2l8 HISTOLOGY 



leaving the limb. The peculiar structure of the cell is remarkable, and 

 is seen only in the auditory organs of insects. Its cell body is found in 

 the ganglion (Siebold's or the supra-tympanal ganglion in this case), 

 and the efferent processes from several of these cells form the nerve 

 fibers that pass into the body. The afferent pole is elongated into a 

 moderately long fiber, on the end of which is seen the huge auditory end- 

 organ of the cell which is often larger than the cell body itself. Its 

 interior is occupied by space drawn out in the axis of the fiber and lined 

 with a cuticular shell that is open on the end toward the cell body, but 

 closed distally and thickened into a conical mass. The central portion 

 of the afferent fiber coming from the cell is directed into the open end 

 of this peculiar end-organ, and ends as a nerve fibril that projects freely 

 into the body. Its free portion in the cavity is called the axial filament, 

 and its vibrations are caused by the sound waves working through the 

 tympanum as a medium, or, perhaps, directly. These vibrations produce 

 the stimulation of the nerve cell. The distal portion of this sensory 

 cell-organ is further extended to form a means of attachment for the 

 cell to the tympanal surface. This is called the terminal filament. The 

 presence of other nuclei in the scolophores of some insects might lead 

 one to believe that the whole apparatus was not unicellular, and that other 

 cells than the ganglion cell had taken part in the formation of the scolo- 

 phore. The axial filament at least is a part of this cell. When no 

 special tympanum exists, as in some lower larvae (see below), the terminal 

 filament may act as a tympanum itself. 



As has been said, there are two groups of these cells. One is known 

 as Siebold's ganglion, and its end-organs are attached to the trachea. 

 They form a long row of cells of diminishing length, and probably each 

 cell is adapted to respond to a note of different wave length. The other 

 group is known as the supra-tympanal ganglion, and its end-organs are 

 attached to the outer walls of the limb. Their exact function would 

 make an interesting experimental study. 



A somewhat simpler organ of the same kind exists in the limbs and 

 body walls of many other insects. The tympanum may be entirely 

 lacking, and yet the chordotonal organ be fairly well developed. This 

 condition is usually accompanied by a lack of voice in the species, and 

 gives rise to some doubts as to whether the creature can hear or not. 

 If it cannot hear, we must then decide as to whether or not the organ is 

 degenerate, or a rudiment of one that will be used to hear later in the 

 history of the race. 



The apparently simplest form of the chordotonal organ is found in the 

 body tissues of many insect larvae. In this form, as exemplified by the 

 structure of the larva of a fly, Chironomus, the auditory cells occur in 

 many segments of the body in small groups of from one to three or more. 



