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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 415 



sists of apparently four segments in the male and female, in 

 reality there are five as the first segment has become very 

 much reduced, and has fused with the second abdominal seg- 

 ment forming the anterior face of the base of the abdomen 

 (see fig. 8). The segments succeeding the fifth are greatly 

 reduced in the male, and in the female they form the tubular 

 ovipositor which, in repose, is telescoped within the abdomen. 

 The second, third, fourth, and fifth abdominal segments are 

 well developed, and consist of a large tergal plate, which 

 extends laterally to the ventral side. The sternal plates are 

 much reduced, and form a series of narrow plates lying on 

 the ventral membrane along the mid-ventral line. The 

 spiracles are situated on the lateral margins of the tergal 

 plates. The sclerites of the abdomen which are exposed are 

 strongly setose, especially the fourth and fifth dorsal plates, 

 but they do not bear macrochaebse. 



IV. INTERNAL STRUCTURE. 

 1. The Muscular System. 



The muscular system of the fly is similar to that of 

 Volucella, described by Kuuckel d'Herculais (1881), and of 

 the Blow-fly, described by Lowne and Hammond, and conse- 

 quently they will be but briefly described. The muscles may 

 be divided into the following groups : 1. Cephalic, 2. Thoracic, 

 3. Segmental, 4. Those controlling the thoracic appendages, 

 and 5. Special muscles. 



1. The cephalic muscles will be considered in the detailed 

 description of the head. 



2. The thoracic muscles are enormously developed and 

 almost fill the thoracic cavity. They are arranged in two 

 series. The dorsales (figs. 13 and 15, do.) are six pairs of 

 muscle-bands on each side the median line, attached posteriorly 

 to the postscutellum and mesophragma, and anteriorly to 

 the prescutum and anterior region of the scutum. The 

 sternodorsales (st.do.) are vertical and external to the dorsales 

 and are arranged in three bundles on each side. The first 



