Since the description of Ptilophorus dufourii (Latreille, 1818), the unknown immature stages and biology of Ptilophorinae is one of the longest persisting gap in the research of the wedge-shaped beetles (Ripiphoridae). Here we describe the primary larva of P. dufourii based on SEM and CLSM images. Its modified mouthparts unsuitable to grasp prey rule out predaceous habits and also processing any solid substrates (e.g., wood or other fresh plant tissues). In contrast, the weakly sclerotized and very small body, the enlargement of the sensorial appendage of the antennae, and simplified mouthparts strongly suggest parasitism and consumption of liquid food. We provide a comparison with the only known larva of Pelecotominae, another possible basal lineage of Ripiphoridae, and discuss potential synapomorphies and differences between larvae of both subfamilies. A sistergroup relationship between Ptilophorinae and Pelecotominae is suggested by the presence of only two stemmata, spatulate femoral setae, a scale-like lateral cranial surface, and an enlarged antennal sensorial appendage, and a clade Ripidiini + Ripiphorinae by a characteristic boat-shape of the primary larvae. The host of P. dufourii remains unknown. We reviewed published host records, but none of them is based on traceable evidence and none of them appears credible. Observations of adult behaviour of P. dufourii are documented. Taxonomy and systematics of Ptilophorus and Ptilophorinae are discussed. The Cretaceous Spinotoma ruicheni Hsiao et Huang, 2017 is transferred from Pelecotominae to Ptilophorinae in accordance with diagnostic characters given in the original description and current definitions of both subfamilies.