Of the DIPTERA or two-winged flies, many species occasionally attack man. Of these, a few are outstanding pests, many of them may also serve to disseminate disease, a phase of our subject which will be considered later. We shall now consider the most important of the group from the viewpoint of their direct attacks on man. Psychodidæ or Moth-Flies The PSYCHODIDÆ or Moth-flies, include a few species which attack man, and at least one species, _Phlebotomus papatasii_, is known to transmit the so-called "three-day fever" of man. Another species is supposed to be the vector of Peruvian verruga. The family is made up of small, sometimes very small, nematocerous Diptera, which are densely covered with hairs, giving them a moth-like appearance. The wings are relatively large, oval or lanceolate in shape, and when at rest are held in a sloping manner over the abdomen, or are held horizontally in such a way as to give the insect a triangular outline. Not only is the moth-like appearance characteristic, but the venation of the wings (fig. 163, d) is very peculiar and, according to Comstock, presents an extremely generalized form. All of the longitudinal veins separate near the base of the wing except veins R_2 and R_3 and veins M_1 and M_2. Cross veins are wanting in most cases. Comparatively little is known regarding the life-history and habits of the Psychodidæ, but one genus, _Phlebotomus_, contains minute, blood-sucking species, commonly known as sand-flies. The family is divided into two subfamilies, the PSYCHODINÆ and the PHLEBOTOMINÆ. The second of these, the PHLEBOTOMINÆ, is of interest to us. THE PHLEBOTOMINÆ--The Phlebotominæ differ from the Psychodinæ in that the radical sector branches well out into the wing rather than at the base of the wing. They are usually less hairy than the Psychodinæ. The ovipositor is hidden and less strongly chitinized. The species attacking man belong to the genus _Phlebotomus_, small forms with relatively large, hairy wings which are held upright, and with elongate proboscis. The mandibles and maxillæ are serrated and fitted for biting. According to Miss Summers (1913) there are twenty-nine known species of the genus _Phlebotomus_, five European, eleven Asiatic, seven African and six American. One species only, _Phlebotomus vexator_, has been reported for the United States. This was described by Coquillett, (1907), from species taken on Plummer's Island, Maryland. It measures only 1.5 mm. in length. As it is very probable that this species is much more widely distributed, and that other species of these minute flies will be found to occur in our fauna, we quote Coquillett's description.