1892.] IN CERTAIN LEPIDOPTERA. 191 



again pointed out, this was a structure common to all nervures, and 

 the ribbing of the nervures is always strongest near the base of the 

 wings. 



Scudder himself suggests that the sound is produced by the small 

 erect scales on the superficies of the two wings that overlap rubbing 

 against each other; but this is obviously inadequate to produce a 

 clicking sound audible twenty yards off, and it is of universal 

 occurrence that in the parts of wings that overlap the scales are 

 short and differently formed, so as to decrease the friction ; though 

 the rubbing of the wings one against another might be sufficient to 

 account for the slight rustling or hissing sound made by many of 

 the Vanessidce when held close to the ear. 



Ageronia arethusa, Cram, c? • 



Base of fore wing and part of thorax. 



a, pyrifoi'in membranous sac attached to fore wing ; b, chitinoiis hooks of sac ; 



c, cbitinous hooks of thorax. 



On detaching and clearing a fore wing o{ Ageronia arethusa (fig. 3), 

 I found there was a small pyriform membranous sac attached to the 

 base of the inner margin of the fore wing, open anteriorly, and with 

 a pair of curved chitinous hooks with spatulate extremities lying 

 freely in front of it. It was obvious that this could not come into 

 contact with any of tlie nervures of the hind wing, and that no 

 structure attached to the hind wing could act on it; and as there 

 seemed to be a projection on the thorax in the immediate neighbour- 

 hood, I cleared and denuded of scales a half insect with the wings 

 still attached to the thorax, and could then see under a low power 

 of the microscope that there was a pair of strong chitinous hooks 

 attached to the thorax, and that when the fore wing was moved up 

 and down the spatulate ends of the chitinous hooks attached to the 

 wing played against these, being released when the wing reached a 

 certain angle, and I suggest that this is the cause of the cUcking 

 sound, the hooks acting as a tuning-fork and the membranous sac 

 as a sounding-board. 



In this case the structure exists in both sexes, and we must conclude 

 that there is a mutual wish to attract, and that perhaps it is also 

 used as a means of inspiring fear, in accordance with Bigs- Wither' s 



14* 



