INSECTS IN GENERAL. 



INTERNAL STEUCTURE OF INSECTS. 



THE NERVOUS SYSTEM. 



1.1 



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 above the 

 other. The lower thread 

 swells at intervals into lit- 

 tle knots of nervous matter, 

 called ganglia. In insects 

 of an elongated form, such 

 as some of the Neuroptera 

 (e. g. Corydalis), and the 

 larvse of the Lepidoptera, 

 there is a ganglion at each 

 segment of the body, ma- 

 king thirteen in all ; but in 

 most mature insects the 

 ganglia become more or less 

 consolidated. In the But- 



Nervous M.ystnui ol insects, explained iu the test. 



terfly (Papilio), there are ten ganglia, counting the brain as one; in the 

 Bee (Apis), there are eight; iu the May-beetle (Meloloutha), there are 

 five, and in the Cicada there are bat two. The upper of the two ner- 

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

 ganglia. These two threads seem to rej)reseut the double and more 

 compact cord which constitutes the si>inal marrow of the higher or ver- 

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

 nerves of motion, and the lower and ganglionic thread, the nerves of 

 sensation. The fibres which compose these cords separate at the ante- 

 rior extremity of the body, so as to embrace the oisophagus or gullet, 

 above wbich they again unite to form the cerebral ganglion or brain, 

 which is somewhat larger than the other ganglia. From the nervous 

 cords, and chiefiy from the ganglia., fine lateral threads are emitted, 

 which are distributed to the adjacent parts. 



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

 is called the cerebro-spinal system of nerves, and are sometimes called 



