In the first place the mouthparts of the two sexes are different—the male lacks the mandibles which are present in the female. This makes it necessary for them to procure their food from different sources, the male obtains his from flowers, while the female lives by puncturing the skin and sucking the blood of warm-blooded vertebrates. Thus it is evident that during the time spent in procuring food the sexes cannot remain together. From an economic standpoint the female most concerns the student and she is often taken for study without an attempt being made to procure the male. At this point I can say collect females around horses, cattle and other animals, and males on flowers; but this is not enough, for knowing the general habits of insects we are certain that there is a common ground where the two sexes may be found together. One finds this common ground in the vicinity of water, where their transformations take place and where their eggs are laid, also in various other places, which we shall take occasion to discuss as we proceed. The females of all our local species of Chrysops with Tabanus pumilus and nivosus come buzzing around the collector in numbers, and at such times may be taken easily with a net. Other species of Tabanus come near enough that the sound of their wings is recognizable, but are so active that it is almost impossible to procure them. During the time the female is ovipositing the male is often sitting near by on the foliage. At Georgesville, Ohio, June 4th, I observed C. mœchus ovipositing on foliage overhanging a mill-race; soon after specimens of the male sex were observed resting on the upper leaves of the same plant on which females were ovipositing. In a few minutes collecting, a dozen or more specimens of each of the sexes were procured. The only males of C. indus I have ever taken were procured at Columbus, on the border of a small pond, where the females were ovipositing. The sexes of many species of Tabanus often alight on the bare ground of paths or roads that run through or along woods. At Cincinnati, June 10th, in company with Mr. Dury, we procured large numbers of the sexes of different species resting on some furrows that were plowed around a woods to prevent the spread of fire. We also took the same species resting in paths and roads that ran through the woods. Some of these same species were also taken from low-growing foliage in sunny places among the trees. At Medina, Ohio, males and females of T. vivax and trimaculatus were taken while resting in a road that ran through a dense woods.