130 ENTOMOLOGY 
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, 
Fic. 164. 
leaving only a disorderly net- 
work. ‘The fat-cells sometimes 
contain an albuminoid — sub- 
stance, and usually the fat-body 
B includes considerable quantities 
Fat-cells of a caterpillar, Pieris. 4: of uric acid or its derivatives, 
cells filled with drops of fat; B, cell 
freed of fat-drops, showing nucleus— frequently in the form of con- 
After Ko.se. : - 
spicuous concretions. 
Functions.—The physiology of the fat-system is still ob- 
scure. Probably the fat-body combines several functions. In 
caterpillars and other larvee 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- Fic. 165. 
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 Section through fat-body of a silkworm, 
Serie (ene ; c, m showing nucleated cells, loaded with drops 
) = act : a 
the blood-system; fat-cells “7” 
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 
