ok The few diptera whose mouth-parts have been the object of the anatom- ical studies, the results of which are noted in the following pages, were chosen, on the one hand, with especial reference to their presenting a series beginning with a species possessing simple, separate and fully developed mouth- parts, and ending with a species of which the complexity of the mouth-parts was due to coalescence and incomplete development of their different elements, and, on the other hand, with partial reference to forms whose mouth-parts were of sufficient length to render their study by sections, made with the microtome, of value in determining their relative lengths, their positions and their attach- ments. With the above-mentioned objects in view species of the genera Cuk.i; Bombylius, Installs, and Muxca were chosen. Upon the anatomy of the mouth- parts of Culcx the following notes probably add most to what has been previously recorded. No study of the development of the mouth-parts has been made in preparing the following paper; the results are anatomical only. The work necessary for the preparation of this paper was done by me, as student, in the Laboratory of the Zoological Institute in Leipzig, and I gladly take this opportunity of expressing my sincerest thanks to its director, my honored instructor, Professor Leuckart, for the advice and encouragement which he has given me in my studies. HISTORICAL. Until FABRICIUS, in J7<5, first called attention to the importance of the mouth-parts in the classification of insects, little or no valuable progress had been made in the study of their comparative anatomy. Observations on the anatomy of the mouth-parts of single insects are to be found in the works of many writers, previous to the above-mentioned date, but they can be best regarded as forming only a part of the history of the anatomy of single insects, and, so far as they treat of insects which are further discussed in this paper, they will be mentioned later. 1