AND ON THE HOVERING HABIT OF ERISTALIS TENAX. 517 



The fourth slide contains two 2:)ortions of a membrane which 

 entirely closes the internal part of the spiracle ; the smaller 

 portion shows its expanded condition, the larger fragment its 

 contracted appearance ; it is an expansion of the tracheal 

 system and in the living insect forms a perfect air-chamber. 

 If the second slide is examined carefully fragments of this mem- 

 brane may be observed still adhering thereto ; the larger portion, 

 however, has necessarily been removed in order to display the 

 folded membranes. 



The opinion I have arrived at is briefly as follows. The 

 air-chamber is kept constantly inflated by the movements of 

 the fly. When the insect requires to produce the musical 

 sound, it has simply to bring the free edges of the pleats close 

 together, and to expel air with suflicient force to set them in 

 vibration. The force employed to do this is derived from the 

 large bundles of muscles which almost completely fill the cavity 

 of the thorax, among which the finer ramifications of the tracheal 

 system are abundantly distributed. The air contained in these 

 would be driven forward very gradually into the air-chamber, and 

 thus replace the quantity discharged to produce the note. Probably 

 when the note is long sustained some of the numerous sclerites 

 may assist the operation by reducing the cavity of the thorax. 



If this explanation is correct it is self-evident that no constant 

 ^ibration of either wings, poisers, scales, or even rapid motion of 

 the legs is necessary, one long-sustained, slow, steady movement 

 alone being suflicient to produce the musical note.' 



If the movements of the fly are restrained without injury to 

 the insect — as for successful experiment it should be — it can be 

 caused to give forth its musical note. To accomplish this you 

 need only use the net in which you capture the insect ; gently 

 but firmly compress it until visible movement ceases. The sense 

 of touch will convey the unmistakable impression that some 

 organs are in a state of rapid \ibration. With the cessation of 

 the note the vibration discontinues, and thus, I submit, corro- 

 borative evidence in support of the explanation given is supplied. 



It only remains to point out the difierences disclosed by these 

 dissections from Dr. Burmeister's description. They are four in 

 number, viz. : 



(1) There are two rods, or plates if you prefer the term, and 

 not one. 



(2) That the folded membrane occupies both sides of the 

 stigma ; not one side only as implied. 



