Simple Summary Gyrostigma rhinocerontis (Hope), the rhinoceros bot fly, is a rare obligate intestinal parasite of white and black rhinoceroses that can cause severe myiasis and secondary infection, leading to enormous economic and scientific loss. As the main sensory organs of flies, the antennae provide insects with critical information about the environment, playing significant roles in their key activities. The antennal characteristics of G. rhinocerontis remain largely unexplored, probably due to the extreme rarity of adult specimens in collections. In this study, the antennae of G. rhinocerontis were thoroughly examined using light and scanning electron microscopy. The morphology, including detailed ultrastructure, of antennal sensilla are presented. As the largest species of Oestridae Leach, not surprisingly, G. rhinocerontis has significantly larger antennae with more sensilla and sensory pits than any other Oestridae species, which could be an adaptation to locate their rare and endangered hosts. Gyrostigma rhinocerontis (Diptera: Oestridae) is a rare obligate intestinal parasite of both white and black rhinoceroses, which can induce severe myiasis, cause secondary infection, and lead to enormous economic and scientific loss. Antennae are the main sensory organs of G. rhinocerontis, which may have evolved a series of specialized adaptive structures to facilitate the exploitation of their hosts. Here, we thoroughly examine the antennae of G. rhinocerontis via light and scanning electron microscopy. Only microtrichia and chaetic sensilla were observed on the scape and pedicel, and the latter is enlarged, half-enveloping the postpedicel. Four types of sensilla (trichoid sensilla, basiconic sensilla, coeloconic sensilla, and clavate sensilla) and sensory pits are detected on the postpedicel. A set of coeloconic sensilla and a chaetic sensillum are located on the arista. Distribution, type, size, and ultrastructure of antennal sensilla are presented. The antennae of G. rhinocerontis are the largest among Oestridae species, with the most sensilla and the most sensory pits. These antennal characteristics could be correlated to their adaptation for more sensitive and accurate olfactory organs, used to locate their rare and endangered hosts. Accordingly, this morphological evidence supports that the host is an important driving factor in the diversity of antennal morphology in the bot flies.
