124 



[November, 



circular in profile in X. polyodon ; lanceolate and clothed above with 

 smooth hair iu C. nupta ; or very elongate and spatulate in Plusia 

 gamma. Placed behind at the origin, concealed by this protuberance, is 

 the opening of a spiracle. Another adjunct is a little triangular 

 membranous valve (r), fringed with hair; attached anteriorly at the 

 entrance of the cavity, and connected by a muscular ligament to the 

 base of the hind-wing, in such a way as to participate in its movement. 

 I have thought this might contribute in some way to the power of 

 hearing, by sudden and rapid condensation of the air in the cavity, 

 during the flight of the insect. 



The external cavity («) is similarly terminated by a membrane 

 (at m), which, proceeding from the front of the auditory adit, closes 

 the external ear vertically and obliquely. Its surface is convex, and 

 outline elliptical. It is divided vertically into distinct portions, indi- 

 cated by one (C. nupta) or more (X. polyodon) chitinous pieces (w) 

 placed on the disc. Of the two parts, that outward from the pro- 

 jecting piece or pieces is distinguished by a milky opacity : the 

 most inward (shaded in the figure) on the contrary, is extremely tense, 

 beautifully iridescent, and of great tenuity, the slightest touch causing 

 it to rend and collapse like a withered flower petal. This is, I conceive, 

 the sentient portion (membrana tympanica). In a careful dissection of 

 a large moth it is just possible, when the posterior parts have been re- 

 moved with the scissors and needle-point, to detach successfully a 

 portion of this membrane from the thoracic muscles, so as to reveal 

 the structural mechanism of the internal ear. The figure shows this 

 operation successfully performed in Catocala nupta. We then see 



disclosed a cellular air-bladder (i), 

 which the membrane shuts off from 

 the auditory canal. To the iimer-side 

 of the elongate, club-shaped, horny 

 piece (w), is attached a slender white 

 chord, traceable to a little membranous 

 vesicle, cylindrical in shape and dis- 

 tended with fluid, to which it unites. 

 (This evidently corresponds to the 

 "membranous labyrinth" of Siebold, 

 and " thin-skinned bladder filled with 



EXPLANATION OF FIGURE. i. >' £ nr-n \ 17 ii 



water ot Muller). Irom the upper 



o Meatus, 



6 Auditory cell with thin tymr.nn^i mem- extremity of this vcsiclc another short 



brane reinovcd, showing vesirlc con- >' 



,>. tainiiigflrudaiKl its attachments. procCSS is SCUt tO the Upper homy 

 (6, c, d, E) An Eustachian tube formed ofr il J 



^ichith" us piece. "i^^-gin of the chamber, where it 



71 Acoustic nerve. appears lo ramify ; and from the 



