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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 519 



dorso-lateral oblique, the internal lateral oblique, the longi- 

 tudinal ventro-lateral, the ventro-lateral oblique and ventral 

 oblique muscles. Each segment as it comes forward takes a 

 firm grip ventrally by means of the spiniferous pad. By the 

 time the last spiniferous pad has become stationary the 

 mandibular sclerite has left its anchorage, and by the con- 

 traction of the lateral and intersegmental muscles, which 

 takes place from before backwards, the lengths of the 

 segments of the larva are increased serially and the anterior 

 end begins to move forward again, when the whole process 

 is repeated. 



3. NERVOUS SYSTEM. 



The central nervous system of the larva (PI. 32, fig. 23) 

 has attained what would appear to be the limit of ganglionic 

 concentration and fusion. The boat-shaped ganglionic mass, 

 which lies partly in the fifth segment, but the greater portion 

 in the sixth segment, is a compound ganglion and represents 

 the fusion of eleven pairs of ganglia similar to that which 

 Leuckart (1858) describes in the first larval stage of Melo- 

 phagus ovinus, but which, however, has not undergone so 

 great a degree of concentration as in M. domestica. This 

 ganglionic mass, which for convenience and brevity I shall 

 call the ganglion (Lowne's (< neuroblast ") does not exhibit 

 externally any signs of segmentation, the interstices between 

 the component ganglia being filled up with the cortical tissue, 

 whose outer wall forms a plain surface. In horizontal and 

 sagittal sections, however, the component ganglia can be 

 recognised and their limits are more clearly defined. The 

 ganglion is surrounded by a thick ganglionic capsular sheath 

 which is richly supplied with trachea?, and appears to be con- 

 tinuous with the outer sheath of the peripheral nerves. Two 

 pairs of large trachea? (fig. 24) are found entering the gang- 

 lionic sheath, an anterior pair (tr. ') which runs in between 

 the cerebral lobes, and a lateral pair (tr. ") entering the gan- 

 glion beneath these lobes. In the young larva the cortical 



