TAXONOMY ol' MUSCOIDEAN FLIES — TOWNSEND $J 



bifurcation, and therefore the true origin of the apical crossvein, is 

 apparent. In such cases the term should be used. It should not be 

 used in those other genera in which no point of bifurcation is indi- 

 cated, this having not arisen during their development. In such 

 cases the term "fourth vein" is correctly applicable to the whole, 

 whereas the term "apical crossvein" can not be so applied, especially 

 in Hyalomyia, Phorantha, Alophora, Beskia, Sciasma, Stomatodexia, 

 and many other genera. The latter class of genera represents a 

 lesser specialization than the class showing a stump or wrinkle, and 

 therefore is an older type, and indicates a more ancient assemblage 

 of forms. Following Williston, the last three sections of fourth vein, 

 when latter exhibits no furcation, but is more or less angularly bent, 

 may very appropriately be termed the antepenultimate, penultimate, 

 and ultimate. 



Halteres and tcgithc. — The sense of audition is acutely developed 

 in insects, at least in the majority of the forms, as evidenced by the 

 sounds they produce. There is nothing to indicate that the Mus- 

 coidea are in any way an exception. Air waves which produce no 

 effect whatever upon our ears doubtless register impressions upon 

 the auditory nerve-ends of Diptera. Auditory organs are located 

 near or at the base of the wings in Diptera, Coleoptera, Lepidoptera, 

 Neuroptera, Orthoptera, and Hemiptera. They are less perfect in 

 the Lepidoptera, Neuroptera, and Orthoptera, and only faintly repre- 

 sented in the Hemiptera, in which four orders other chordonotal 

 structures have succeeded and more or less supplanted them. In the 

 Culicidse, and perhaps in other of the nemocerous groups, the an- 

 tennal hairs are auditory. This has been established in the male 

 mosquito. 



In the Diptera the nerve supplied to the halter is next in size to the 

 optic nerve, the latter being the largest nerve in the body. At the 

 base of the halter is a number of vesicles arranged in four groups, 

 to each of which groups the nerve sends a branch. These vesicles 

 are perforated and contain a minute hair, and the vesicles of the 

 upper groups are protected by chitinous hoods. 



Sharp (Cambridge Nat. Hist., vi, p. 448) says of the halteres: 

 "They possess groups of papillae on their exterior surface, with a 

 chordotonal organ inside the base. Each halter is provided with 

 four muscles at the base, and can, like the wings, execute most rapid 

 vibrations. Seeing that they are the homologues of wings, it is 

 remarkable that in no Diptera are they replaced by wings, or by 

 structures intermediate between these two kinds of organs." This 

 is because they have taken on special functions. 



