218 THE CAUSES WHICH PROPAGATE 



contains an intricate structure of membrane and tracheae ; and 

 here the main air-pipes, if I mistake not, enter the abdomen by 

 a single tube, which again separates into the two main pipes of 

 the abdomen immediately behind the auditory org-an. In the 

 Cabbage Moth there are just beneath this cell two horizontal 

 semicircular membranes (Fig. 9, d) attached posteriorly, with a 

 fringe of minute nervous (?) filaments on their edge. The 

 internal convolutions of this assemblage of cells that on either 

 side cap the auditory chamber, are as grotesque as their outward 

 configuration ; and their rude representation of two hunting- 

 horns, placed mouth to mouth, would remind us of the cochlea, 

 were not we already convinced of the different situation of the 

 membraneous labyrinth. Their function is in part, I suppose, 

 that of the eustachian tubes, and they possibly reproduce the 

 mastoid cells. 



I had for some time been familiar with the aspect of the 

 auditory organ in the Noctuiua before I began to attack the 

 silk-spinning group of the Bombycina. In these the accessory 

 of convoluted cells is obsolete, so that the mirror bearing on its 

 disc a horny styliform piece may be wholly exposed by dissecting 

 the abdomen as previously, and then cutting it longitudinally, 

 so as to view the structure from within. The component parts, 

 however, in a majority of our indigenous species are exceedingly 

 minute, and little more is to be gained than that the ear here 

 presents a transition from the complex form of Noctuina to the 

 far simpler structures of the grasshoppers and CicadiB. This is, 

 too, what we might expect from a comparison of the habits of 

 the moths composing these two great divisions. The Noctuinae 

 when disturbed are restless and fleet of wing; the Bombycinae 

 slow of perception and apathetic to touch; many, as the beau- 

 tiful Tiger Moths and Ermines, remaining sunk in coma with 

 their limbs limp and listless, even after impalation. Yet in the 

 latter group alone, we have remarked certain individuals to 

 possess sound-producing organs, a perquisite we previously have 

 associated with this simpler form of ear, and from whence we 

 might likewise infer their noises are intended for communication, 

 being probably employed by the sexes in pairing. 



In the stout-bodied moths with looping caterpillars these 

 organs are likewise rudimentary. In the Scolloped Oak [Cro- 



