Male genitalia of Charimachilis (Insecta: Archaeognatha) and the status of archaeognathan “paleoforms”

Abstract

The paper continues the exploration of the morphological and functional diversity of male genitalia in Archaeognatha by the study of an undescribed Charimachilis species. Exoskeletal details are documented by drawings and SEM micrographs. The lack of tubular setae on gonapophyses and penis suggests sperm transfer by direct male-female contact (not via sperm deposited on a thread), while genitalic morphology is not suggestive of the male clasping the female. For a broader evolutionary interpretation of archaeognathan male genitalic characters and mating behaviour, we revisited the issue of the “paleoforms”: Charimachilis, Ditrigoniophthalmus, Mesomachilis, and Turquimachilis had been categorized as such, i.e. as the putatively basalmost offshoots of crown-group Archaeognatha (Machiloidea = Meinertellidae + potentially non-monophyletic Machilidae). A recent cladistic analysis, however, had found all four genera subordinate in a monophyletic Machilidae. After revision of characters and addition of zygentoman outgroup taxa, we re-analysed this dataset, finding that except for the monophyly of Meinertellidae the dataset provides no convincing resolution of machiloid relationships. The status of the paleoforms is thus unclear with currently available data. Yet, based on comparison with zygentomans, mating with use of a sperm-thread appears plesiomorphic for Archaeognatha. We also briefly discuss whether the distribution of elements of the vestiture (e.g. scales, setae, and non-articulated trichoid sensilla) can support our previously published tentative hypothesis of the archaeognathan (and insect) penis being formed by sternal, coxal, and gonapophyseal elements of abdominal segment 10.