NERVOUS SYSTEM. 195 



on the muscles and the minutest ramifications of tracheae 

 for respiration. These inter-ganglionic nerves come off ge- 

 nerally at a greater distance from the ganglia in insects and 

 Crustacea, and in worms, than in the myriapods and the 

 arachnida. The last pair (A. 13,) of ganglia are already ap- 

 proached to those which precede them (A. 12.) In the pupa 

 state of this moth (Fig. 83. B,) the approximation of the 

 segments has not only shortened the total length of the co- 

 lumns, but has caused them to assume a puckered or curved 

 appearance, where they lie free between the ganglia ; the 

 shortening of the whole trunk during the metamorphosis 

 thus obviously takes place more quickly than the correspond- 

 ing changes of the nervous system. The oesophageal ring is 

 diminished, the cephalic ganglia are enlarging and extend- 

 ing transversely for the myriad of developing eyes, and the 

 thoracic pairs of ganglia are approaching to each other, pre- 

 paratory to their uniting together at the part best suited 

 to send nerves to the yet undeveloped thoracic members. 

 The third pair of ganglia often advance to unite closely with 

 the second pair, the fifth to unite with the fourth pair, and 

 the seventh pair to unite with the sixth, during the passage 

 to the imago state, and several, or the whole, of the ganglia 

 which succeed these in the cavity of the abdomen, entirely 

 disappear, while the motor and sensitive nerves still con- 

 tinue to come off from the same parts of the columns, as 

 seen in the nerves of the perfect insect (Fig. 83. C. 7, 8.) In 

 the imago, or perfect state of the insect, (Fig. 83. C,) the 

 loose inter-ganglionic portions of the columns, which were 

 zig-zag in the pupa, have assumed a straight and shorter 

 form, the two last pairs (12, 13,) have coalesced into one 

 ganglion, and advanced from their original position, the cine- 

 ritious matter has disappeared from two pairs of the abdo- 

 minal ganglia (7, 8,) without affecting the original origins of 

 their nerves ; four pairs of ganglia (6, 5, 4, 3,) have coalesced 

 at two points of the thorax, to supply nerves to the muscles 

 of the legs and wings ; the second and first pairs of ganglia 

 (2, 1,) have approached in the head, and diminished the dia- 

 meter of the oesophageal ring, the cephalic ganglia (C. 1,) 

 have enlarged and extended transversely, to form the ex- 

 panded optic lobe in each orbit, and the accessary nerves 

 j(C. b, b,) and the great sympathetic (C. a,) running back- 



o 2 



