(92) 



532 C. 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 tracheae. 



Two chief blood-siiinses 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 lies 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 

 globules of a fatty character. 



7. THE IMAGINAL Discs. 



As in other cyclorrhaphic Diptera, the imaginal discs of 

 some of which have been described by Weismann (1864), 

 Kunckel d'Herculais (1875-78) and Lowne, the imago is 

 developed from the larva by means of these imaginal rudi- 

 ments, which are 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 tracheae and in some cases 

 are innervated by fine nerves. Although the imaginal discs 



