250 THE CAUSES WHICH PEOPAGATE 



centrally placed in the protliorax ; the third, uniting- the two 

 forms, lies at the anterior part of the mesothorax, and surpasses the 

 first and second in bulk ; a fourth ganglion lies on a horny sternal 

 piece in the metathorax. The position of the structures claim- 

 ing- to be auditory, as in Lepidoptera, Acridiidse, and Cicadidse, is 

 at the opening of a spiracle, here the metathoracic. Forming the 

 internal superior boundary of the prothorax is a chitineous saddle- 

 shaped piece, as in moths ; but the cellular structure is wanting 

 centrally, where it merely forms 2^ point d'apjrtd for the insertion 

 of the thoracic muscles. At its extremities, where it abuts on 

 the metathoracic bladders that lie at the wino--roots and serve 

 their inflation, it is intimately connected with small chitineous 

 rods directed inwardly, while a white bladder (»), with a pitted 

 surface and seemingly enclosing a fluid, arises as a white tube 

 that communicates with the external air at a point where the 

 external angular pieces of the thoracic dermis unite their apices, 

 and then, passing along inferiorly to the saddle-piece, joins the 

 apex of the chitineous rods and distends to an ovate form, after 

 which, again attenuating, it ends in a nerve which runs to the 

 ganglion in the metathorax. A minute membrane lies just 

 superiorly to the termination of the white tube, and contiguous 

 to the spiracle, concealed in a depression. It is ovate in form, 

 and pellucid. The only well-develojied part here, nevertheless, is 

 the ganglion, or bladder. 



In conclusion I should mention the implement I have found 

 most essential in those investigations of these auditory structures 

 I have myself undertaken, is a pocket lens of three glasses ; a 

 microscope, from its obtuseness in regard to opaque subjects, has 

 proved of little service unless of the very best, and then it in no 

 ways stands to the former in the relation of an astronomical 

 telescope to a star-finder, as here the chief labour is manipula- 

 tion, and the higher power can only be employed to reveal the 

 tissues of the parts, not to unravel their composition and 

 relations. 



Should any seek to know more of the capabilities of these 

 lowest of ear-structures than is to be gleaned from crude 

 anatomical description, resort may be made to observation and 

 cautious experiment. And having notated a series of insect 

 passages, and the excitation under which they were produced, 



