THE HALTERES OF THE BLOW-FLY. 609 



two cavities of the capitellum are more simple in Eristalis, and 

 are shown in PI. XLII., 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 scalse 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 scalse 

 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 papillae of the scapal and basal plates each 

 contain an auditory rod (Horstift), and that the basal and 

 scapal papillas differ, inasmuch as the former are closed 

 externally and each of the latter opens by a narrow slit on the 

 surface. 



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