6o6 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 (PI. XLIIL, Fig. i, 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-lly, it appears probable that these canals present 

 :i 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 (PI. 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 scalar and cupola. It is continued into 

 the scape in the septum, between the anterior and posterior 

 anals, and contains a small tracheal vessel. This septum 



