IMAGINAL DISCS 145 



The pericanliuin lic-s in tlic foui- posterior segments of the 

 body, and is delimited ventrally irom the general body-cavity by a 

 double row of large characteristic pericardial cells. These cells 

 have a fine homogeneous structure and are readily distinguished 

 from the adjacent adipose tissue cells, whose size they do not 

 attain. The pericardial cavity contains a profuse supply of fine 

 tracheal vessels which indicates a respiratory function. A similar 

 condition occurs in the blow-riy larva, and Imms (1907) has de- 

 scribed a rich pericardial tracheal supply in the larva of Anopheles 

 maculipennis, as also Vaney (1902) and Dell (1905) in tlie laiva of 

 Psychoda punctata. 



The adipose tissue-cells (fig. QQ, f.c.) form the very prominent 

 " fat-body." They are arranged in folded cellular laminae that lie 

 chiefly in the dorso-lateral regions of the body, and in section have 

 the appearance shown in the figure. The cells have a similar 

 structure to those of the adult fly ; they are 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-sinuses can be distinguished — the pericardial 

 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 ; pos- 

 teriorly, it encloses the ganglion and the convoluted visceral mass 

 above which it opens into the pericardial sinus between the peri- 

 cardial 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. 



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 



H. H.-F. 10 



