Simple SummaryFemale frog-biting midges exclusively feed on blood from frogs. They are attracted by the calling of male frogs and search for specific feeding sites on their host’s body. To feed, these blood-feeding midges use a very short proboscis that is composed of six piercing structures and an enclosing component. We analyzed the morphology of the mouthparts using SEM and compared these with the well-studied proboscises of other blood-feeding flies. Females ofCorethrellashare more similarities to the very small blood-feeding short-proboscid biting midges, black flies and sand flies than to their more closely related long-proboscid mosquitoes. We interpret our findings in the functional context of a very short piercing proboscis and its possible specialization to pierce frogs.AbstractFemales of frog-biting midges (Corethrellidae) obtain their blood meals from male calling frogs. While the morphology of the feeding apparatus is well studied in hematophagous Diptera that impact humans, frog-biting midges have received far less attention. We provide a detailed micromorphological examination of the piercing blood-sucking proboscis and maxillary palpus in threeCorethrellaspecies using scanning electron microscopy and histological semi-thin sectioning. We also compare the sensilla found on the proboscis tip and the palpus ofCorethrellawith other piercing blood-sucking Diptera.Corethrellaspp. have a proboscis length of about 135 µm, equipped with delicate mandibular piercing structures composing the food canal together with the labrum and hypopharynx. Their proboscis composition is plesiomorphic and more similar to other short-proboscid hematophagous Culicomorpha (e.g., Simuliidae), in contrast to the phylogenetically more closely related long-proboscid Culicidae. As in other short-proboscid taxa, the salivary canal inCorethrellaspp. transitions into an open salivary groove with one mandible forming a seal, whereas in Culicidae the salivary canal is closed until the tip of the proboscis. We discuss the possible functional constraints of very short, piercing blood-sucking proboscises (e.g., dimensions of host blood cells) that may limit the size of the food canal.