104 



ENTOMOLOGY 



B 



FIG. 164. Fat-cells of a cater- 

 pillar, Pieris. A, cells filled with 

 drops of fat; B, cell freed of fat-drops, 

 showing nucleus. After KOLBE. 



usually the fat-body includes considerable quantities of uric acid or its 

 derivatives, frequently in the form of conspicuous concretions. 



Functions. The physiology of the fat-system is still obscure. Prob- 

 ably the fat-body combines several functions. In caterpillars and other 

 larvae it furnishes a reserve supply of nutriment, at the expense of which 



the metamorphosis takes place; the 

 amount of fat increases as the larva 

 grows, and diminishes in the pupal stage, 

 though some of it lasts over to furnish 

 nourishment for the imago and its germ 

 cells. The gradual accumulation of uric 

 acid and urates in the fat-body indicates 

 an excretory function, particularly in 

 Collembola, which have no Malpighian 



tubes. The intimate association between the ultimate tracheal branches 

 and the fat-body has led some authorities to ascribe a respiratory func- 

 tion to the latter. A close relation of some sort exists also between the 

 fat-system and the blood-system; fat-cells are found free in the blood, 

 and the blood 'corpuscles originate in the thorax and abdomen from tis- 

 sues that can scarcely be distinguished 

 from fat-tissues. The corpuscles (leu- 

 cocytes, or phagocytes) which in some 

 insects absorb effete larval tissues 

 during metamorphosis have been by 

 some authors regarded as wandering 

 fat-cells. Cells constituting the peri- 

 cardial fat-body are attached to the 

 lateral muscles (alary muscles) of the 

 dorsal vessel, but almost nothing is 

 known as to their function. 



(Enocytes. Associated with the 

 fat-body proper and with tracheae as 

 well are the peculiar yellow cells 

 known as cenocytes (Fig. 166), that 



occur in abdominal segments of larvae. These cells are enormous in size 

 as compared with all other insect-cells excepting ova, and are essentially 

 isolated from one another, though grouped among tracheal branches into 

 loose clusters, one on each side of a spiracle-bearing segment. 



After arising from the primitive ectoderm the cenocytes never divide, 



FIG. 165. Section through fat-body 

 of a silkworm, showing nucleated cells, 

 loaded with drops of fat. 



