Introduction Oudemans has described several species of mites of the genus Sennertia, most of them only after their heteromorphic deutonymphs (or hypopi). These species live as adults in the nests of Hymenoptera, especially bees, and are transported phoretically from nest to nest by means of their heteromorphic deutonymphs which attach themselves to the adult bees. Since the time that Oudemans published his material numerous new species of Sennertia have been described in several parts of the world. In order to recognize these species, generally known only after their hypopial stage, it has become necessary to utilize new morphological criteria. Therefore a new and more complete description of the old species is needed. The present paper is devoted to the redescription of the hypopi of Sennertia described by Oudemans and which are deposited in the Oudemans collection at the Rijksmuseum van Natuurlijke Historie at Leiden. Sennertia Oudemans, 1905 Sennertia koptorthosomae (Oudemans, 1901) (figs. 1-2) Trichotarsus koptorthosomae Oudemans, 1901b: 81, pl. 3 figs. 53-54 (in part). Sennertia koptorthosomae; Oudemans, 1924: 329. Dr. L. van der Hammen has sent us five slides of the species "Sennertia koptorthosomae". Three of these slides are labelled "Koptorthosoma tenuiscapa Westw., ♀, Java, Oct. 1900. J. D. Alfken". The catalogue numbers are nos 3, 4 and 5, respectively. One is labelled "Koptorthosoma tenuiscapa Peradeniya, Ceylon 02.02.1904. E. Ernest Green leg; catal. no 1". The fifth slide is labelled "Nest van Koptorthosoma ... Medan. Deli. 1919. Dr. W. Roepke leg. Cat. no. 2.11".