 tube to the other, and admitting of being unrolled with a 

 little addi-efs. By this the fides of the tube are conflantly 

 maintained circular, fo that the palFage of the air is always 

 free. Yet all the tracliese are not provided with this part in 

 their whole length ; fome, which Cuvier calls veficular, are 

 dilated at certain intervals, fo as to form merely membranous 

 pouches r:i. t provided with this elailic fupport. 



The tracheae communicate externally by fmall lateral holes 

 pierced, on eath iide of the body, and called ftigmata ; or 



fome- 



