60 ON THE ANATOMY OF THE FLY. 



mass of nearly pure uric acid. That passed afterwards is a turbid 

 fluid, sometimes almost clear, and very irritating when applied to 

 any tender part of the skin. This fluid when acidulated with 

 hydrochloric or nitric acid deposits an abundance of crystals of 

 uric acid. I believe this acid is held in solution by ammonia. 

 The excrement of the fly when heated over a lamp gives off a 

 strong urinary smell. 



No trace of these organs exists in the larva, which may be ac- 

 counted for by its inactive mode of life. The bile tubes appear 

 to be its only excretory organs. 



Section VI. The Integument of the Thorax. 

 PLATE V. 



The integument of the thorax consists of no less than fifty-one 

 pieces, some of which are extremely complex in form. The 

 number is, however, much reduced if the smaller pieces con- 

 nected with the articulation of the wings and halteres, the 

 orifices of the respiratory organs, and the twelve forming the 

 cox8G, are omitted. As these will be described in their respect- 

 ive places, and as five of the seventeen which remain are pairs, 

 but twelve will need description here. These belong to four 

 distinct segments, the first being the ventral and lateral plates of 

 the nfth or last cephalic segment. 



The ventral plate or sternum of the fifth segment forms a very 

 remarkable organ, which I have named the cephalo-sternum (Plate V, 

 Fig. 9). It consists of a saddle-shaped body with a pair of thin lobes 

 directed forwards. The body is opaque and strengthened by ridges which 

 suppoit the lobes. These are thin and almost transparent ; they are 

 about l-200th of an inch in diameter and are covered by a multitude of 

 setas, about l-600th of an inch in length, each having its base sur- 

 rounded by a thickened ring, which gives the lobes a honeycombed 

 appearance. In front of each lobe is a small leaf-like plate of opaque 

 integument. The upper surface of the lobes is in apposition with a tuft 

 of nerve filaments, which covers it, and which may be distinctly seen to 

 terminate in nerve cells. The use of this organ is at present unknown, 

 but it is probably an organ of special sense. 



