332 Transactions of the Society. 



45 degrees with the axis of the body, no buzzing is produced, 

 though efforts to do so are frequent as evidenced by the vibration 

 of the wing shoulder. 



Further information is obtained on reference to Plate IX. This 

 represents an external lateral view of the wing cavity in the side 

 of the mesothorax after removal of all muscular parts. In this 

 figure identical numbers indicate identical parts in Plate VIII. 



G is the insertion of the wing-muscles ; the area 8 consists of 

 a broad chitinous plate attached to the thorax by its superior edge ; 

 the lower margin is connected to the rib 20 by the clear transparent 

 membrane F ; across this membrane runs the stretching-piece or 

 "extensor" C; this is articulated at one extremity to the rib 20, 

 and at the other to the lower margin of the sclerite 8. At approxi- 

 mately the centre of 8 there is a deep indentation I), terminating 

 at an apodeme to which a large nerve is connected. 



It will be seen that the rib 12 forms the upper margin of a 

 fenestrum B, which is covered by a tense transparent membrane 

 at the point A. On this rib is an excrescence with an undulated 

 surface against which the projection 19 (Plate VIII) rests when the 

 wings are closed. The impact of the percussor 11 (Plate VIII) takes 

 place at the point 23 on the thickened margin of the sclerite. (In 

 Calliphora the point of impact is somewhat removed from the 

 margin and there is a markedly thickened isolated area upon which 

 the percussor strikes.) 



From the undulating surface of the excrescence A and its 

 direct connexion with B, which is virtually a tympanum, it might 

 be supposed that the sound was • produced by the movement 

 of 19 (Plate VIII) over A, but Test 3 appears to dispose of that 

 possibility as the pressure there applied at the point 15 (Plate VIII) 

 has for effect to remove 19 (Plate VIII) from contact with A. On 

 the other hand, it is obvious that owing to the arrangement of the 

 " extensor" C, any variation of pressure at the point A has for 

 effect to vary the tension of the membrane F, and thus modulate 

 the note produced. 



It is evident that the organs of phonation of Eristalis described 

 above are of a higher order than the stridulating organs of 

 Coleoptera and Orthoptera, and a little consideration will tend to 

 the belief that it is not impossible that they fulfil the dual function 

 of emitting and receiving sound. According to the laws of 

 acoustics a stretched membrane will vibrate under the influence of 

 external notes which are approximately in unison with the note it 

 can itself emit ; it is therefore not unreasonable to infer that the 

 resonant areas (8, F, Plate IX) will respond to the buzz of other 

 individuals and thus form one of the elements of an auditory 

 apparatus. 



If it be further borne in mind that, in Eristalis and perhaps in 

 other genera the note is susceptible of a somewhat syren-like 



