STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 019 
dorso-lateral oblique, the internal lateral oblique, the longi- 
tudinal ventro-lateral, the ventro-lateral oblique and ventral 
oblique muscles. Hach 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 (Pl. 382, 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 
creat 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 tracheee, and appears to be con- 
tinuous with the outer sheath of the peripheral nerves. Two 
pairs of large tracheee (fig. 24) are found entering the gang- 
lionic sheath, an anterior pair (¢7.’) which runs in between 
the cerebral lobes, and a lateral pair (¢7.”) entering the gan- 
glion beneath these lobes. In the young larva the cortical 
