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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 419 



longitudinal band of nerve tissue, which runs dorsal to them, 

 and behind the metathoracic ganglia swells out into a gang- 

 lionic mass (A.G-.), which represents the abdominal ganglia. 

 In this median dorsal band there is a median dorsal fissure 

 stretching posteriorly from above the middle of the meso- 

 thoracic ganglia. The dorsal regions of the mesothoracic 

 and metathoracic ganglia show ganglionic swellings. From 

 the antero- dorsal sides of the prothoracic ganglia a pair of 

 prothoracic dorsal nerves (pr.d.) arise and supply the muscles 

 of that region, including those of the anterior thoracic 

 spiracle. The nerves supplying the mesothoracic legs 

 (ms.cr.) arise from the postero-ventral sides of the meso- 

 thoracic ganglia. Between the mesothoracic ganglia there 

 is a median ganglionic mass, situated slightly dorsal, from 

 the middle region of which the nerve-fibres of the large pair 

 of dorsal mesothoracic nerves (m.s.d.) arise ; Lowne, in the 

 blowfly, calls these prothoracic. The roots of these nerves 

 are broad dorsoventrally. These nerves innervate the 

 sterno-dorsales muscles of the middle region. In this 

 median mesothoracic nerve centre, posterior to the origin of 

 the dorsal mesothoracic nerves, the fibres of a pair of nerves, 

 the accessory dorsal mesothoracic nerves (ac.ms.), have their 

 origin ; these appear externally to arise dorsal to the roots of 

 the mesothoracic crural nerves. The dorsal metathoracic 

 nerves (mt.d.), which innervate the halteres, and are the 

 largest pair of thoracic nerves, have their origin from the 

 median dorsal band in front of the metathoracic ganglia, so 

 that they appear to be almost mesothoracic in origin. The 

 metathoracic crural nerves (mt.cr.) arise from the posterior- 

 ventral sides of the metathoracic ganglia. Posterior to these 

 a pair of slender nerves, the accessory dorsal metathoracic 

 nerves, have their origin, and innervate the muscles at the 

 posterior end of the thorax. 



The dorsal band becomes much thinner posterior to the 

 abdominal ganglion, and runs into the abdomen as a median 

 abdominal nerve (ab.n.]. In the thorax two pairs of abdo- 

 minal nerves arise. In the abdomen the abdominal nerves 



