196 NERVOUS SYSTEM. 



ward? in the median plain above the alimentary canal, have 

 assumed an encreased development, as shown by Lyonet 

 and Straus, and thus more intimately united all the seg- 

 ments and parts of the body by encreasing these bonds of 

 connexion between all the organs and functions of vegetative 

 or organic life. The same kind of change in the whole 

 condition of the nervous system, effected by the metamor- 

 phosis of the insect, is seen in Straus' figure of the adult or 

 imago state of that system in themelolontha vulgaris (Fig. 83. 

 D,) where the usual concentration of the nervous matter in 

 the head and thorax has proceeded to a greater extent than 

 in the papilio. The ganglia of the abdomen are most fre- 

 quently preserved through all the stages of life in the lepid- 

 opterous and the hymenopterous insects, and in those which 

 have the segments of the abdomen the least altered from 

 their larva condition by the process of metamorphosis. But 

 in this coleopterous insect (Fig. 83. D,) where the adult form 

 of the whole body is very remote from that of a caterpillar 

 or of an annelide, all the ganglia have disappeared from the 

 short round abdomen, and have accumulated in three con- 

 tiguous masses in the middle of the thorax, from which the 

 nerves radiate to the organs of motion, and extend back- 

 wards into all the segments of the abdomen. The cephalic 

 ganglia (1, 2,) have also encreased above and below the oeso- 

 phagus, the cerebral lobes (1,) passing laterally into the 

 large compound eyes, and the great sympathetic longitudinal 

 series of supra-cesophageal ganglia (,) with their accessary 

 lateral filaments and ganglia (b } ) have advanced still more in 

 their development. The greatest change of the nervous 

 system, however, from its original larva condition, effected 

 by the metamorphosis in insects, is that presented by the 

 pentatoma, the cicada, and some others where all the ganglia 

 of the columns have accumulated in two points, above and 

 below the oesophagus, in the head and in the middle of 

 the thorax ; thus nearly approaching to the highest condi- 

 tion presented by this symmetrical nervous system in the 

 most elevated tribes of Crustacea, and to the cyclo-gangliated 

 character so general in the molluscous classes. Although 

 during this rapid series of changes the last and the penulti- 

 mate pairs of ganglia are generally the earliest to ap- 

 proach and unite, they are at first distinct pairs like those 



