AND ON THE HOVERING HABIT OF ERISTALIS TEN AX. 515 



and opening it carefully, found its posterior and inner lip, which 

 is directed towards the commencement of the trachea, to be 

 expanded into a small crescent-shaped plate, upon which are 

 nine parallel very delicate horny laminae, the central one being 

 largest, while those on each side became gradually smaller and 

 lovrer; and it is, he is persuaded, in consequence of the air 

 being forcibly driven out of the trachea and touching these 

 laminae that they are made to vibrate and sound precisely in 

 the same way with the glottis of the larynx." 



The quotation need not be prolonged ; we are, I imagine, on 

 firm ground as to the point of origin of the sound produced 

 by Eristalis tenax. 1 think, however, ithat it will be capable 

 of being demonstrated that Dr. Burmeister was not strictly 

 accurate in his description of the musical organ, unless the 

 theory of evolution has been exemplified by this fiy in a most 

 remarkable manner since his observations were made. 



In order to do this to the satisfaction of the members of our 

 Club, I have sent six slides of dissections of the parts to be 

 dealt with in these remarks; if they are thought worthy of a 

 place in the cabinet of the Club, I shall be pleased to have 

 them placed where they may possibly be more useful than 

 remaining in mine almost unnoticed. 



The first slide contains the anterior and posterior thoracic 

 spiracles, also three of the organs from the abdomen ; they are 

 mounted without pressure, and arranged to give an external 

 view of the parts. There are also two portions of the exquisite 

 arborescent fringe which guards the aperture, mounted under 

 pressure to admit of higher power being employed for their exam- 

 ination. It will be observed that each posterior stigma is entirely 

 surrounded with the fringe of appendages. In this respect they 

 differ from the corresponding organs in others flies that I have 

 examined. There is nothing especially remarkable about the 

 anterior spiracle, consequently it wilL disappear from subsequent 

 remarks on slides. 



The second slide contains two posterior thoracic spiracles, 

 taken from difierent flies — thus they may be regarded as tests 

 for any difference in those organs ; they are mounted to give an 

 internal view, without pressure, in order that their normal form 

 may be preserved. A wonderful and very beautiful addition is 

 displayed, to which I now desire to direct your attention. 



