In describing the mechanism of the heart we have already considered in a general way the mode of circulation of the blood.
The heart pumping the blood into the aorta, the nutritive fluid passes out and returns along each side of the body; distinct, smaller streams passing into the antennæ, the legs, wings (of certain insects), and into the abdominal appendages when they are present. All this may readily be observed in transparent aquatic insects, such as larval Ephemeræ, dragon-flies, etc., kept alive for the purpose under the microscope in the animalcule box.
Carus, in 1827, first discovered the fact of a complete circulation of the blood, in the larva of Ephemera. He saw the blood issuing in several streams from the end of the aorta in the head and returning in currents which entered the base of the antennæ and limbs in which it formed loops, and then flowing into the abdomen, entered the posterior end of the heart. Wagner (Isis, 1832) confirmed these observations, adding one of his own, that the blood flows backward in two venous currents, one at the sides of the body and intestine, and the other alongside of the heart itself, and that the blood not only entered at the end of the heart, but also at the sides of each segment, at the position of the valves discovered by Straus-Dürckheim.
Newport maintains that the course of the blood is in any part of the body, as well as in the wings, almost invariably in immediate connection with the course of the tracheæ, for the reason that “the currents of blood in the body of an insect are often in the vicinity of the great tracheal vessels, both in their longitudinal and transverse direction across the segments.”
The circulation of the blood in the wings directly after the exuviation of the nymph or pupa skin, and before they become dry, has been proved by several observers. As stated by Newport, the so-called “veins” or “nervures” of the wings consist of tracheæ lying in a hollow cavity, the peritracheal space being situated chiefly under and on each side of the trachea.
Fig. 382.—Circulation of the blood in hind wing of Periplaneta orientalis: the arrows indicate the usual direction of the blood currents.—After Moseley.
Newport gives the following summary of the observations of the early observers, to which we add the observations of Moseley. “A motion of the fluids has been seen by Carus in wings of recently developed Libellulidæ, Ephemera lutea and E. marginata, and Chrysopa perla; among the Coleoptera, in the elytra and wings of Lampyris italica and L. splendidula, Melolontha solstitialis and Dytiscus.” Ehrenberg saw it in Mantis, and Wagner in the young of Nepa cinerea and Cimex lectularius. Carus detected a circulation in the pupal wings of some Lepidoptera, and Bowerbank witnessed it in a Noctuid (Phlogophora meticulosa); Burmeister observed it in Eristalis tenax and E. nemorum, and Mr. Tyrrel in Musca domestica, but it has not been observed in the wings of Hymenoptera.
Bowerbank observed that in the lower wing of Chrysopa perla the blood passes from the base of the wing along the costal, post-costal, and externo-medial veins, outwards to the apex of the wing, giving off smaller currents in its course, and that it returns along the anal vein to the thorax. He found that the larger veins, 1
408 in. in diameter, contained tracheæ which only measured 1
2222 of an inch in diameter; but in others the tracheæ measured 1
1340, while the cavity measured only 1
500 of an inch. He states, also, that the tracheæ very rarely give off branches while passing along the main veins, and that they lie along the canals in a tortuous course. (Newport, art. Insecta, p. 980.)
Bowerbank, also, in his observations on the circulation in the wings of Chrysopa, “used every endeavor to discover, if possible, whether the blood has proper vessels, or only occupied the internal cavities of the canals; and that he is convinced that the latter is the case, as he could frequently perceive the particles not only surrounding all parts of the tracheæ, and occupying the whole of the internal diameter of the canals, but that it frequently happens that globules experienced a momentary stoppage in their progress, occasioned by their friction against the curved surface of the tracheæ, which sometimes gave them a rotatory motion.”