INSECTS 



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. In- 

 sects do not become "fat" in external appearance. Their 

 fatty products are held in a special organ called the, fat- 

 body. 



The fat tissue of a caterpillar consists of many small, 

 flat, 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. 158). 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, 



[292] 



