THE INTERNAL ANATOMY OF INSECTS 



125 



Successive stages in the coalescence of the thoracic and abdominal 

 ganglia can be seen by a study of the nervous system of the larva, 

 pupa, and adult of the same species, a distinct cephalization of the 

 central nervous system taking place during the development of the 

 insect. Varying degrees of coalescence of the thoracic and of the 

 abdominal ganglia can be seen by a comparative study of the nervous 

 systems of different adult insects (Fig. 140). 



The transverse band of fibers that unite the two members of a pair 

 of ganglia is termed a commissure. In addition to the commissures 

 that pass directly from one member of a pair of ganglia to the other, 



there is in the head a com- 

 missure that encircles the 

 oesophagus in its passage 

 from one side of the brain 

 to the other, this is the sub- 

 osophageal commissure (Fig. 

 141). 



The nerves that extend 

 3 central chain of 



to the different 



cesophageal commissure; s~g, suboesophageal rfq r f ^ hoHv Prp nflr f- 

 ganglion; pg, paired ganglion (After Lienard). * 



of the central nervous sys- 

 tem ; the core of each nerve fiber being merely a process of a ganglionic 

 cell, however long it b 



may be. 



~-oe 



Fig. 141. Lateral view of the oesophagus of a f rom 

 caterpillar, showing the suboesophageal com- 

 missure; b, brain; oe, oesophagus; sc, sub- ganglia 



oes st a \ i'__ r OP 



b. THE GESOPHAGEAL 

 SYMPATHETIC NER- 

 VOUS SYSTEM 



In addition to the 

 central nervous sys- 

 tem as defined above 

 there are three other 

 nervous complexes 

 which are commonly 

 described as separate 

 systems although 

 they are connected 

 to the central nervous 

 system by nerves. 

 These are the oeso- 

 phageal sympathetic 



Fig. 142. Lateral view of the nerves of the head in the 

 larva of Corydalus; a, antennal nerve; ao, aorta; ar 

 paired nerves connecting the frontal ganglion with the 

 brain; b, brain; cl, clypeo-labral nerve; con, connective; 

 cr, crura cerebri; fg, frontal ganglion; fn, frontal nerve; 

 i, unpaired nerve connecting the frontal ganglion with 

 the brain; I, labial nerve; Ig, the paired ganglia; md, 

 mandibular nerve; m, p, q, s, u, z, nerves of the cesopha- 

 geal sympathetic system; mx, maxillary nerve; o, optic 

 nerves; oes, oesophagus; ph, pharynx; pn, pharyngeal 

 nerve; r, recurrent nerve; sc, subcesophageal commis- 

 sure; sg, 'suboesophageal ganglion; st, stomagastric 

 nerve; v, ventricular ganglion (From Hammar). 



nervous system, the ventral sympathetic nervous 



