1 Paleontological Journal, 2022, 56(3): 338–348. Russian version: Paleontologicheskii Zhurnal, 2022(3): 105–116. REVIEWS AND DISCUSSIONS Don't draw a sword against a louse (Chinese proverb) Don’t mind a louse in a pie: a good cook can bake two (Russian proverb – Vladimir Dal) Crawlers of the scale insect Mesophthirus (Homoptera: Xylococcidae) on feathers in Burmese amber – wind transport or phoresy on dinosaurs? D. E. Shcherbakov* Borissiak Paleontological Institute, Russian Academy of Sciences, Moscow, 117647 Russia *e-mail: dshh@narod.ru Received October 21, 2021; revised December 15, 2021; accepted December 15, 2021 Abstract—Mesophthirus engeli (Mesophthiridae incerti ordinis), described as a feather-feeding parasite of dinosaurs, has recently been reinterpreted as the early instar nymph (crawler) of a primitive scale insect. Mesophthirus has no specific similarities to bird lice, although life on feathers should modify parasites in a similar way. Based on re-examination of photographs of the type specimens of M. engeli, the subfamily Mesophthirinae stat. nov. is assigned to the archaic extant family Xylococcidae s.l. (recorded since Hauterivian), next to the extant subfamilies Xylococcinae and Stigmacoccinae. Xylococcid honeydew is an important food resource for birds and other arboreal vertebrates. Like their modern relatives, Mesophthirinae lived under the bark of trees and produced copious honeydew. Early birds and their precursors, small feathered dinosaurs, may well have fed on honeydew and the mesophthirines themselves. Scale crawlers are adapted to the wind transport and phoresy on insects and vertebrates, so finding Mesophthirus on feathers is natural. Keywords: Sternorrhyncha, Coccomorpha, crawlers, piercing-sucking mouthparts, stylets, chaetotaxy, phytophagy, Psocoptera, Mallophaga, booklice, chewing lice, parasitism, birds, trees, honeydew DOI: 10.1134/S0031030122030121 Gao et al. (2019) described Mesophthirus engeli Gao et al. (Mesophthiridae) as feather-feeding parasite of dinosaurs, similar to booklice and chewing lice, but of uncertain ordinal position. Grimaldi and Vea (2021) demonstrated that Mesophthirus is the early instar nymph (termed crawler, or just larva) of a primitive scale insect. Let us consider the arguments in favor of the first and second interpretations. We compare Mesophthirus, on the one hand, with scale crawlers, and, on the other hand, with chewing lice, descendants of the booklice Liposcelididae, which have passed to parasitism on the feathers and hair of tetrapods. Parasitism on the wing membrane of bats and pterosaurs causes parallel changes in unrelated groups (Ponomarenko, 1976; Shcherbakov, 2017). Living on feathers is a no less specific kind of parasitism, which should also lead to similar transformations in parasites. This means that if Mesophthirus was feather-feeding, then it should have been at least somewhat similar to chewing lice, even if it was not their direct relative.