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 scalae. 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 scalae exhibit numerous regularly 

 arranged organs on their surface, which are known as pores 

 or papillae, 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 antennae. This fact was pointed out by Braxton Hicks 

 [292] ; but Loew [293] 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. XLIIL, 

 Figs, i 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 scalae 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 scalae and cupola. These have also been called pori- 

 ferous plates, but it is always easy to see what is meant by the context. 



