246 THE CAUSES WHICH PROPAGATE 



base of the hind wing-, and in a way to participate in its move- 

 ment. When the insect takes wing, this valve, like the poisers 

 of the fly, leaving its horizontal position of rest, beats up and 

 down above the ear-hole, so that the tip of its fringe just grazes 

 the round protuberance {iv), suggesting its purport as a mag- 

 nifier of the sonoral atmospheric piilses, either by a sudden 

 condensation of the air in the ear-hole, or as a deaf man puts his 

 hand to his ear to collect the vibrations. 



The auditory canal [a] is closed by a tympanum [m), very 

 similarly in all these moths, and shows many striking points of 

 agreement with its counterpart in the grasshoppers, but with 

 this difference — that its structure is far more delicate. The 

 tympanal membrane (Plate IV., Fig. 6) proceeds from the front 

 of the auditory canal near its entry, and closes the external ear 

 vertically and obliquely, so as to be somewhat presented back- 

 wards. Its surface is convex, and its outline elliptical. It is 

 divided vertically into two distinct portions, indicated by one 

 [Nupfa) or more {Pol^oclon) horny, chitineous pieces [d) , placed 

 on the disc. Of these two parts, that outwardly from this pro- 

 jecting knob, or knobs, is distinguished by a milky opacity in 

 the membrane; the most inward {m), on the contrary, is ex- 

 tremely tense, and of great tenuity, the slightest touch causing 

 it to rend and collapse like a withering flower-petal. Another 

 striking character is seen in a beautiful iridescence, or mother- 

 of-pearl refraction of light, that appears to portray a rough 

 surface and concealed acoustic power, for this is doubtless the 

 sentient portion of the memhrana tympanica. 



In a careful dissection it is just possible, in a large moth, 

 when all the other parts have been removed posteriorly with 

 the scissors and needle-point, to successfully detach a portion of 

 the tympanum from the muscles of the thorax, so as to reveal 

 the structural mechanism of the internal ear (^). Fig. 6 shows 

 the operation performed in the Red Underwing, and the various 

 parts then exposed, which form a striking resemblance with the 

 corresponding elements of the internal ear of the grasshopj^ers. 

 It is found that the removal of the tympanum has disclosed a 

 small air-vesicle it shuts off from the auditory canal. To the 

 inner side of the elongate club-shaped horny piece on the disc 

 we see attached a slender white cord, which it is easy to trace 



