Structure and Life-history of the Holly-fly. 265 
which are calcareous concretions, each enclosed within 
a nucleated cell. They present a rough surface, and an 
internal concentric lamination resembling that of a starch- 
grain. Similar concretions are found in the proglottides of 
a tape-worm. 
The fate of the concretions of the holly-fly larva is 
interesting. They persist throughout the larval stage, 
gradually increasing in size, but disappear soon after 
upation. In an old puparium the internal tissues are 
devoid of lime-salts, but the cuticle effervesces strongly as 
soon as its inner surface is touched with acid. It seems 
likely that the substance of the concretions is absorbed 
and re-deposited in the cuticle.* Occasionally a few 
concretions persist and are carried over into the fly. 
Fig. 4. 
Cells of the fat-body. Two calcareous corpuscles are shown, 
one in surface-view, the other in optical section; the fat- 
globules have been omitted from one cell. (x 625.) 
The Gonads.—Two spherical bodies (fig. 3) lying between 
the coils of the intestine are the gonads; they remain 
practically unchanged throughout the larval period. 
Larval Stages.—The larva of the holly-fly moults twice 
before pupation ; there are thus three larval stages. The 
first lasts from July to December; the second from 
December to February, and the third from February to 
the middle or end of March. At times of moult the skin 
splits along the ventral side, which is unusual in insects. 
At pupation the larva is 35-4 mm. long. 
The peculiar features of later Larve.—A larva of the 
second or third stage differs from the larva which has just 
* A similar transference probably occurs in other Dipterous 
leaf-miners, ¢.g. in Acidia heraclei. 
