464 



44 



IXTER^^AL STRUCTUEE QF INSECTS. 



THE NERVOUS SYSTEM. 



The nervous system of in- 

 sects consists of a double 

 cord extending the length 

 of the body, and lying upon 

 the inferior or ventral side 

 of the internal cavity. The 

 two threads which compose 

 this cord do not lie side by 

 side, but one abo\T the 

 other. The lower threjid 

 swells at intervals into lit- 

 tle knots of nervous matter, 

 called ganglia. In insects 

 of an elongated form, such 

 as some of the !N^europter;i 

 (e. g. Corydalis), and the 

 larvte of the Lepidoptera, 

 there is a ganglion at each 

 segment of the body, making 

 thirteen in all ; but in most 

 mature insects the ganglia, 

 become more or less consoli- 



Xervoiis system of insects. Explained in tlio text. dated. In the JjUttertly 



(Papilio), there are ten ganglia, coimting the brain as one ; in thie. Bee 

 (Apis), there are eight ; in the May-beetle (Melolontha), there are fi\ e ; 

 and in the Cicada there are but two. The upper of the two nervous 

 threads runs nearly in contact with the lower, but is destitute of gang- 

 lia. These two threads seem to represent the double and more compact 

 cord which constitutes the spinal marrow of the higher or vertebrated 

 animals. The upper simple thread is supposed to furnish the nerves 

 of motion, and the lower and ganglionic tliread, the nerves of sensation. 

 The fibres Avhich compose these cords separate at the anterior extremity 

 of the body, so as to embrace the oesophagus or gullet, above which 

 they again unite to form the cerebral ganglion or brain, which is some- 

 what larger than the other ganglia. From the nervous cords, and 

 chiefly from the ganglia, fine lateral threads are emitted, which are dis- 

 tributed to the adjacent parts. 



The nerves thus far described represent what, in the higher animals, 

 is called the cerebrospinal system of nerves, and are sometimes called 



