Mideopsis roztoczensis Biesiadka & Kowalik, 1979 (Acari, Hydrachnidia, Mideopsidae) is a common and often abundant component of macroinvertebrate communities in Palearctic streams. Although the external morphology and genetic structure of this species are relatively well documented, knowledge of its internal anatomy remains limited, despite its importance for an integrative understanding of morphology and evolutionary diversity. The present study utilized histological techniques to investigate and describe the internal structures of M. roztoczensis based on newly collected material from Serbia. Histological analyses revealed the organization of several internal organs, including an extensive blind sac-like excretory organ that was morphologically distinct from the midgut, while a hindgut was not identified. Additionally, other structures, including dermal glands, the midgut, salivary glands, and the synganglion, were characterized. The observed variation in internal organization, particularly the morphology of the excretory organ and the number of paired dermal glands, provides additional characteristics in comparative morphological studies of water mite. This study contributes to our current knowledge of the internal anatomy of Hydrachnidia and highlights the value of histological approaches for future integrative studies of water mites.
