NERVOUS SYSTEM. 31 



by which they can be opened and closed at pleasure. 

 The tracheae themselves are exceedingly delicate 

 tubes, formed of two membranes, and between these 

 there is a slender spiral fibre which serves to keep 

 the tube constantly distended. They are not always 

 simple tubes, however, but in many insects give rise 

 to large air- sacs, which occasionally appear externally 

 as transparent spots. 



The reader who has accompanied me through all 

 the preceding details will probably be rejoiced to 

 learn that we have only one other set of organs to 

 notice before finally taking leave of this part of our 

 subject. But this set of organs is one of the 

 highest importance, as it is in obedience to its man- 

 dates that all the other parts of the body perform 

 their several functions, and it is only by its agency 

 that the Insect is rendered conscious of the objects 

 that surround it. The apparatus which plays this 

 important part in the (Economy of the Insect is the 

 nervous system. 



In most insects this consists of a series of knots of 

 nervous matter, called ganglia, which runs along the 

 centre of the lower part of the body immediately 

 above the ventral plates. Of these ganglia the largest 

 is found in the head, forming a sort of brain, from 

 which the nerves of the eyes and antennae are given 

 off. A second ganglion is found beneath the oeso- 

 phagus, which communicates with the upper one by 

 two filaments passing down on each side of the oeso- 

 phagus, so that the latter is surrounded by a sort of 

 collar of nervous matter. From the lower ganglion 

 issues a pair of fine nervous filaments, which unite it 

 with the first ganglion of the thorax, and all the other 

 ganglia communicate with each other in the same 



