424 0. GORDON HEWITT. 



The Rectal Glands. — The four rectal glands {rect.gl.) 

 are arranged in two pairs, two on each side of the rectal 

 cavity. Each rectal gland (fig. 25) has a conical or pyriforra 

 apex with a swollen circular base. It is composed of a single 

 layer of large columnar cells {r.gl.), the papilla being hollow, 

 with the cavity in communication with the general body 

 cavity. It is covered externally by a perforate chitinous 

 sheath {sh.), which is continuous with the intiraa of the rectum. 

 A number of trachese {to\) enter the cavity of each gland, and 

 fine tracheJB may be seen penetrating the wall. The cavity 

 of the gland is filled with a loose tissue of branching cells. 

 As the gland is capable of pulsation there is no doubt a 

 constant interchange of blood between the cavity of the gland 

 and the body cavity (which is a hsemoccel). By this means 

 waste products may be extracted from the blood by the 

 large gland cells and excreted into the rectum through the 

 pores on the external sheath of the gland. The rich supply 

 of tracheae probably assists the cells in the process of excre- 

 tion, as we find the trachete very numerous, and intimately 

 connected with the malpighian tubules. 



4. The Respiratory System. 



The respiratory or tracheal system is developed to a very 

 great extent in the fly and occupies more space than any 

 other anatomical structure. Only by dissection of the freshly- 

 killed insect can one obtain a true conception of its impor- 

 tance. It consists of tracheal sacs of varying size having 

 extremely thin walls and tracheas which may arise from the 

 sacs, or, in the case of the abdominal tracheae, independently 

 from the spiracles. 



The Anterior Thoracic Spiracles (figs. C and lS,a.th.). 

 — Each is alarge vertical opening behind the humeral sclerite 

 and above the anterior legs. It is surrounded by a chitinous 

 ring, the peritreme and the opening is guarded by a number 

 of dendritic processes which prevent the entrance of dust 

 and other foreign bodies. It leads into a shallow chamber or 



