In discussing the tracheae we remarked on the importance of their function and on their abundant presence in the body. Equally conspicuous, and perhaps scarcely less important in function, is the fat-body, which on opening some Insects, especially such as are in the larval stage, at once attracts attention. It consists of masses of various size and indefinite form distributed throughout the body, loosely connected together, and more or less surrounding and concealing the different organs. The colour varies according to the species of Insect. This fat-body is much connected with fine tracheal twigs, so that an organisation extending throughout the body is thus formed. It may be looked on as a store of nutritious matter which may be added to or drawn on with great rapidity; and it is no doubt on this that many of the internal parasites, so common in the earlier stages of Insects' lives, subsist before attacking the more permanent tissues of their hosts. There is some reason to suppose that the fat-body may have some potency in determining the hunger of the Insect, for some parasitised larvae eat incessantly. The matter extracted from the food taken into the stomach of the Insect, after undergoing some elaboration—on which point very little is known—finds its way into the body-cavity of the creature, and as it is not confined in any special vessels the fat-body has as unlimited a supply of the nutritive fluid as the other organs: if nutriment be present in much greater quantity than is required for the purposes of immediate activity, metamorphosis or reproduction, it is no doubt taken up by the {137}fat-body which thus maintains, as it were, an independent feeble life, subject to the demands of the higher parts of the organisation. It undoubtedly is very important in metamorphosis, indeed it is possible that one of the advantages of the larval state may be found in the fact that it facilitates, by means of the fat-body, the storage in the organisation of large quantities of material in a comparatively short period of time. A considerable quantity of fat tissue is found in the pericardial sinus, where it is frequently of somewhat peculiar form, and is spoken of as pericardial cells, or pericardial tissue. Some large cells, frequently of pale yellow colour, and containing no fat, are called oenocytes by Wielowiejski. They are connected with the general fat-body, but are not entirely mingled with it; several kinds have been already distinguished, and they are probably generally present. The phagocytes, or leucocytes, the cells that institute the process of histolysis in the metamorphosis of _Muscidae_, are a form of blood cell; though these cells are amoeboid some writers derive them from the fat-body. The cells in the blood have no doubt generally an intimate relation with the fat-body, but very little accurate information has been obtained as to these important physiological points, though Graber has inaugurated their study.[60]