234 THE CAUSES WHICH PROPAGATE 



ORGAN OF HEARING IN THE ACRIDIID^ (PLATE II. ^ FIGS 1 AND 3 ; 

 PLATE VI., FIGS. 1 AND 2.) 



That saltatoriiil insects should possess capacious ears, is in 

 keeping with, what we witness in mammalia such as hares, kan- 

 garoos, and squirrels, which similarly proceed by leaps and bounds ; 

 and yet often it is scarcely possible to say whether it be sight 

 or hearing that caiises the grasshoppers to start up over the 

 greensward before our advancing footsteps. But as regards the 

 structure of certain auditory cavities in these last we have ample 

 detail, thanks to the researches of Miiller, Siebold, and others. 

 We find in nearly all an evident recess, with an ovate, lunate, or 

 only linear opening (Plate II., Fig. 3, a), situated at the hinder 

 and lateral portion of the first dorsal arc of the hind body, or 

 abdomen, partially covered by the elytra. This cavity is closed 

 interiorly by a posteriorly iridescent thin and oval membrane 

 [membrana tympanicd) (Plate II., Fig. 1a, mm), which parts it 

 from the first abdominal air-bladder (/'). On its disc (Fig. 1b), 

 certain brown punctuate discolorations are seen, which mark 

 the position of two raised chitineous pieces on its internal 

 surface, the larger angular, the other small and triangular in 

 plan. To the more projecting angular piece is attached a 

 snow-white vesicle distended with a clear fluid that sends off a 

 thin arm to be inserted in the smaller piece situated towards the 

 centre of the disc. This tender vesicle is penetrated by a nerve 

 (;^), representing the acoustic proceeding from the third (fourth) 

 thoracic ganglion (Plate VII., Fig. 2). Lastly, in the chitineous 

 settino- that surrounds the iridescent membrane where it dilates 

 anteriorly and inferiorly, is a minute round or oval oj^ening 

 (Plate II., Fig. 1b, e), forming a communication between the 

 air-bladder and the external air, adapted, it would seem, to the 

 part of an eustachian tube by immediately introducing air 

 behind the membrane. 



The organisation and function of these organs has been but 

 slowly recognised. Naturalists of the last century considered 

 them as the source of stridulation. Swammerdam, in Holland, 

 thus vaguely states the grasshopper to have two peculiar small 

 drums, like the drum of our ear, which being struck by help of 

 two lunated cartilages, vibrate the air in such a manner as to 



