The male postabdomen and genital apparatus of †Mengea tertiaria, a strepsipteran amber fossil (Insecta)

The male genital apparatus of a fossil insect including internal soft parts is described in detail for the first time. The conditions found in an approximately 42-My-old specimen of dagger Mengea tertiaria embedded in Baltic amber are compared to what is found in other extinct and extant strepsipterans, notably members of the basal 'Mengenillidae' (probably paraphyletic). The postabdomen of dagger Mengea is very similar to what is present in other fossil and extant members of Strepsiptera. Only few structural features vary within the group, but it differs strongly from the apparatus in other holometabolan lineages. The slender, exposed tergite IX, the complete absence of parameres, a sperm pump formed by a strongly developed muscularis (layers of mainly circular muscle fibres) around the proximal part of the ejaculatory duct and the presence of four specific muscles are potential autapomorphies of the Strepsiptera. A presumptive strepsipteran ground plan feature found in dagger Mengea is the nearly straight, simple penis, which is also present in dagger Protoxenos, dagger Cretostylops, Bahiaxenos, Mengenilla, Eoxenos and Congoxenos. This strongly suggests that males of dagger Mengea (and other stem group strepsipterans) copulated in a very similar way as males of extant members of the group with free-living females (e. g. Mengenilla). In contrast, the penis of stylopidian males, which copulate with females parasitizing in pterygote hosts, is hook shaped. A sister group relationship between dagger Mengea and Strepsiptera s.s. (extant groups) is supported by a distinctly weaker sclerotization of the abdominal tergites, compared to the corresponding sternites. The study of other stem group strepsipterans using mu-computer tomography should have high priority. This technique has a great potential to facilitate morphological reconstruction and phylogenetic placement of amber fossils.