The size of the corpuscles has been ascertained by Graber, who found that the diameter of the circular blood-disks of the leaf-beetle, Lina populi, is 0.006 mm.; of Cetonia aurata and Zabrus gibbus, 0.008 to 0.01 mm.; and those of certain Orthoptera (Decticus verrucivorus, Ephippiger vitium and Œdipoda cœrulescens), 0.011 to 0.014 mm. The longest diameter of the elongated corpuscles of Carabus cancellatus is 0.008 mm.; of Gryllus campestris, Locusta viridissima, Cossus ligniperda, Sphinx ligustri (pupa), and others, 0.008 to 0.01 mm.; of Caloptenus italicus, Saturnia pyri, Anax formosus, and others, 0.011 to 0.014 mm.; of Ephippiger vitium, Œdipoda cœrulescens, Pezotettix mendax, Zabrus gibbus, Phryganea, and others, 0.012 to 0.022 mm.; in Stenobothrus donatus and variabilis, 0.012 to 0.035 mm. The largest known are those of Melolontha vulgaris, which measure from 0.027 to 0.03 mm.
Fig. 381.—Blood corpuscles, or leucocytes, of insects: A, a-g, of Stenobothrus dorsatus (the same forms occur in most Orthoptera and in other insects). B, a, leucocyte of the same insect with the nucleus brought out by ether; b, another of serpentine shape. C, leucocytes of the same insect after a longer stay in ether. D, leucocytes of the same after being in glycerine 14 days.—After Graber.
As regards the nature of the corpuscles, Graber concludes that they are more like the cells of the fat-bodies than genuine cells. That they are not true cells is shown by the fact that after remaining in their normal condition a long time they finally coalesce and form cords. After shrivelling, or after the blood has been subjected to different kinds of treatment, the nucleus is clearly brought out (Fig. 381).
Besides the blood corpuscles there have been detected in the blood round bodies which are regarded as fat-cells. They are circular, and for the most part larger than the blood corpuscles, have a sharp, even, dark outline, and an invariably circular nucleus. (Kolbe.)
The blood of Meloe, besides the amœboid corpuscles, according to Cuénot, contains abundant fibrinogen, which forms a clot; a pigment (uranidine), which is oxidized and precipitated when exposed to the air; a dissolved albuminoid (hæmoxanthine), which has both a respiratory and nutritive function; and, finally, dissolved cantharidine.
The corpuscles arise from tissues which are very similar to the fat-bodies, and which, at given times, separate into cells. The position of these tissues is not always the same in different insects. In caterpillars, they occur in the thorax, near the germs of the wings; in the saw-flies (Lyda), in all parts of the thorax and abdomen; in larval flies (Musca), in the end of the abdomen, just in front of the large terminal stigmata. The place where the blood corpuscles are formed is usually near, or in relation with, the fat-bodies. But while the fat-bodies mostly serve as the material for the formation of the blood-building tissues, in caterpillars the tracheal matrix also, and, in dipterous larvæ, the hypodermis serve this purpose. (Cæsar Schaeffer in Kolbe. See also Wielowiejski, Ueber das Blutgewebe.)
Other substances occur in the blood of insects. Landois (1864) demonstrated the existence of egg albumen, globulin, fibrin, and iron in the blood of caterpillars. Poulton found that the blood of caterpillars often contained chlorophyll and xanthophyll derived from their food plants. A. G. Mayer has recently found that the blood (hæmolymph) of the pupæ of Saturniidæ (Callosamia promethea) contains egg albumin, globulin, fibrin, xanthophyll, and orthophosphoric acid, and Oenslager has determined that iron, potassium, and sodium are also present. (Mayer.)
c. The circulation of the blood
Every part of the body and its appendages is bathed by the blood, which circulates in the wings of insects freshly emerged from the nymph or pupal state, and even courses through the scales of Lepidoptera, as discovered by Jaeger (Isis, 1837).