CHAPTER III 



THE INTERNAL STRUCTURE OF MUSCA DOMESTICA 



The Muscular System. 



The muscular system of the fly is similar to that of Volucella, 

 described by Kunckel d'Herculais (1881), and of the blow-fly, 

 described by Lowne and Hammond, and consequently it will 

 be but briefly described. The muscles may be divided into 

 the following gi-oups : 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 (see p. 58). 



2. The thoracic muscles are enormously developed and almost 

 fill the thoracic cavity. They are arranged in two series. The 

 dorsales (figs. 17, 18) are six pairs of muscle-bands on each side 

 of the median line, attached posteriori}^ to the postscutellum and 

 mesophragma, and anteriorly to the prescutum and anterior region 

 of the scutum. The stemodorsales (st.do.) are vertical and ex- 

 ternal to the dorsales and are arranged in three bundles on each 

 side. The first two pairs have their upper ends attached to the 

 prescutum and scutum, and their lower ends inserted on the 

 mesostemum ; the third pair is attached dorsally to the scutum 

 and ventrally to the lateral plate of the postscutellum above 

 the spiracle. As Hammond has shown in the blow-fly (1881), 

 all these muscles are mesothoracic. The dorsales by contraction 

 loosen the alar membrane and so depress the wing, the stemo- 

 dorsales have the opposite effect. 



