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Show 222 CLASS III. INSECTA. Insects, which form the third class of articulated anima~ provided with articulated legs, have, besides, a dorsal ves· sel analogous to the vestige of a heart, but totally desti· tute of any branch for the circulation( I). They respire by (1) An~tomists are greatly divided with respect to the nature of this organ; some consider it as a true heart; others, among- whom is the Baron Cuvier, deny it this quality, an opinion which appears to us to be fully confirmed by the admi. rable researches of M. Marcel de Serres-" Memoire sur le Vaisseau Dorsal des Insectes"-published in the Mem. du Mus. d'Hist. Nat. According to the latter it secretes fat, which is subsequently elaborated irt the adipose tissue which sur. rounds it. Lyonet says that it contains a gummy substance of an orange colour, Some very recent observations appear to establish the existence of certain very small vessels; but in addition to the fact that this circulation must be very partial, Insects would still greatly differ, in this respect, from the Crustacea, inasmuch as the blood does not return to the heart. M· Straus in his report-Bullet. Univers., de M. le Baron de Ferussac-on a Memoir of M. Herold on this subject, has inti· mated his own opinion on the matter as deduced from his anatomical invcstiga· tiona of the Melolontha. "The dorsal vessel," says that gentleman, "is the true heart of Insects, being, as in the higher animals, the locomotiV'e organ of the blood, which, instead of being contained in vessels, is diffused throughout the general cavity of the body. This heart occupies all the length of the back of the abdomen, and terminates anteriorly by a single non-ramified artery which carrie~ the blood into the head where it diffuses it, and whence it returns into the abdomen in consequence of its accumulation in the head, to again enter the heart; to this all the circulation in Insects is reduced, they having merely a single artery without branches and no veins. The alre of the heart are not muscular as is asserted by Herold: they are mere fibrous ligaments which keep the dorsal vessel in its place. The heart, that is to say the abdominal part of the vessel (in the Melolontlta vulgaris) is divided, internally, into eight chambers separated from each other by two converging valvul~ which allow the transmission of the blood from behind forwards, and from one chamber to another, into the artery which INSECTA. 223 Of two principal trache:e, extending, parallel to each !Deans • oth er, throughout the whole. length of the body, havtng cen-t intervals, from wluch proceed numerous branch~, ctroesr,r eas ponding to external openm• gs or sti•g mata ( 1 ) , wh 1' c h a d-th b ad but which prevent it from retrograding. At the lateral and an-run. s to rte fe e ac' h chamber, are two transverse fissures wh '1 c h commum.c ate W.l t h ter1or pa 0 • • • I b.l inal cavity and through wh1ch the blood contamed m the atter enters the a uom · · d -1 • 11 · h I' tl · · h t Each of these apertures ts proV1 cu, mterna y, W!t a 1t c semt-cu·- the ear· . h' h I 1 Which presses on it durmg the systole of the heart. From t 1s s ort cu ar va ve . . d descr1·p t'1 0 n 1·t will be seen, tha. t when the po.s ter1.o r chamber dilates, the bloo conta.m e d ·n the abdominal cavtty penetrates mto 1t by the transverse fissures of 1 • • • h. h we have spoken, and which we call aunculo-ventnculalres. When the cWha ICm ber contracts, the blood finding no ex•t t m' to t h e a bdo ~m•n a.} cav1• ty fio rces th. e intel"'ventricular valve, passes into the second chamber wh1ch dtlates to receive 1~, and which, at the same time, receives a certain quantity of bl?od by the true a~rlculo ·ventricular apertures. When the second chamber rece1~es the con.tractin?' impression, the blood passes into the third, whic~\ ~lso rece1ves a portion of 1t through the lateral openings, and thus th~ blood 1s f~rced from one chamber to another into the artery. It is these successtve contractiOns of the chambers of the heart that we perceive through the skin of caterpillars." The heart of the Crustacea Dccapod11, Squillre, Limuli, Aranea:, &c., as I have been as~ured by the same profound observer, also contains similar valvul~. It is encl~sed m a sort o~ ~a~ or pericardium, which, according to him, acts in lieu of an a.uncle .. These dtvJs1ons or chambers of the dorsal vessel are what Lyonet terms atles or wmgs; he also s:nv that the dorsal vessel extended to the head, and terminated there in the manner already described: but he did not see the orific~s and val~ula: ~entioned by ~traus. The definition of the dorsal vessel given by tlus naturalist, eV'1dently proves, that, whatever be its internal formation, it is not a true heart. Besides, these observa· tionsdo not teach us the true nature of the liquid .it contains, nor how it becomes difl'used throughout the other parts of the body to effect their nutrition. It is however certain from the observations of Lyonet, that all the parts of the body communicate wi~h the c(fl'ps graisseuo» by means of fibrllli. The trache~ give off branches which extend to the extremities of tl1e various appendages of the body. The action of the air may occasion the ascension of the nutritive juices in the interstices, fot·ming a sort of capillary tubes. (1) The number of segments in the body of the Myriapoda being undetermined, that of their stigmata is the same, and frequently extends to above twenty. In the Hexapoda it is frequently eighteen, nine on each side. This computation, how· ever, is rather true with respect to the animal as a larV'a than in its. perfect state. Caterpillars, the larv~ of the Coleoptera and those of various other Insects, have one pair of stigmata on the first segment, or the one that bears the first pair of le&s; the second and the third are destitute of them, owing, I presume, to the development of the wings which occurs in these rings, and renders the presence of respiratory apertures useless in that particular place. The fourth and each l.f the seven following annuli exhibit a pair: but in coleopterous Insects in their perfect sta~e, besides the two anterior stigmata concealed in the cavity of the pro-tho~·a~, wluch had not been perceived, we observe two others, situated between the orJgm |
