AUDITORY TISSUES 



219 



nv. 



The terminal filament is long and slender, and is attached to some part 

 of the body cuticle. This brings a tension upon the central ganglion. 

 In many cases a ligament is devel- 

 oped leading from another part of 

 the body wall and attached to the 

 ganglion cell mass. The short 

 length of this ligament brings the 

 tension between two parts of the 

 body wall in the same segment 

 and relieves the delicate nerve 

 and ganglion, as well as the cen- 

 tral ganglion, of nearly all strain. 

 At the same time it allows the 

 ligament-filament cord to vibrate 

 under tension, and thus stimulate 

 the axial filament which lies in 

 the scolophore in the middle sec- 

 tion of the compound tympanic 

 cord (Fig. 197). 



x-v c . i i r FIG. 197. Chordotonal organ of a Chironomus 



On account of its lack of an in- larv a. ,, !.- !.._. i ^ 



ternal air space and an externally 

 differentiated tympanum, we must 

 look upon this last auditory organ 

 as the simplest insect form, especially that one which lacks a ligament. 



One more insect form should be briefly examined, as exhibiting the 

 most highly specialized form of the insect's auditory organ. This 

 is the " ear " of the " grasshopper " or locust. This organ consists 

 essentially of the same auditory cells, with their terminal filaments rest- 

 ing against a very large tympanum. The tympanum is stretched 

 across the enlarged opening of an abdominal trachea that it closes, 

 with the exception of a small pore left to allow of an equal 

 distribution of air pressure. This allows the membrane to vibrate 

 freely. Resting against the tympanum, and attached by their terminal 

 filaments to two horny irregularities on its surface, are the auditory 

 nerve cells or scolophores. The nerve, which comes from the ganglion 

 formed by their assembled cell bodies, passes over the inner surface 

 of the tympanum and into the body, where it enters a central ganglion. 



A strong peculiarity of all the above insect auditory organs is the 

 small number and high specialization of the auditory nerve cells or 

 scolophores, which, with the possible exception of the mosquitoes, are 

 found in all of them. In the mosquito the perceptory nerve cells are 

 found in the second basal joint of the antenna, where they form a peculiar 

 ganglion (see Fig. 195). 



nv., nerve; hg., ligament; fil., "ter- 

 minal filament " or cell-organ of sound percep- 

 tion; aud.c., auditory cells. (After GRABER in 

 Arch.f. mik. Anat.) 



