STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 521 
forward and innervates the posterior region of the pharyngeal 
mass; the anterior region of the latter is supplied by the 
second pair of nerves. These nerves also innervate the 
anterior segments of the body. The first (a) of the three 
pairs of nerves which arise from the stalks of the imaginal 
dises runs to the anterior end supplying the protractor and 
retractor muscles of the pharyngeal mass. ‘The second (b) 
of these three pairs of nerves innervates the muscles of the 
body-wall of the third and fourth segments; the latter segment 
is also innervated by the third (c) of the three pairs of nerves. 
The succeeding nine pairs of lateral nerves are segmentally 
distributed, and innervate the muscles of the body-wall of 
segments five to thirteen. Hach nerve bifurcates on reaching 
the muscles, and these branches further subdivide into very 
fine nerves. 
The nerves, which arise dorsally, and which I have called 
the accessory nerves, are interesting. The first pair (d. a.’) 
which arises about mid-way along the dorsal side of the 
ganglon, accompanies the pair of nerves supplying the 
seventh segment. The second (d.a.”), which is an unpaired 
nerve, bifurcates in the seventh segment, and the resulting 
nerves proceed to the body-wall in association with the nerves 
supplying the eighth segment. The third and_ posterior 
dorsal accessory nerve (d.a.’’’) bifurcates in the seventh seg- 
ment. Hach of the resulting nerves undergoes a second 
bifurcation; the dextral nerve, bifurcating in the eighth 
segment, accompanies the nerves supplying the ninth seg- 
ment; the sinistral nerve bifurcates between segments 
eight and nine, and the resulting nerves proceed to the tenth 
segment. None of the remaining lateral nerves appear to be 
accompanied by an accessory nerve, of which there are four 
pairs only. ‘The ganglionic sheath is penetrated by trachee, 
some of which arise from the ganglion in association with the 
nerves which they accompany to the body-wall. Two of 
these trachez are shown (fig. 24, ¢.). Similar fine trachez 
arise with the three posterior pairs of lateral nerves, and 
on account of their similarity to accessory nerves I at first 
