606 THE SENSES AND SENSORY ORGANS. 
small branches which are distributed to two groups of chordo- 
tonal organs which have not hitherto been described, or which 
have been confounded with Lee’s group of chordotonal organs. 
The Form and Structure of the Halteres vary within narrow 
limits in different families of the Diptera. They are some- 
times straight, as in Tipula; sometimes curved, as in Eristalis 
and Leptis. Sometimes the scape passes almost imperceptibly 
into the conical capitellum, as in Leptis and Tipula, or it may 
be sharply defined where it joins the subspherical head, as in 
Musca. 
In all those insects which I have examined, the halter ap- 
pears, however, to have a very similar structure. The scape 
usually consists of two distinct tubes. These appear to me to 
correspond with the marginal nervure, and the remigium of the 
wing respectively. The cavities of these tubes intercommuni- 
cate in the capitellum, but they are distinct in the scape of the 
halter. 
Canals.—The marginal nervure is in front and below, and the 
submarginal or remigial nervure is above and behind. I shall 
term the cavity of the former the anterior canal, and that of 
the latter the posterior canal, of the scape. The anterior and 
posterior canals of the scape communicate with distinct blood 
sinuses in the base of the halter. It is not easy to trace 
these; but the posterior canal is seen to be in close relation 
with a membrane (Pl. XLIII., Fig. 1, mb) which supports 
the special nerve end organs. In Eristalis (Pl. XLII., Fig. 3) 
it divides into two branches, one under each scala; and 
although I have been unable to demonstrate the same division 
in the Blow-fly, it appears probable that these canals present 
a similar arrangement in that insect. The anterior canal is 
more simple; it extends from a blood sinus in the scabellum 
to the anterior surface of the capitellum (Pl. XLII., Fig. 3). 
The Cavity of the Scabellum—There is a third cavity which 
contains the nerve and its ganglia, and which communicates 
with the cavities of the scale and cupola. It is continued into 
the scape in the septum, between the anterior and posterior 
canals, and contains a small tracheal vessel. This septum 
