Embryonic development of Tenomerga mucida and Satonius kurosawai – First embryological information on Archostemata and Myxophaga (Coleoptera) does not support their sister group relationship

For the first time we provide embryological information for Archostemata and Myxophaga, two very small but phylogenetically crucial suborders of Coleoptera. For this purpose we used Tenomerga mucida of Cupedidae, the largest archostematan family, and Satonius kurosawai of Torridincolidae, the largest family of Myxophaga. The egg of T. mucida is cylindrical with a well-developed micropylar projection on the anterior pole of the egg. A long embryo forms on the ventral side of the egg, and with its progressive growth the segmentation rapidly proceeds, with an embryogenesis of the long-germ type. The elongated embryo then immerses into the egg, and taking a position in the egg along its anteroposterior axis, it continues to develop until blastokinesis occurs. Undergoing this process, the embryo appears on the egg surface, and with the progressive definitive dorsal closure, it acquires its definitive configuration. The first instar larva, which lacks an egg tooth, hatches, biting the chorion open with its robust mandibles. The egg of S. kurosawai is oval and lacks a micropylar projection. A small embryo forms on the ventral surface. It stretches along the egg surface, increasing its length, and successively forming the segments, with its caudal end passing the posterior end of the egg and anteriorly almost reaching near the anterior pole. In contrast to T. mucida , the embryogenesis is of the short-germ type. With the blastokinesis taking place and the definitive dorsal closure proceeding, the embryo acquires its definitive configuration. The first instar larva hatches by tearing the chorion longitudinally. Comparing the information obtained such as regarding the micropylar projections and germ types with those of other groups of beetles and Neuropterida, we discuss the phylogenetic relationships of coleopteran suborders. Our embryological study does not fit well with the sister group relationship between Archostemata and Myxophaga as suggested by recent large-scale molecular phylogenetic studies, i.e., requiring additional evolutionary steps and differing profoundly in the developmental pattern. In contrast, they are fully compatible with patterns suggested based on morphology, especially “Archostemata + (Adephaga + (Myxophaga + Polyphaga))”.