Immediately behind the wings of flies two curious knobbed organs are to be observed; these are considered to be rudimentary hinder wings by entomologists, and are called the halteres. Bolles Lee and others of the French scientists call them _balanciers_. This latter name I consider the correct one, for these organs unquestionably preside over alate equilibrium: they are true balancers. I do not propose to enter into any discussion as to whether these organs are rudimentary wings or not; suffice it to say that they appear to me to be organs fully developed and amply sufficient to serve the purposes for which they were created. Whether or not in the process of evolution there has occurred a change of function, is a point which will not be discussed in this paper. As they now exist, I deem them to be auditory organs of Diptera (flies, gnats, etc.). The semicircular canals are, to a great extent if not entirely, the seat of equilibration in man. Any derangement or disease of these canals interferes with equilibration; this is well shown in Ménière's disease, in which there is always marked disturbance of the equilibrating function. If the balancers of a horsefly be removed, the insect at once loses its equilibrium; it cannot direct its flight, but plunges headlong to the ground. The same can be said of _Chrysops niger_--in fact, of the entire family of Tabanidæ, of the gall gnat (_Diplosis resinicola_, Comstock), and of the March flies (_Bibionidæ_). These widely differing flies constitute the material from which I have derived my data; I will venture to assert, however, without fear of contradiction, that what has been said about the flies mentioned above is equally true of all flies. When the knobbed end of the balancers of the horsefly (_Tabanus atratus_, Comstock)[19] are examined with the microscope, the cuticle will be found to be set with minute hairs or setæ; some of these hairs penetrate both cuticle and hypoderm, are hollow, and receive into their hollows delicate nerve-fibrils. These nerve-fibrils pass inward toward the centre, and enter ganglia, which in turn are in immediate connection with the great nerves of the balancers. There is but one nerve in the insect's body that is larger than the balancer nerve, and that is the optic nerve; hence, it is natural to infer that the balancer nerve leads to some special sense centre. This centre in my opinion is, unquestionably, the seat of the auditory function. [19] Consult Comstock, _loc. cit. ante_, p. 455.