We have already, on p. 405, called attention to these organs, but they also have an intimate relation to the fat-body.
Kowalevsky (1892) remarks that the disposition of these cells varies much in different insects and even in the same animal. Thus, in the Diptera and the ordinary flies there are found around the lower part of the dorsal vessel 13 pairs of large pericardial cells which lie next to a crowded bed of small cells forming a compact mass around the anterior part of the dorsal vessel. In caterpillars, notably silkworms, from the compact layer of pericardial cells which surround the heart, pass off trunks which are directed towards the lateral walls of the body, also forming close networks around the tracheæ and then passing down into the abdominal cavity of the body of the larva.
In the larvæ of certain Hymenoptera, the trunks which pass off from the pericardial region form a loose cord, a sort of fatty tissue covering the entire body cavity.
This tissue, adds Kowalevsky, entirely differs from œnocytes, or from the so-called glandular body whose formation in Gryllotalpa has been described by Korotaiev, and in Bombyx mori by Tichomiroff. In a recent work wherein has been collected everything known regarding these last-named cells, Pékarsky proves that they are unique in nature and cannot be regarded either as fat-cells, or as pericardial cells, or even as formative leucocytes.
As to the structure of the pericardial cells, Kowalevsky adds that they are always attached to muscular fibres passing off from the heart, and that they lie, so to speak, upon them. In the locusts the muscular fibres supporting the pericardial cells appear distinctly like little staves or sticks. The attachment of the pericardial cells to the muscular fibres has been observed by Cuénot and reproduced by him in his work, but his description somewhat differs from that observed by Kowalevsky in the locust (Acrydium migratorium).
As to the nature of the acid excretions which are formed in the pericardial cells, in spite of his attempts to solve the problem, Kowalevsky has been unsuccessful. The only observations in this direction are those of Letellier on the pericardial glands of lamellibranch molluscs, which he found to contain hypouric acid, and it is probable, says Kowalevsky, that the acidity of the pericardial cells in insects is due to the presence of the same acid.
Leucocytes or phagocytes in connection with the pericardial cells.—It is thought by Schäffer that the leucocytes or phagocytes may be free or wandering fat-body cells. They play an important part in metamorphosis, while they absorb or feed upon the remains of the larval organs, and thus prove of use in the building up of the organs of the adult insects.
While the faculty of phagocytosis is wanting in the urinary tubes, Balbiani and more recently Cuénot have expressed the opinion that the pericardial cells of insects may have the power of absorbing hard bodies, “acting as a phagocytic gland.” This, however, is called in question by Kowalevsky, from studies made on different insects. On introducing powdered carmine into the body of an insect it has not been absorbed by the pericardial cells, as they have not been colored red. It is the leucocytes which absorb the grains of carmine, and which, after having dissolved them, transmit them to the pericardial cells. Hence, then, the pericardial cells have not the phagocytic power of which Cuénot speaks.
Returning to his own observations on hard bodies introduced into insects, or large globules introduced under the form of a milk emulsion, Kowalevsky has found that these bodies were absorbed in the first place by the free-swimming leucocytes, and in the second place by whole groups or nests of leucocytes situated in different parts of the body, principally on the threads of the adipose body. In the Orthoptera the absorption is immediately effected by means of the cells of the membrane which separates the pericardium from the cavity of the body underneath the heart. The regions where the hard bodies are absorbed in great number coincide with the regions of formation of the blood corpuscles. In his researches on the larvæ of Hyponomeuta and other Lepidoptera, Schäffer describes these regions as forming a sort of island. The nests where the blood globules are formed are the most active centres of phagocytosis.
