RESPIRATOR V SYSTEM 41 



and tubules show. Lowne's view that, in the blow-fly, they are 

 of the nature of a hepato-pancreas is untenable on both morpho- 

 logical and physiological grounds. 



The rectal glands. 



The four rectal glands (rect.gl.) are arranged in two pairs, 

 two each side of the rectal cavity. Each rectal glanfl (fig. 16) 

 has a conical or pyriform apex with a swollen circular base. It 

 is composed of a single layer of large columnar cells {r.gl.) ; the 

 papilla is hollow and its cavity is in communication with the 

 general body cavity. It is covered externally by a perforate 

 chitinous sheath (sh.), which is continuous with the intima of 

 the rectum. A number of tracheae (t7\) enter the cavity of each 

 gland, and fine tracheae may be found 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 haemocoel). By this means waste pro- 

 ducts may be extracted fi-om 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 excretion, as we find the 

 tracheae very numerous, and intimately connected with the 

 malpighian tubules. 



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 development and 

 importance. It consists of tracheal sacs of varying size, ha\dng 

 extremely thin walls and tracheae which may arise from the sacs, 

 or, in the case of the abdominal tracheae, independently fi:-om 

 the spiracles. 



The Anterio7' Thoracic Spiracles (figs. G, 17, a.th.). Each is 

 a large vertical opening behind the humeral sclerite and above 

 the anterior legs. It is surrounded by a chitinous ring, the 



