THE HALTERES OF THE BLOW-FLY. 609 
two cavities of the capitellum are more simple in Eristalis, and 
are shown in P]. XLIL, Fig. 3. 
The Structure of the Septum—The septum consists of a 
basal layer of elongated cells which Bolles Lee describes as 
modified hypodermal cells. These cells closely resemble those 
which form the attachment of some of the muscles of the 
thorax, which I have described as fibrillated hypodermal cells 
(p. 283), but I have been unable to find any traces of muscle 
fibres in the septum. On the distal surface of this fibro- 
cellular layer there are numerous fat-cells and multi-nucleated 
cell-strings. Both surfaces of the septum and the whole inner 
surface of the capitellum are covered by a single layer of thin 
pigmented cells, and similar cells line the canals of the scape 
and are continued over the basilar membrane of the scala and 
cupola in the scabellum. 
The pigment they contain is orange-coloured and in minute 
granules. The pigment in the capitellum is not evenly dis- 
tributed, but forms numerous circular spots visible through 
the pellucid chitinous integument. 
b. Special Sensory Structures. 
Historical.—In the years 1856-57 Braxton Hicks [292, 294] 
first described the sensory organs at the base of the halteres. 
This author only examined the chitinous structures, although 
he drew attention to the great nerve of the halter. He 
described the three special plates, the cupola, and the scale 
as containing rows of vesicles. In 1860 Leydig [269] under- 
took the reinvestigation of these vesicles; he regarded the 
vesicles as closed externally and open internally, and described 
auditory hairs unconnected with the vesicles as the true nerve 
end organs. 
In 1882 Graber, in his paper on chordotonal organs [285], 
held that the papilla of the scapal and basal plates each 
contain an auditory rod (Hérstift), and that the basal and 
scapal papille differ, inasmuch as the former are closed 
externally and each of the latter opens by a narrow slit on the 
surface. 
40—2 
