THE EXTERNAL ANATOMY OF INSECTS 91 



renders the insect a mute; while the destruction of the other mem- 

 branes, the timbal remaining intact, simply reduces the sound. 



The chief function of the opercula is doubtless the protecting of 

 the delicate parts of the musical organ; but as they can be lifted 

 slightly and as the abdomen can be moved away from them to some 

 extent, the chambers containing the vibrating parts of the organ can 

 be opened and closed, thus giving a rhythmic increase and decrease of 

 the loudness of the call. 



e. THE SPIRACULAR MUSICAL ORGANS 



There has been much discussion of the question whether insects, 

 and especially Diptera and Hymenoptera, possess a sound -producing 

 organ connected with the spiracles or not. Landois ('67) believed 

 that he found such an organ and figures and describes it in several 

 insect^. It varies greatly in form in different insects. In the Diptera 

 it consists of a series of leaf -like folds of the intima of the trachea; 

 these are held against each other by a special himiming ring, which 

 lies close under the opening of the spiracle; and is foimd within two 

 or all four of the thoracic spiracles. These membranous folds of the 

 intima are set in vibration by the rush of air through the spiracles. 



In the May-beetle, according to Landois, a buzzing organ is found 

 near each of the fourteen abdominal spiracles. It is a tongue-like 

 fold projecting into the lumen of the trachea under the base of the 

 closing apparatus. On its upper surface it is marked with very fine 

 arched furrows. He concludes that this tongue is put in vibration by 

 the breathing of the insect, and hence the buzzing of the flying beetle. 



If insects produce sounds in the way described by Landois. they 

 have a voice quite analogous to our own. But the validity of the 

 conclusions of Landois has been seriously questioned; the subject, 

 therefore, demands further investigation. See also Duncan ('24). 



/. THE ACUTE BUZZING OF FLIES AND BEES 



Many observers have found that when the wings of a fly or of a bee 

 are removed or held so that they can not vibrate the insect can still 

 produce a sound. The sound produced under these circumstances is 

 higher, usually an octave higher, than that produced by the wings. 

 It is evident, therefore, that these insects can produce sounds in two 

 ways ; and an extended search has been made for the organ or organj 

 producing the higher note. 



