The exoskeleton of the male genitalic region in Archaeognatha, with hypotheses on the early evolution and the morphological interpretation of genitalia in insects

The ventral exoskeleton of abdominal segments 7 – 9, including the phallic organs (belonging to segment 9 or 10), is described for five archaeognathan species: Machilis hrabei and Lepismachilis notata (Machilidae-Machilinae), Pedetontus unimaculatus (Machilidae-Petrobiinae), Petrobiellus takunagae (Machilidae-Petrobiellinae), and Machilinus sp. (Meinertellidae). In the focus are the segmental patterns of sclerites and formative elements, and fine structures of the cuticular surface. The results are compared with earlier descriptions of these body parts in Archaeognatha. Hypotheses of homonomy (transsegmental and male-female) and homology at the level of Ectognatha (= Insecta) are proposed and insect-wide terminologies applied. Morphological interpretations are revised, if required, with a focus on the segmental assignment and other aspects of the male genital opening and phallic organs. A data matrix of 39 male genitalic characters is composed as a source of information for subsequent phylogenetic and taxonomic work on Archaeognatha. Some discussions on character evolution are given; few apomorphies agree with previous molecular results of a clade Petrobiellinae + Meinertellidae, but phylogenetic conclusions remain limited due to poor data for outgroup comparison (mainly for Zygentoma). We compare and discuss the occurrence of genitalic specialities (= structural differences compared to pregenital segments) on segments 7 – 9 in both sexes. The new data shows that male Archaeognatha exhibit many genitalic specialities on segment 9 and few on segment 8, whereas females show many on segments 9 and 8 and on the posterior part of segment 7; the male specialities are largely a subset of the female ones, except for structures categorised as phallic in the male being largely absent in the female (with possible exceptions). Based mainly on the genitalic specialities common to both sexes, we discuss two discrete scenarios for the early sex-shared evolution of the genitalic region in stem-Insecta: (1) The ‘aquaeductal hypothesis’ proposes that water-uptake from crevices was the initial driving force of structural specialities that today mainly serve for genitalic functions. (2) The ‘sensorial hypothesis’ proposes that improving the sensorial exploration of the substrate was the driving force.