§ 341. 433 
THE INSECTA. 
are always extravascular, and in this way bathes all the organs.® The 
uewly-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 ques- 
tion, ]ut 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. 
RESPIRATORY 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 vavities are only prolongations of 
the visceral 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. 
9The 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. 13) 
and Newport (Philos. Trans. 1834, p. 395, Pl. 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- 
spiral artery of the Myriapoda (§ 284), requires 
further research, for it may be questioned if such 
an organ, found only in certain groups of Insecta, 
is really a vessel. 
387 
10 Very dissimilar and contradictory opinions 
have been published on these pulsatory organs. 
Behn (Miuller’s Arch. 1835, p. 654, Taf. XIIT. fig. 
18, 14, or Ann. d. Sc. Nat. IV. 1835, p. 5) has de- 
scribed them with Coriza, Ploa, Naucoris, Nepa,, 
and Ranatra, as thin, movable lamellae attached 
to the inner wall of the tibiae. Verloren (Mém. 
loc. cit. p. 82, Pl. VI. fig. 24, 25) has confirmed 
these observations with the Cicadidae, although 
neither L. Dufour (Ann. d. Sc. Nat. IV. 1835. 
p. 313) nor Wesmaél (Bullet. de VAcad. du 
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; La- 
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, as 
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, through endosmosis and exosmosis, 
in the transference of air from the water into the 
tracheaen system. Duges (Traité de Physiol. IL 
p. 549) is therefore correct in terming them Bran- 
chies trachéales. 
