176 On the Sound Organs of the Green Cicada. 



the greater portion of each becoming transformed into a pair 

 of tense, delicate, translucent membranes, which constitute 

 the inferior boundaries of air-spaces. The tension-membranes 

 of the metathorax are the smaller, and look downwards 

 and backwards ; those of the first abdominal segment, much 

 larger, look downwards and forwards. A chitlnous ridge 

 and band separate the two pairs transversely. In the 

 ventral middle line a short, stout ridge connects inferior 

 medial projections, a blunt spur from the metathorax, and a 

 semi-circular plate from the abdominal segment, and serves for 

 the linear attachment of the great abdominal muscles. Both 

 pairs of these delicate membranes are likewise protected by 

 two large chitinous plates, which arise externally to the legs 

 in the metathorax, and are enormously larger than corres- 

 ponding folds in the female. These plates quite cover in 

 the white membranes. 



A pair of huge muscles take their origin close together 

 below, along the visceral aspect of the median ridge referred 

 to, and with great antero-posterior extension proceed 

 upwards and outwards, diverging as they rise, to terminate 

 about a quarter of an inch below the rattle-membranes in 

 broad, plate-like, rigid terminals. From these tendinous slips 

 pass to the rattle-membranes. 



Capacious air-spaces, which act as resonators, are formed 

 by absorption or suppression of peri- visceral and muscular 

 elements in the regions affected. The p-eneral boundaries of 

 the air-space are — in front the muscles and viscera of the meso- 

 thorax ; below the two pairs of tension-membranes ; behind 

 the muscles and viscera of the hinder abdominal segments; and 

 above the rattle-membranes. But this space is subdivided 

 into successive resonators as follows : — (1) Antero-lateral 

 recesses, bounded by the rattle-membranes above, the 

 muscles and their terminal plates within, and the anterior 

 tension-membranes below ; (2) median recess between the 

 diverging muscles ; and (3) the vast drum-like cavities 

 behind the muscles and above the great tension-membranes,, 

 excavated at the expense of the normal abdominal contents. 



The modus oioerandi is apparently this. The muscles 

 contract, and by their tendons set the rattle-membranes in 

 a motion which is perfectly free. Vibratory motion would 

 have been hindered or prevented altogether by a direct 

 insertion of the massive muscles into the rattle-membrane. 

 The vibrations of the membranes produced by the friction 

 of the horny ridges is communicated to the air of the resona- 



