ON INSECT SOUNDS. 343 



and thin chitinous membranes whether present in the form of 

 curtain or leaf or in semi-cylinder. The state of tension in which 

 these membranes are maintained by the action of muscle fibres 

 attached to them exerts an essential influence upon the tones 

 produced. 



The expired air sets these membranes or elastic bands in 

 vibration. Prof. Landois says that by cutting off all external 

 organs of motion (legs, wings, &c.) of a blue bottle or of eristalis, 

 and laying the insect on its back on the surface of water, its trunk 

 jerks forward with every utterance of sound. It is therefore by 

 compression of air through the vocal apparatus affected by the 

 movement of the trunk that sound is produced. If the vocal 

 apparatus be sealed up, the trunk movement ceases. Professor 

 Landois does not think that inspiration of air causes any sound. 

 How does the air gain entrance into the insect's body ? The 

 extremity of the tracheal tube behind each stigma is supplied with 

 a lever apparatus which when acted upon by muscles presses on 

 the tube and narrows its calibre. By this arrangement the insect 

 can at will open or shut the passage of entrance. At inspiration, 

 entrance is made free and air passes into the body, but the 

 tracheal tubes cannot in consequence of their inner chitinous 

 lining membrane, propel this air "further. Therefore the closing 

 apparatus is worked, the muscles of respiration contract, and thus 

 the air is driven on through the bronchial tree, to its finest 

 ramification, and to the respiratory cells at their ends. If there 

 were no apparatus to close the tracheae, the air would simply pass 

 outwards without being of any service to the internal functions of 

 the insect's body. But by this apparatus the insect can regulate 

 the quantity of air which it requires to take in. Thus when it 

 wants to fly, it can fill its air tubes full, by inspiring quickly and 

 not allowing the air to escape, and so pump air into its reservoirs 3 

 thus filling its bellows, so to speak, with air wherewith to utter 

 sounds while it flies. As the principal stigmata are always 

 situate on the insect's thorax, the contractions of the muscles of 



