The most abundant of the food reserves stored by the caterpillar is fat. With insects, however, fat does not accumulate among the muscles and beneath the skin. sects do not become "fat" in external appearance. Their fatty products are held in a special organ called thefat- bodv. The fat tissue of a caterpillar consists of many small, fiat, irregular masses of fat-containing cells scattered all through the body cavity, some of the masses adhering in chains and sheets forming a loose open network about the alimentary canal, others being distributed against the muscle layers and between the muscles and the body wall. The cells composing the tissue vary much in size and shape, but they are always closely adherent, and in fresh material it is often difficult to distinguish the cell bound- aries. Specimens prepared and stained for microscopic examination, however, show distinctly the cellular struc- ture (Fig. I58). Each cell contains a darkly-stained nucleus (Nu), but the nuclei are seen only where they lie in the plane of the section. The protoplasmic area about the nucleus in each cell appears to be occupied mostly with hollow cavities of various sizes (a), but in life each cavity contains a small globule of fatty oil. The proto- plasmic material between the oil globules contains also glycogen, or animal starch, as can be shown by staining with iodine. Both fat and glycogen are energy-forming compounds, and their presence in the fat cells of the caterpillar shows that the fat-body serves as a storage organ for these materials during the larval life. The stored fat and glycogen will be consumed during the period of metamorphosis, when the insect is deprived of the power of feeding and receives no further nourishment from the alimentary canal. The transformation processes will then depend upon the food materials that the caterpillar has stored in its own body; and the success of the pupal meta- morphosis will depend in large measure on the quantity of these food reserves. A starved caterpillar, therefore,
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