OF SOUNDS. 469 



instrument of the sound. To convince myself of this, I closed both the 

 spiracles with gum, and then urged the fly to vibrate its wings, but it 

 was scarcely to be induced to do so, yet when it occasionally tried it, no 

 sound was produced; only after an interval, when the spiracle was freed 

 from its stoppage by means of violent volatile motions, was the sound 

 renewed. There is no doubt, therefore, that the air streaming from 

 the spiracle is the cause of the sound, and that a body which by this 

 draught of air is brought into vibration, must necessarily stand in 

 connexion with the spiracle. I therefore cut out one of the spiracles, 

 opened it carefully, separating the angles of the incision, and soon found 

 what I sought, namely, the vibrating body, and not one only, but very 

 many. That lip of the spiracle, namely, which lies posteriorly, and also 

 somewhat inwardly, and which is lengthened upon its inner side, that is 

 turned towards the commencement of the trachea, is formed into a small 

 Hat half-moon-shaped plate ; upon this plate there are nine parallel very 

 delicate horny leaves, the superior free sharp edges of which are bent 

 somewhat downwards, so that the anterior one inclines a little over 

 the rest. They are also higher towards the trachea, and towards the 

 margin of the spiracle lower, and the central one is the largest, from 

 which on each side they gradually become smaller and lower. Upon 

 the air, which is driven with force out of the trachea, touching these 

 laminae, they are made to vibrate and sound precisely in the same 

 manner as the vibrating of the glottis of the larynx. Thus, consequently, 

 there is no insignificant analogy between the spiracles and the larynx, 

 particularly of birds. To convince myself that it was merely the 

 posterior spiracles of the thorax which emitted sounds, I likewise 

 inspected the anterior ones, but found in them not the least trace of the 

 just-described laminae at the inner side of the posterior lip *. 



We can now comprehend the reason of the change of tone on the loss 

 of the wings. The vibrations of the contracting muscles can no longer 

 be so intense in consequence of the loss of the organs made to vibrate, 

 and in consequence of the weaker contractility, the air cannot be expired 

 with the same degree of force. The tone is therefore weaker than 

 when the wings were present; also, as Chabrier supposes, some air may 

 escape through the open trachea of the wings which are cut off. 



* Chabrier, in his Essai sur le Vol des Insectes, p. 45, &c., likewise explains the hum of 

 insects as produced by the air streaming out of the thorax during flight : he also speaks 

 of lamina? which lie at the aperture of the spiracle ; but I cannot recognise from his 

 description whether he saw these or others. 



