16 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
The clear parts of the tubules usually contain only a small amount of minutely 
granular matter, but occasionally parts of them are filled with crystals of the 
form shown in figure 5, B. The larger of these, which reach a diameter also of 
about 40 microns, are clearly concretions about a central body. Wherever 
they occur the epithelium of the tubule is much reduced. 
THE FAT BODY 
The fat tissue of the apple maggot occupies much space in the body cavity, 
and gives the larva its opaque white appearance. Two large masses of fat cells, 
however, in the anterior part of the body are particularly conspicuous on account 
of their pale yellow color. These arise above the salivary glands on each side and 
meet along the middle of the back. The rest of the tissue forms loose, white 
masses of a delicate texture, lying along the sides of the body, among the loops 
of the alimentary canal, and on the floor of the abdominal segments. 
The fat cells are very small in young larvae, and at first contain but little 
fat. During the third larval stage, however, they increase rapidly in size and 
their cytoplasm becomes filled with large oily globules. By the middle of this 
stage the fat cells are mostly from 43 to 46 microns 
in diameter (fig. 6, A, B), and, at the end of it, 
they have increased to more than twice this size, 
being now from 100 to 120 microns in diameter, and 
are so charged with fat that, in surface view (C), 
they appear to be mere spheres of oil globules with 
a surface film of granular protoplasm. When the 
maggot is contracting to form the puparium the 
fat cells reach their maximum size, 200 microns or 
more in longest diameter (D), while the cytoplasm 
is reduced to branching strands between the volu¬ 
minous oily inclusions. The granular appearance 
of the cytoplasm has become more pronounced. 
It has been asserted by some writers that the 
great vacuoles in the fat cells at this stage do not 
contain fat. Berlese {4) has stated that this is true 
of the fat cells of the blow-fly, and Koehler {20) 
says the same for those of the honeybee. P6rez 
{89), however, claims that the vacuoles in the fat 
ally found in other parts of the cells of the blow-fly always contain fat, and Bishop 
tubules, the largest 40 microns in ^ g g game 0 f the fat cells of the honeybee, 
dismotcr ' ** 
The present writer found that tests with Soudan 
III and osmic acid on the fat cells of both the honeybee and the apple maggot 
give very positive reactions in favor of fat in the vacuoles at all stages. 
Much of the fat in the fat body of insects with complete metamorphosis 
disappears during the early part of the pupal period. But it is now known that 
the fa't body of such insects is an organ of much greater physiological activity 
and importance than its name would imply. Though its cells form mostly fat 
during the larval stage, which crowds their cytoplasm with oily vacuoles, they 
store up also glycogen, in some species, and they elaborate proteid or albuminoid 
bodies which finally accumulate in large numbers in the cytoplasm. For this 
reason Berlese {4) has appropriately called the fat cells trophocytes. 
The presence of glycogen in insects has not been generally noted, but Bataillon 
and Couvreur {2) have shown for the silkworm, and Straus {44) for the honeybee, 
that the larva stores up glycogen in increasing amount till the end of its life, and 
that this glycogen is then rapidly consumed during the pupal stage. These in¬ 
vestigators, however, did not attempt to locate the glycogen in the body of the 
Fig. 5.—A, crystals from the terminal 
enlarged parts of the larval Malpig¬ 
hian tubules; B, crystals occasion- 
