THE HALTERES OF THE BLOW-FLY. 611 
to the plates and papilla. 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 1th 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 w in diameter in the basal 
plate or cupola. Each has beneath it a vesicle containing a 
highly refractive central spot (Pl. XLII., Fig. 4). The trans- 
parent areas have an average diameter of 25 mw, and are 
situated in rows on convex longitudinal ridges, between which 
setee 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 
(Pl. XLII., 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 [800, 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. XLII., Figs. 4 and 5. 
The best preparations I have made showing the scapal 
and basal organs are from halteres fixed in osmium peroxide. 
