Abstract: This paper describes the morphological ultrastructure of the previously unknown final larval instars of the obligate myrmecophilous rove-beetle species Lomechusa pubicollis. Diagnostic larval characters for the genus Lomechusa are proposed for the first time. At present, this paper is the only up-to-date, detailed and richly illustrated description of the external structures of larvae representing the Lomechusina subtribe. The features of L. pubicollis larvae described here correspond with the abiotic (e.g. absence of ocelli, white body) or biotic conditions (e.g. stumpy body, short legs, absence of urogomphi, dense and asymmetrical chaetotaxy, membranous cuticle) of the anthill, including the passive lifestyle resembling that of the host larvae Formica truncorum, in conjunction with a unique behaviour pattern in the myrmecophileant relationship. Morphological similarities between mature larvae of L. pubicollis and workers of Formica truncorum, the host of this myrmecophile, are also listed. The morphological structure of L. pubicollis larva with the so far best-known larvae of another myrmecophile from the Lomechusini tribe Pella laticollis living in the peripheral zones around the nest, was compared. The different lifestyles of the two species is reflected in the morphological structure of their larvae, particularly: chaetotaxy, structure of setae, ocelli, structure of mouthparts, legs and urogomphi. The characteristics of the morphological structure peculiar to L. pubicollis larva in conjunction with well-developed chemical mimicry, they enable complete adoption and integration in the ant nest, which at such an advanced level appears to be unique compared to other staphylinid myrmecophiles.