The aim of the study was to describe the morphological structure, including chaetotaxy, of the previously unknown early (L₁) and late (L_2-3) larval instars of Gyrophaena boleti and to present certain aspects of their behaviour, mainly associated with their means of collecting food. G. boleti is probably the only member of its genus that breeds inside tubes of the open hymenium of the polycarp Fomitopsis pinicola. It was established that certain morphological characters and aspects of the behaviour of the larvae are an expression of the species' adaptation to its preferred host. Thus such characters as body shape, some shortened structures on the head and thorax, the shape of the setae and the length of the urogomphi are the result of synchronic evolution of the larvae with its specific microhabitat, i.e. the narrow tubes of the hymenium. Morphological differences between early and late larval instars of G. boleti involve 17 characters, including 10 new ones that have not previously been noted in gyrophaenines. The level of activity of the tergal gland system varies depending on the age of the larvae, which is probably linked to differences in the structure of the setae on abdominal tergite VIII. The function of these setae was explained for the first time, and a hypothesis was put forth regarding a recognition-aggregation function of tergal gland secretion with respect to individuals within species. The morphological structure of the mature larvae of G. boleti was compared with that of G. nana, which is the only Gyrophaena larva that has been sufficiently well illustrated, and 16 traits differentiating these species were distinguished.