LAUSR

Simple Summary The family Leiodidae include beetles adaptated to many various habitats and foods ranging from saprophagous by obligatory mycophagists to true ectoparasites. The phylogeny of the family is complex and still requires research. Larval stages have different characters to adults and provide a lot of phylogentically valuable data. However, there are still gaps in our knowledge about immatures of the family, amongst others, in the tribe Leiodini, which is connected with endogean fungi. We provide the first detailed study of morphology of larval stages of Leiodes cinnamomea, which is an obligatory feeder and a pest of the economically valuable truffle. Our study found that larvae had unique morphological characteristics. This uniqueness, with larvae of all stages having a specific arching behaviour, is probably more common in this genus and is connected with endogean mycophagy and with other morphological characteristics. We present our hypothesis, based on our observation of the behaviour and morphology, that larvae of L. cinnamomea use a mosaic strategy for beetles living in ephemeral and in solid fungi. The study also showed that many other morphological features, such as chaetotaxy and length ratios of other parts of the body, confirmed the phylogenetic relationship that puts Leiodini close to Scotocryptini. Abstract Detailed redescription of morphology for three larval instars of truffle-feeder Leiodes cinnamomea, documented on scanning micrographs and illustrations, is provided. Systematic context, observations on biology and unique characteristics of larvae of the only well-described representative of Leiodini are discussed. Exoskeletal invaginations (pseudomycangia), granulation on the head and the median longitudinal row of sclerotized plates on tergites VII–IX—the unique structures among leiodid larvae—were confirmed, described and documented. A mosaic strategy for beetles living in ephemeral and in solid fungi is discussed.