130 



ENTOMOLOGY 



FIG. 164. 



B 



Fat-cells of a caterpillar, Pieris. A, 

 cells filled with drops of fat; B, cell 

 freed of fat-drops, showing nucleus. 

 After KOLBE. 



structure of the fat-tissue is often difficult to make out because 

 the cells are usually filled with globules of fat (Fig. 165), 



while old cells break down, 

 leaving only a disorderly net- 

 work. The fat-cells sometimes 

 contain an albuminoid sub- 

 stance, and usually the fat-body 

 includes considerable quantities 

 of uric acid or its derivatives,, 

 frequently in the form of con- 

 spicuous concretions. 

 Functions. The physiology of the fat-system is still ob- 

 scure. Probably 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 Col- 

 lembola, which have no Mal- 

 pighian tubes. The intimate 

 association between the ulti- 

 mate tracheal branches and 

 the fat-body has led some 

 authorities to ascribe a res- 

 piratory function to the lat- 

 ter. A close relation of 

 some sort exists also be- 

 tween the fat-system and 

 the blood-system ; fat-cells 

 are found free in the blood, 

 and the blood corpuscles originate in the thorax and abdo- 

 men from tissues that can scarcely be distinguished from 



FIG. 165. 



Section through fat-body of a silkworm,, 

 showing nucleated cells, loaded with drops 

 of fat. 



