(81) 



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 

 discs runs to the anterior end supplying the protractor and 

 retractor muscles of the pharyngeal mass. The second (6) 

 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. Each 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 

 ganglion, 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. Each 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 tracheae, 

 some of which arise from the ganglion in association with the 

 nerves which they accompany to the body- wall. Two of 

 these tracheae are shown (fig. 24, t.). Similar fine tracheas 

 arise with the three posterior pairs of lateral nerves, and 

 on account of their similarity to accessory nerves I at first 



