THE HALTERES OF THE BLOW-FLY. 605 
by the action of its own muscles, so that the head and scape 
vibrate in a conic surface which has its apex within the thorax, 
in the axis of the organ produced. Sections of the plane of 
its movement would probably vary from a circle to a straight 
line passing through the various phases of Lissajous’ curves, as 
the plane of movement depends upon the composition of two 
vibrations—one in the vertical and one in the horizontal plane. 
The Scabellum exhibits externally two semicircular ridges, 
one above and one below, which extend on to the scape. They 
are frequently termed scapal organs (s.s. ; s.c’.). They will be 
distinguished here as the superior and inferior scale. At the 
base of the superior scala is a hemispherical swelling, termed 
the basal plate by Graber, which I shall distinguish as the 
cupola (c). 
The cupola and the two scale exhibit numerous regularly 
arranged organs on their surface, which are known as pores 
or papille, according to the view taken by different observers 
of their nature. These will be designated as scapal or basal 
organs, according to their position.* 
The Nerve Supply.—The halter receives the largest nerve in 
the insect, next to those which supply the compound eyes and 
the antenne. This fact was pointed out by Braxton Hicks 
[292]; but Loew [298] contended that Hicks had not estab- 
lished the fact that this nerve is a sensory nerve, and not a 
motor one supplying the muscles of the halter. Such a con- 
tention has now, however, no value, as it has been abundantly 
proved that the nerve to the halter ends in a series of distal 
ganglia which are related to special sensory organs (PI. XLIII., 
Figs. 1 and 2). The nerve divides into two branches—a small 
one which enters an organ which I shall term Lee’s chordotonal 
organ, and a large one, which subsequently divides into three 
branches and ends in three special sets of organs, the scapal 
and basal organs, which are situated in the scale and cupola 
respectively. Besides these, there are also undoubtedly two 
* The terms scapal and basal organs have been frequently applied by 
various writers to the scala and cupola. These have also been called pori- 
ferous plates, but it is always easy to see what is meant by the context. 
