O32 CG. GORDON HEWITT. 
very large, with reticular protoplasm containing fat globules, 
and there may be more than one nucleus in a single cell. As 
in the fly, the fat-body is closely connected with the tracheal 
system by means of a very rich supply of trachez. 
Two chief blood-sinuses can be distinguished—the peri- 
cardial sinus, which has already been described, lying in the 
dorsal region in the four posterior segments, and the great 
ventral sinus. The latter hes between the outer sheaths of 
the major cephalic imaginal discs and extends anteriorly into 
and about the pharynx ; posteriorly it encloses the ganglion 
and the convoluted visceral mass, above which it opens into 
the pericardial sinus between the pericardial cells. 
The blood which fills the heart and sinuses and so bathes 
the organs is an almost colourless, quickly coagulable fluid, 
containing colourless, nucleated, amoeboid corpuscles and small 
elobules of a fatty character. 
7. Tue Imacinat Discs. 
As in other cyclorrhaphic Diptera, the imaginal discs of 
some of which have been described by Weismann (1864), 
Kunckel @’Herculais (1875-78) and Lowne, the imago is 
developed from the larva by means of these imaginal rudi- 
ments, whichare gradually formed during the later portion of 
the larval life. They do not all appear at the same time, for 
whereas some may be in a well-developed state early in the 
third larval instar, others do not appear: until the larva 
reaches its resting period or even later. ‘The imaginal discs 
appear to be hypodermal imaginations though their origin is 
difficult to trace in all cases; in many instances they are con- 
nected with the hypodermis by means of a stalk of varying thick- 
ness. The imaginal disc or rudiment may consist of a simple 
or of a folded lamina of deeply-staining columnar embryonic 
cells, as in the wing discs, or of a number of concentric rings 
of these cells, as in the antennal and crural. discs. They are 
usually closely connected with the trachez and in some cases 
are innervated by fine nerves. Although the imaginal dises 
