According to Kowalevsky, the pericardial cells, and the garland-shaped, cellular cord consist of cells, whose function it is to purify the blood, and to remove the foreign or injurious matters mingled with the blood.
Fig. 379.—Diagram of the circulatory organs in the head of the cockroach, seen from above: A, ampulla; V, antennal vessel; M, chief muscular cord; m, muscular band; Bs, wall of the blood sinus; am, opening of the aorta (a); rg, anterior sympathetic or visceral ganglion; hg, hinder visceral ganglion; F, F, facetted eyes; o, vestigial ocellus; G, G, brain; S, œsophagus.—After Pawlowa.
Ampulla-like blood circulation in the head.—In the head of the cockroach occurs, according to Pawlowa, a contractile vascular sac at the base of each antenna. The cavity has a valvular communication with the blood space below and in front of the brain, and muscle-fibres effect systole and diastole. Each sac is beyond doubt an independently active part of the circulatory system. These organs also occur in Locusta and other Acrydiidæ, and Selvatico has described similar structures in Bombyx mori and certain other Lepidoptera.
Pulsatile organs of the legs.—Accessory to the circulation is a special system of pulsatile organs in the three pairs of legs of Nepidæ, generally situated in the tibia just below its articulation with the femur, but in the fore legs of Ranatra, in the clasp-joint or tarsus, just below its articulation with the tibia. First observed by Behn (1835), Locy has studied the organ (Fig. 380) in Corixa, Notonecta, Gerris, besides the Nepidæ. It is a whip-like structure attached at both ends, with fibres extending upward and backward to the integument of the leg, separate from the muscular fibres and does not involve them in its motions, and is not affected by the muscles themselves. “As the blood-corpuscles flow near the pulsating body they move faster, and around the organ itself there is a whirlpool of motion.” The beating of these organs aids the circulation in both directions, and when the motion ceases, the blood-currents in the legs stop; the rate of the pulsating organ is always faster than that of the heart, and the action is automatic.
Fig. 380.—Pulsating organs in Hemiptera: A, Belostoma nymph, B, legs of Corixa. C, Ranatra, adult, to show the exceptional position of the pulsating organ in the fore legs. D, pulsatile organ in tibia of Ranatra.—After Locy.
b. The blood
The blood of insects, as in other invertebrates, differs from that of the higher animals in having no red corpuscles. It is a thin fluid, a mixture of blood (serum) and chyle, usually colorless, but sometimes yellowish or reddish, which contains pale amœboid corpuscles corresponding to the white corpuscles (leucocytes) of the vertebrates, though they are relatively less numerous in the blood of insects. The yellow fluid expelled from the joints of certain beetles (Coccinella, Timarcha, and the Meloidæ) is, according to Leydig, only the serum of the blood. In phytophagous insects the blood is colored greenish by the chlorophyll set free during digestion. The blood of Deilephila euphorbia is colored an intense olive-green, and that of Cossus ligniperda is pale yellow. (Urech.) The blood of case-worms (Trichoptera) is greenish. In some insects it is brownish or violet. The serum is the principal bearer of the coloring material, yet Graber has shown that in certain insects the corpuscles are more or less beset with bright yellow or red fat-globules, so as to give the same hue to the blood.
The leucocytes.—The corpuscles are usually elongated, oval, or flattened oat-shaped, with a rounded nucleus, or are often amœbiform; and they are occasionally seen undergoing self-division. When about to die the corpuscles become amœbiform or star-shaped. (Cattaneo.) Their number varies with the developmental stage of the insect, and in larvæ increases as they grow, becoming most abundant shortly before pupation. The blood diminishes in quantity in the pupal stage, and becomes still less abundant in the imago. (Landois.) The quantity also varies with the nutrition of the insect, and after a few days’ starvation nearly all the blood is absorbed. Crystals may be obtained by evaporating a drop of the blood without pressure; they form radiating clusters of pointed needles. The freshly drawn blood is slightly alkaline. (Miall and Denny.)