Most Labidostomatidae bear lateral protuberances called pustules on their idiosoma, which have been considered as "enigmatic" structures. Here, the fine structure of the pustules of Labidostoma luteum is described using scanning and transmission electron microscopy. It is shown that the pustules represent peculiar exocrine glands which extrude their secretions through a specialized cuticular region which is shaped like the hat of a mushroom. This region includes a space located between the epicuticle and the procuticle and is regarded as a reservoir from which secretions likely are evacuated passing through the inner epicuticle. The inner epicuticle is incompletely penetrated by numerous tubular indentations. The peculiar structure of the pustules is shown to be a specialized region of the complex cuticle of the mite being provided with pore canals which terminate under cuticular indentations and a peculiar tunnel system. The latter is organized in cuticular ribs by spaces between inner epicuticle and procuticle. The function of the pustules and the tunnel system remains unknown, but inter- and/or intraspecific communication or a defensive role seem likely.