THE WING ORGANS. 621 



is united with the marginal nervure by a small transverse 

 branch ; each pore exhibits a dark brown margin, enclosing 

 a small circular opening, around the margin of which there is 

 a clear space surrounded by an outline which exhibits a double 

 contour. 



' This collection of pores is prolonged in either direction in 

 a single line of similar pores. After clearing the wing with 

 potash and compressing it, a very distinct view of its interior 

 was obtained. A thick tracheal stem is observed.' After 

 describing the position of the vessel, he continues : ' Closely 

 related to this tracheal tube there is a nerve of moderate size 

 which gives off a series of branches to the pores, resembling 

 the nerves to the crista of the Locustidse in their arrangement. 

 These branches terminate in end organs, which are related 

 to vesicles beneath the integument. The vesicles are some- 

 what elongated, and lie almost at right angles to the ejid 

 organs. 



' The vesicles are the clear spaces surrounded by a double 

 contour already mentioned. Each vesicle lies under a cuticular 

 pore the transparent spot surrounded by a dark margin in 

 surface views. 



' The nerve terminals consist of a nucleated enlargement, 

 which is to be regarded as a ganglion-cell followed by a second 

 fusiform enlargement, in one of which there is a thread-like 

 structure ; this is not, however, very distinct.' 



Graber concludes that if his observations are correct, this 

 is a chordotonal thread, and adds : ' Opposite to the place 

 where the vesicle is related to the cuticular pore, several 

 diverging bands spring from a single point, and are inserted 

 into the integument (Fig. 81, A, a), giving them the character 

 of chordotonal organs.' 



From Graber's drawing and my own observations on the 

 Blow-fly, I think it indubitable that the bright spot which he 

 regards as a pore is really due to the presence of a corpuscle 

 beneath the integument similar to that which I have figured 

 from the great chordotonal organs of the halteres. In the 

 Blow-fly the wing organs are situated on the remigium, or base 



