STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 435 



The penis (figs. 7 and 9) lies internally on the ventral side 

 of the abdomen, dorsal to the ventral arches of the fifth and 

 seventh segments. It is composed of several sclerites. A 

 median sclerite {c.pe.), the anterior and ventral edge of which 

 is ronghly semicircular in outline, forms the body of the 

 penis. This is produced laterally to form two alar processes ; 

 at the bases of these processes the lateral extremities of the 

 dorsal arch of the eighth segment articulate with the body of 

 the penis; the extremities of the processes are attached to 

 the lateral extremities of the ventral sclerites of the seventh 

 segment, the secondary forceps. The penis proper consists 

 of a hollow cylindrical tube, the theca, which receives the 

 ejaculatory duct. The tlieca articulates with the body of the 

 penis by means of a pair of small chitinous nodules (" cor- 

 netti" of Berlese) ; posterior to the attachment the thecals 

 constricted slightly. Below the aperture for the entrance of 

 the ejaculatorj'' duct, the theca is produced into a ventrally 

 directed curved process, the inferior apophysis {t.ajJ.) ; above 

 the aperture a short cylindrical process, the superior apo- 

 physis {s.ap.), arises. The anterior end of the theca is con- 

 tinued as a slightly inflated hyaline structure, the glans 

 {p.gl.), at the curved extremity of which the ejaculatory duct 

 opens. 



V. The Internal Steucture op the Head. 



The skeletal framework and tracheal system of the head 

 have already been described. It remains, therefore, to give 

 an account of the musculature of the head and pharynx, and 

 also an account of the oral lobes. 



The posterior region of the head (tig. 1) not occupied by 

 tracheal sacs is usually filled up with small multinucleate fat- 

 cells (/.c), which are also occasionally found in the proboscis. 

 The frontal sac or ptilinium {Pt.) fills up the anterior portion 

 of the head not occupied by air-sacs. Its crescentic opening, 

 the lunule, has already been described. It is attached to the 



