^ 341. 



THE INSECTA. 



433 



are always extravascular, and in this way bathes all the organs.® The 

 newly-prepared nutritive fluid passes through the walls of the digestive 

 canal in which it is found, into the visceral cavity, and thence directly into 

 the blood. Latterly, this extravascular circulation has been called in quea* 

 tion, but its presence may be easily and directly observed with very many 

 perfect Insecta and their larvae. The vascular walls supposed to have 

 been seen at certain points, are, undoubtedly, the result of some error of 

 observation or interpretation.*^* This is also true of the pulsatile organs 

 supposed to have been observed in the legs of many water-bugs, and which 

 were thought to affect the circulation.'^"* 



CHAPTER VII. 



EESPIRATORY SYSTEM. 



§ 341. , 



The Insecta respire, in all their conditions of life, by means of a system 

 of Tracheae which are spread through the entire body and penetrate all the 

 organs. This system of air-vessels either opens externally by stigmata 

 through which the atmospheric air is introduced directly, or they have no 

 external communication, but derive the air from the water by means of 

 lamelliform or tubular prolongations with which the tracheae terminate, 

 and which have often been compared to branchiae. <*' In the first case, they 

 are called Pulmonary tracheae, and in the second, Branchial tracheae. 



8 In the antennae, the legs, the filaments of the 

 tail, and other appendages, the arterial and ven- 

 ous currents are contiguous. But in the wings 

 they are are isolated ; and although they may be 

 observed in the nervures of the wings, yet these 

 last should not therefore be regarded as true blood- 

 vessels, for their cavities are only prolongations of 

 the viscera! cavity, as is shown by the fact that 

 they are sometimes traversed at the same time by 

 branches of tracheae. In the memoir of Verloren 

 (loc. cit. p. 76) will be found a very complete ac- 

 count of all the reasons opposing the presence of 

 vascular walls in Insecta. 



9 The same should probably be said about the 

 thin walls which Bowerbank, and Newport (loc. 

 cit.) think they have observed with Ephemera 

 concerning the two lateral currents which run to- 

 wards the posterior extremity of the abdomen. 

 Another vessel which, according to Treviranus 

 (Zeitsch. f. Physiol. IV. p. 182, Taf. XIV. fig. 1.3) 

 and Newport (Philos. Trans. 1834, p. 395, PI. XIV. 

 fig. 9, and Cyclop, loc. cit. p. 980), is found in the 

 larvae and imagines of Lepidoptera above the 

 ganglionic chain, and is the analogue of the supra- 

 81)U-al artery of the Myriapoda (^ 284), requires 

 further researcli, for it may be questioned if such 

 an organ, found only in certain groups of Insjota, 

 is really a vessel. 



87 



10 Very dissimilar and contradictory opiniona 

 have been published on these pulsatory organs. 

 Behn {Mailer's Arch. 1835, p. 554, Taf. XIII. flg. 

 13, 14, or Ann. d. Sc. Nat. IV. 1835, p. 5) has de- 

 scribed them with Corixa, Ploa, Naucoris, Nepa, 

 and Ranatra, as thin, movable lamellae attached 

 to the inner wall of the tibiae. Verloren (M^m. 

 loc. cit. p. 82, PI. VI. fig. 24, 25) has confirmed 

 these observations with the Cicadidae, although 

 neither L. Ditfour (Ann. d. Sc. Nat. IV. 1835, 

 p. 313) nor Wesmael (Bullet, de I'Acad. de 

 Bruxell. III. p. 158) has been able to discover 

 them in the water-bugs above cited. It is possible 

 that these apparent pulsations are produced simply 

 by the contractions of neighboring muscular fibres. 



1 See Burmeister (Handb. &c. I. p. 179 ; Lor- 

 cordaire, Introduct. &c. II. p. 89 ; and Newport, 

 Cyclop, loc. cit. p. 983). These organs have not the 

 structure of true branchiae, and the blood is not 

 subjected in their interior to the respiratory act, a» 

 is shown by the small quantity of this fluid which 

 traverses "them. These false branchiae are evi- 

 dently designed to receive air, or, to speak more 

 properly, to act, tlirough endosmosis and exosmosis^ 

 in the transference of air from the water into tbe 

 tracheaen system. Dugis (Trait* de PhysioL H. 

 p. 549) is therefore correct in terming them Brctn- 

 chies trackiales. 



