520 W. H. HARRIS ON ERISTALIS TSAAX. 



respect appear to differ from those of the larger scale now to 

 be referred to. 



The appendages of the larger scale, unlike those of the smaller, 

 are of true dermal origin ; they arise from and are continuous 

 with small elevations which border the organ. They consist of 

 a central shaft, which for some little distance from the base gives 

 off numerous short branches ; these in turn divide and give rise 

 to from two to six very long tapering branches, entire throughout 

 their length ; collectively they produce the effect of an exquisite 

 fringe. They are at their maximum length in the centre of the 

 scale, decreasing somewhat on either side. 



If a perfect fly is examined there will be found, near the part 

 where the membranes are folded to form the two scales, a curious 

 organ to which I now desire to draw attention. I have never 

 seen any allusion to it in any books that I have had access to. 

 To this organ I attribute the property which enables the fly 

 to accomplish the feat of hovering, and I propose to call it, 

 provisionally, the "plume." 



The sixth slide contains an isolated plume. This consists of 

 two parts — viz., a strong chitinous basal part, and a membranous 

 hollow expansion which gives rise to innumerable long hairs ; 

 it presents a beautiful feathery appearance, and from its size 

 would appear capable of presenting considerable resistance to 

 the passage of air. The whole organ can be extended at right 

 angles to the thorax ; in this position it is free from the alulets, 

 or it may be compressed against the side of the fly, and probably 

 withdrawn under shelter of the scales. It is attached to one of 

 the small sclerites which so freely abound near tlie base of the 

 wings and are so difficult to understand, 



I regard the appendages of the four alulets and the two feathery 

 plumas as essential, and sufficient to enable the fly to maintain its 

 apparently motionless position in the air. It is highly probable 

 that the plumes move rhythmically with the wings, being alter- 

 nately extended and withdrawn ; but I have never been able to 

 devise a plan to assure myself of this. The struggles of a fly 

 under restraint may, and probably do, differ from those it makes 

 in its natural state of freedom, and it is so very easy to fall into 

 error. I must, therefore, leave this an open question, resting 

 satisfied for the present with the little I have done towards the 

 elucidation of two very interesting subjects. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. VIII., No. 53, November 1903. 



