330 Transactions of the Society. 



to devise, and I submit that it constitutes the real buzzing appar- 

 atus of the Diptera. 



The species finally selected for close observation was Eristalis 

 tenax, the common drone-fly, as it is plentiful and easily caught 

 throughout the summer and early autumn ; it is moreover a large, 

 tough fly, easily handled without injury, and above all it is a very 

 strong and persistent buzzer. So strongly does it buzz that, when 

 held, the violence of the vibration will make the finger tips tingle. 



With the aid of Plates VIII and IX, I proceed to describe the 

 complex mechanism which is to be found at the junction of wing 

 and thorax and also such portions of the external lateral hollow of 

 the mesothorax as are germane to my subject. 



Plate VIII represents a dorso-lateral view of a specimen having 

 the wing extended at right angles to the body with a slightly down- 

 ward inclination. 1 is the wing, 2 the alulet, 3 the ante-squama, 

 4 the squama, 5 and 6 the anterior nervures ; the part between 

 7 and let is the muscular attachment of the wing to the thorax 9 ; 

 12, 20, 21 are two prominent chitinous ribs which traverse diagon- 

 ally the hollow in the side of the mesothorax in which the wing- 

 joint is accommodated when the wings are closed ; 8 is a broad area, 

 which will be further described in connexion with Plate IX. 



The parts 15, 16, 18 form together with 14 and 17 a very 

 complex articulation which is brought into use when the wings are 

 folded ; 15 is a curved outgrowth from one of the nervures and 

 bears within its curve 16 and 18, the former connected to the 

 alulet and the latter to the ante-squama. In the act of folding the 

 wings 16 and 18 rotate upon each other, 15 rotates upon 19, there 

 is a flexure of the joint between 17 and 19, and the whole (15 to 

 19) rotates about the point 14. The result of the folding is to 

 bring the ante-squama over the squama and the alulet above both. 

 In this position the part 13 is in close proximity to the under side 

 of the two ribs 20 and 21, the projection 19 lies opposite a 

 specialized area on the lower posterior surface of the rib 21 below 

 the point 12, and the knob-like percussor 11 lies close to and 

 slightly in front of the upper portion of the rib 20 at the point 23. 

 It is to be noted that when the fly is at rest, the parts of the wing- 

 are nowhere in actual contact with the thorax. 



Having identified these parts and located their relative positions 

 when the wings are folded, observation of living specimens and 

 the application of a few simple tests will show conclusively that 

 the buzz is produced by a rapid vibration of the muscles between 

 7 and la, causing the chitinous percussor 11 to strike the thorax at 

 the point 23. The resonant apparatus consisting of the compound 

 rib 20, 12, 21 and its attached membranes are thus thrown into a 

 state of vibration. 



The motion of 7, la being somewhat in the direction of the 

 axis of the nervure 5, there is little visible vibration of the anterior 



