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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 423 



tubular type. They are of uniform width throughout their 

 whole length, except the slightly swollen blind termination. 

 These blind ends lie one on each side of the ventral and 

 posterior region of the abdomen, generally embedded in the 

 fat-body. They take a sinuous course forwards through the 

 abdomen into the thorax, where they run alongside the ven- 

 triculus. At the sides of the proventriculus they are thrown 

 into several folds, which appear to be quite constant in cha- 

 racter. They pass forwards at the sides of the oesophagus 

 and on entering the cervical region the ducts lose their 

 glandular character, and assume a spiral thickening ; before 

 leaving the cervical region the two ducts unite below the 

 oesophagus, and the single median duct enters the head ven- 

 tral to the cephalothoracic nerve cord, and runs direct to the 

 proximal end of the hypopharynx, at the end of which it 

 opens. A short distance before entering the hypopharynx 

 the salivary duct (fig. 1, sal.d.) is provided with a small 

 valve controlled by a pair of fine muscles (s.m.), which 

 serves to regulate the flow of the salivary secretion. The 

 glands are composed of glandular cells (fig. 22), which are 

 convex externally, and have a fibrillar appearance in section. 

 No vacuoles have been found in the cells. 



The Malpighian Tubes. A pair of malpighian tubes 

 (fig. 21, malp.) arises at the point of junction of the proximal 

 and distal intestines, that is, where the mesenteron joins the 

 proctodaeum. Each maipighian tube shortly divides at an 

 angle of 180 into two malpighian tubules. The malpighian 

 tubules are very long and convoluted, and intimately bound 

 up with the diffuse fat-body, so that it is a matter of consider- 

 able difficulty to dissect them out entire. They have a 

 moniliform appearance and are of uniform width throughout; 

 never more than two cells can be seen in section. They are 

 generally yellowish in colour. As in most insects they are 

 undoubtedly of an excretory nature, as the contents of the 

 cells and tubules show. Lowne's view that, in the blowfly, 

 they are of the nature of a hepato-pancreas is untenable 

 morphologically and physiologically. 



