THE HALTERES OF THE BLOW-FLY. 611 



to the plates and papillae. My own conclusions are entirely 

 at variance with his, and I do not believe that either the basal 

 or scapal sensory organs have any external opening. 



The Basal and Scapal Organs. Surface views of the organs in 

 question can be obtained with a T Vth oil immersion by mounting 

 the entire halter from an insect the tissues of which have been 

 fixed either in chromic acid or absolute alcohol. These exhibit 

 thin transparent areas about 5 to 6 p in diameter in the basal 

 plate or cupola. Each has beneath it a vesicle containing a 

 highly refractive central spot (PL XLIL, Fig. 4). The trans- 

 parent areas have an average diameter of 25 /a, and are 

 situated in rows on convex longitudinal ridges, between which 

 setae arise which cover the whole surface of the cupola. 

 The scapal organs are more complex. In Tipula they are 

 scattered over the surface of the scapal canals, but in Musca, 

 Calliphora and Eristalis they lie in arcades which are trans- 

 verse to the axis of the scapal canal. These arcades are 

 moniliform convex transverse ridges the integument of which 

 is very thin and transparent. Each arcade is supported by a 

 series of semi-rings which are not unlike the pseudo-tracheal 

 semi-ring of the proboscis without a bifurcated extremity 

 (PI. XLIL, Fig. 5). Beneath the integument, and held by the 

 extremities of each semi-ring, is a distinct vesicle containing a 

 central highly-refractive corpuscle. The semi-ring corresponds 

 with the lips of the fissure which Graber, Lee, and Weinland 

 describe as opening externally. I feel convinced that the 

 opening does not exist. 



The fact that the bright spots disappear when the contents 

 of the halter are removed by crushing it, or by the action of 

 caustic alkalies, was mentioned by me in 1870 [62], and Wein- 

 land's figures confirm this statement [300, Figs. 28 to 34], 

 Neither does he represent any slit such as he describes in these 

 figures, in which, if it existed, it could not fail to be distinctly 

 seen. The appearances of the organs seen in surface views 

 are represented in PL XLIL, Figs. 4 and 5. 



The best preparations I have made showing the scapal 

 and basal organs are from halteres fixed in osmium peroxide. 



