102 STRUCTUEE OF EAH. 



on each side of the tympanum, and enables it freely 

 to transmit the atmospheric vibrations. 



These tracheae, though formed on a similar plan, 

 present many variations, corresponding to those of 

 the tympana, and showing that the tympana and 

 the trachege stand in intimate connection with one 

 another. For instance, in those species wdiere the 

 tympana are equal, the tracheae are so likewise; in 

 Gryllotalpa, where the front tympanum only is de- 

 veloped, though both tracheal branches are present, the 

 front one is much larger than the other; and where 

 there is no tympanum, the trachea remains compara- 

 tively small, and even in some cases, according to 

 Graber, undivided. 



The tibia is thus divided into three parts, as shown 

 in the diagram (Fig. 64), the central 

 X5\ portion being occupied by the two 



/|'' ^ tracheae (Fig. 64, tr, tr). 



\i '!) _M_ ^^ "^^^ other two spaces, one (the 



IrX^l lower one in the figure) is occupied 



^^ IT^I ^^ ^^ *^® muscles, nerves, etc., while 

 K % the other is mostly filled wdth blood, 



V ' 1 / which thus surrounds and bathes the 



^^ auditory vesicles and rods {ar). 



Fig. 6i.-section through Thc acoiistic ucrvc — which, next 

 Mecon'i '^^?'' a^bout to the optic, is the thickest in the 

 trachei? ar, tTie\udu body— dividcs soou after entering 

 toryrod. ^-^^ t\\)\ii into two brauchcs ; the one 



forming almost immediately a ganglion, the supra- 

 tympanal ganglion, to which I shall refer again pre- 

 sently; the other passing down to the tympanum, 

 where it expands into an elongated flat ganglion, known 

 after its discoverer as the organ of Siebold (Fig. Qb), 

 and closely applied to the anterior tracheae. 



