2 
INSECTS IN GENERAL. 
INTERNAL STRUCTURE OF INSECTS. 
TIIE NERVOUS SYSTEM. 
fFis- 1.1 
Tlie 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 
larvae 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 system ol insects, explained in the text. 
terlly (Papilio), there are ten ganglia, counting the brain as one; in the 
Bee (Apis), there are eight; in the May-beetle (Melolontha), there are 
five, and in the Cicada there are but 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 represent the double and more 
compact cord which constitutes the spinal 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 oesophagus or gullet, 
above which they again unite to form the cerebral ganglion or brain, 
which is somewhat larger than the other ganglia. From the nervous 
cords, and chiefly 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 
