NERVOUS SYSTEM. 193 



to perforate the connecting nervous columns, between the 

 first and second pairs of ganglia, as in other articulated clas- 

 ses ; so that the first pair only are supra-oesophageal or ce- 

 phalic, and all the succeeding ganglia of the columns are be- 

 neath the alimentary canal. Those ganglia are at first, like 

 those of the worm and the centiped, nearly at equal distances 

 and of equal size, as the segments themselves of the young 

 caterpillar. The columns and the ganglia originally separate 

 on the two sides, early approximate transversely and unite, 

 and a slow movement of the ganglionic matter is at length 

 observed in a longitudinal direction to the parts of the columns 

 where it is most required in the adult condition of the spe- 

 cies. These transverse and longitudinal movements of the 

 nervous matter so accurately described by HEROL.D in the 

 spinal columns of insects, proceed to a very variable extent, 

 according to the degree of metamorphosis from the larva 

 state to which the whole body is subjected in the different 

 adult forms of this class. As in the myriapods, the first and 

 second pairs of ganglia are here contained within the head, 

 and the succeeding pairs are generally placed near the ante- 

 rior limit of the segments to which they belong. The osso- 

 phageal ring thus formed by the columns between the two 

 first or anterior pairs of ganglia is much wider during the 

 voracious larva state of the insects than in their adult con- 

 dition, (see Fig. 83. A, B, C.) The third pair of ganglia, 

 placed in the prothorax, appear to be generally smaller than 

 the fourth, and the fifth, in the metathorax, smaller than the 

 sixth, as in the cossus. The ganglia contained in the abdo- 

 men of the entomoid classes, like the segments of that part 

 of the trunk, are generally the least altered by develop- 

 ment, from their primitive condition ; but these are most 

 changed in insects, and are often obliterated by the process 

 of development, as shown by Straus in the coleopterous in- 

 sects, (see Fig. 83. D,) and by Dufour in many of the hemip- 

 tera. The last pair of ganglia are generally the first to ad- 

 vance forward to unite with the penultimate pair in the larva 

 state. In the annexed figures (Fig. 83. A, B, C,) are seen 

 the common conditions of the nervous system in the larva, 

 the pupa, and the imago state, and the changes produced in 

 that system by the metamorphosis to the perfect state, as 

 observed and .described by Herold, twenty years ago, in the 



PART II. O 



