THE CATERPILLAR: INTERNAL ORGANS. 33 



in tlie lar\a the suhocsophagal ganglion (except such as are absorbed in 

 sit a) , is situated in the front part of the mesothorax ; it is oval and evidently 

 conn)osed of two unequal, anterior and posterior, elements, for slightlv in 

 front of the middle it is pierced by a vertical passage of considerable size, 

 and the i)()rtion in front of this has a slight, independent tumidity ; from near 

 the middle it emits lateral nerves, double at their origin, which pass toward 

 the wings, and just before the hinder end a rather prominent nerve, which 

 runs backward halfway to the abdomen, parallel to the main cord and nearly 

 as large as it, evidently feeding the legs ; besides these there is another 

 similar [)air, also running backward but divaricating a little, which ori£ri- 

 nates from the widest [)art of the posterior portion. Behind this ganglion 

 the cord is very slender and a little before entering the abdomen emits a 

 pair of slender but very distinct nerves, which are directed backward Init 

 divaricate considerably, passing to the inflated longitudinal tracheal canals ; 

 there is no ganglionic enlargement at their origin, but it perhaps indicates 

 the site of the former sixth body ganglion. Immediately on entering the 

 abdomen the nervous cord appears to thicken and forms a pseudoganglion 

 of an elongate, fusiform shape, terminating just before the first abdominal 

 ganglion ; this appearance, however, is produced by the fact that through- 

 out the abdomen the cord is overlaid by an investment, mainly pellucid, but 

 not pellucid enough to allow the true cord to be seen, excepting from 

 beneath ; this investment does not cover the ganglia to an equal extent, but 

 only as a film ; so that the abdominal development of the nervous system is 

 an exceedingly delicate cord, expanding at four different points into lenticu- 

 lar, disk-like ganglia of a small size, but many times exceeding the cord in 

 diameter, the whole enwrapped in this semi-pellucid investment which 

 makes it appear of nearly uniform diameter, excepting in front of the first 

 true abdominal ganglion, where the investment becomes swollen and less 

 pellucid, resembling a greatly elongated ganglion. The purpose of this 

 investment, the "bauchgefass" of the Germans, is not understood, but that 

 it is an integral part of the nervous system has been proved by Leydig, who 

 pointed out that it was an expansion of the floor of the neurileme uniting 

 above. The abdominal ganglia are equidistant in the mature chrysalis of 

 Hamadryas, but vary with age and species ; the first, which is scarcely 

 broader than the cord, and noticeable mainly by its whitish color, is situated 

 in Hamadryas near the end of the second segment ; the second at the begin- 

 ning of the fourth ; the third at the beginning of the fifth, and the last in 

 the middle of the sixth segment ; the last is larger than the others and emits 

 four delicate })Osterior nerves ; each of the aljdominal ganglia is also provided 

 with lateral nerves, similar to, but more delicate than, those of the lar\a. 

 Between all these ganglia, the cord does not at first pass in a straight line 

 but in tolerably strong lateral sigmoid curves, as if the ganglia were being 

 crowded together faster than the cord could contract. 



