RECHERCHES SUR I.KS DIPTÈRES PARASITES 385 



whicli perforate the compound ganglion and occur as vertical 

 strands along its médian Une. Tracheae also penetrate the gan- 

 glion with tliese strands of capsular tissue. 



On the dorsal side of the anterior end of the ganglion is situa- 

 ted a pair of spherical structures (c. /.), which may he termed 

 the « cérébral lobes ». They are united in the médian line dor- 

 sal to the foramen traversed by the œsophagus {œ.). Thèse Cjere- 

 bral lobes are chiefly of an imaginai character, and contain the 

 fundaments of the supraœsophageal ganglia and also of the optic 

 gangliaaf the future fly (fig. 60), Each is surrounded l)y a thin 

 membranous sheath (vA.) and is eonnected witli the major 

 cephalic imaginai dises by the optic stalk (0. s.). 



The nerves arising from the ganglion may be divided into three 

 groups, according to their origin. Eleven pairs of nerves (fig. 53, 

 l-II) corresponding to the eleven pairs of ganglion arise, 

 two from the anterior end and nine from the sides of the gan- 

 glion. Three pairs of nerves [a, h and c) arise laterally from the 

 stalks of the prothoracic and mesothoracic imaginai dises. In 

 ihe médian dorsal line of the posterior half of the ganglion a 

 single pair (r/. a') and two médian unpared {d. a'\ d. a'") ner- 

 ves hâve their origin ; thèse are accessory nerves. 



The fîrst pair of the two anterior pairs of nerves runs forward 

 and innervâtes the posterior région of the pharyngeal mass ; 

 the anterior région of the latter is suj^plied by the second pair of 

 nerves. Thèse nerves also innervate the anterior segments of 

 the body. The first («) of the three pairs of nerves which arise 

 from the stalks of the imaginai dises runs to the anterior end 

 supplying the protractor and retractor muscles of the jîharyn- 

 geal mass. The second {h) of thèse three pairs of nerves inner- 

 vâtes the muscles of the body-wall of the third and fourth seg- 

 ments ; the latter segment is also innervated by the third [c] 

 of the three pairs of nerves. The succeeding pairs of latéral 

 nerves are segmentally distributed, and innervate the muscles 

 of the body-wall of segments five to thirteen. Each nerve bifur- 

 cates on reaching the muscles, and thèse branches further 

 subdivide into vcry fine nerves. 



The nerves which arise dorsally, and which I bave called the 

 accessory nerves, are interesting. The first pair (V/, a!) which 

 arises about mid-way along the dorsal side of the ganglion. 



