The morphological ontogeny of Adoristes ovatus (C.L. Koch, 1839) is described and illustrated. The adult has the interlamellar seta shorter than the lamella and the translamella is usually absent, but can also be incomplete, or present as a thin line. The juveniles are unpigmented, oval in cross-section, with thin and smooth prodorsal and gastronotal setae, and with the anal opening in the posteroventral position, which is typical of xylophages. The legs and claws of juveniles are relatively thick, especially leg I, and the leg setae are smooth or with short barbs. All juveniles have a sclerotized semicircle located anterior to each prodorsal seta le. The larva has 11 pairs of gastronotal setae, the nymphs have 12 pairs, without the d-series. The mean body length of females is larger than males, but varies greatly among samples (445–735 µm) and the largest females can be 1.5 times longer than the smallest males. The sex ratio and the number of gravid females also vary among samples. We provisionally consider Adoristes (Gordeeviella) Shtanchaeva, Subías & Arillo, 2010 a junior synonym of Adoristes Hull, 1916.