Mouthparts of 10 species representing Ceratopogon, Brachypogon, Echinohelea, Alluaudomyia, Stilobezzia, Monohelea, and Downeshelea of the tribe Ceratopogonini were examined by light and scanning electron microscopy. Greatest differences among species occurred in the sclerotized mandibles of females, where the mean number of coarse medial teeth ranged from 6.9 to 14.6, small basal teeth from 0.0 to 2.8, lateral teeth from 0.0 to 6.2, and length of the tooth row in relation to length of the mandible from 16 to 25%. Mandible structure is species specific. Mandibles have an interlocking mechanism midway along the blade that functions as a fulcrum; abduction of their bases causes divergence and retraction of their tips and results in a sawing action by which an incision is opened in the prey. Mandibles of males are reduced in size and are either smooth or have a few spicules at their tip. The labrum, hypopharynx, and lacinia are unarmored and play no part in opening the incision in their prey but are inserted into the wound made by the mandibles. The mandibles remain between the labrum and the hypopharynx during feeding and serve as the floor of the food canal and roof of the salivary canal. The laciniae form the lateral proximal boundaries of the food canal. Cylindrical structures present at the tip of the labrum of females of all species except Echinohelea lanei Wirth are thought to be mechanoreceptors or chemoreceptors or both; they are present, but smaller, in males. Spicules on the lateral edges of the labrum are much longer in males than in females. Spicules are present on the hypopharynx of females of all genera except Alluaudomyia and of males of the four genera ( Ceratopogon, Brachypogon, Echinohelea, and Downeshelea ) examined. Hypopharyngeal spicules are much longer in males than in females of the same species. Capitate sensilla on the distal end of the third segment of the maxillary palpus arise directly from either the unindented mesal surface of the palpus or in a pit of varying depth. Differences in shape of the terminal bulbous portion (head) and length of the stalk of the sensilla make these structures species definitive. In Downeshelea stonei (Wirth), the heads are located below the rim of the pit, but in the other nine species examined, the sensilla have long stalks so that the heads extend well beyond the pit opening. Males and females have approximately equal numbers of sensilla, suggesting that the sensilla have a reduced role in location of prey by the female. On the basis of number of mandibular teeth, length of the tooth row in relation to length of the mandible, and terminal modification of the labrum, Stilobezzia is considered to be the most specialized and Alluaudomyia the least specialized of the seven genera examined.