The morphology of mouthparts, wings and genitalia of Paleozoic insect families Protohymenidae and Scytohymenidae reveals new details and supposed function

Diversity, Form, and Postembryonic Development of Paleozoic Insects

While Mesozoic, Paleogene, and Neogene insect faunas greatly resemble the modern one, the Paleozoic fauna provides unique insights into key innovations in insect evolution, such as the origin of wings and modifications of postembryonic development including holometaboly. Deep-divergence estimates suggest that the majority of contemporary insect orders originated in the Late Paleozoic, but these estimates reflect divergences between stem groups of each lineage rather than the later appearance of the crown groups. The fossil record shows the initial radiations of the extant hyperdiverse clades during the Early Permian, as well as the specialized fauna present before the End Permian mass extinction. This review summarizes the recent discoveries related to the documented diversity of Paleozoic hexapods, as well as current knowledge about what has actually been verified from fossil evidence as it relates to postembryonic development and the morphology of different body parts.

Life history, systematics and flight ability of the Early Permian stem-mayflies in the genus Misthodotes Sellards, 1909 (Insecta, Ephemerida, Permoplectoptera)

Background

The stem-group of Ephemeroptera is phylogenetically important for understanding key steps in evolutionary history of early pterygote insects. However, these taxa have been mostly studied from the taxonomy point of view focused on the pattern of wing venation and often using only classical optical microscopy devices. In-depth studies on detailed morphology of the different body structures are scarcely performed, although the results are critical for elucidation of life history traits and their evolutionary pattern among the basal pterygotes.

Results

New information is presented on the morphology of two species of Misthodotes, which are stem-mayflies from the Early Permian. Based on new results obtained from a re-examination of the type specimens and supplementary material, we infer the life history traits of both the adult and larval stages of these Palaeozoic insects and reconsider previous interpretations. For the first time, we report the structure of the thoracic pleura and the articulation at the base of the wing in a stem-group of Ephemeroptera and compare them with those of extant mayflies. We also provide additional support for the systematic placement of investigated taxa and an amended diagnosis of the genus Misthodotes.

Conclusions

Adult Misthodotes sharovi and Misthodotes zalesskyi had chewing mouthparts, which enabled them to scavenge or feed on plants. The wing apparatus was adapted for slow powered flapping flight and gliding, using long caudal filaments for steering. The wing base does not have rows of articulary sclerites as previously hypothesized for some Palaeozoic taxa but inflexible axilla similar to that found in modern mayflies. The structure of the thoracic pleura is also similar to that in the crown group of Ephemeroptera, while differences in the course of sutures may be explained by an evolutionary trend towards more powerful dorsoventral flying musculature and forewing-based flight (anteromotorism) in modern taxa. There is no evidence for swarming behaviour and mating in the air as occurs in modern mayflies as they had none of the associated morphological adaptations. Putative larvae of Misthodotes can not be unambiguously associated with the adults. They also exhibit some morphological specializations of Protereismatidae like 9 pairs of abdominal tracheal gills supporting their benthic lifestyle with legs adapted to burrowing.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01820-x.

Male postabdomen reveals ancestral traits of Megasecoptera among winged insects

External male genitalia of insects are greatly diverse in form and frequently used in evolutionary context and taxonomy. Therefore, our proper recognition of homologous structures among various groups from Paleozoic and extant insect taxa is of crucial interest, allowing to understand the key steps in insect evolution. Here, we reveal structural details of two Late Carboniferous representatives of Megasecoptera (families Bardohymenidae and Brodiopteridae), such as the presence of separated coxal plates VIII and ventral expansions of coxal lobes IX. Together with the confirmed presence of abdominal styli in some other members of Palaeodictyopterida (Diaphanopterodea) this suggests that early pterygotes may have had traits more archaic than expected. Whether or not these traits point to a stem-group relationship of Palaeodictyopterida to all other Pterygota as suspected by earlier authors remains unclear at this stage. Furthermore, the present study provides an updated comparison of male postabdomen morphology among extant species of wingless Archaeognatha and representatives of early diverging groups of Pterygota from the Late Carboniferous and Early Permian, the Megasecoptera (Palaeodictyopterida), Permoplectoptera (Ephemeroptera) and Meganisoptera (Odonatoptera).

On the morphology of the Late Paleozoic insect families Bardohymenidae and Aspidothoracidae (Palaeodictyopterida: Megasecoptera)

Megasecoptera is a late Paleozoic order of herbivorous insects with rostrum-like mouthparts and slender homonomous outstretched wings. Our knowledge of their morphology is mainly based on wings while other body parts are scarcely documented. Here we focus on the families Bardohymenidae and Aspidothoracidae. A new well preserved specimen of Sylvohymen cf. sibiricus is described and illustrated, particularly the structures of the external male genitalia previously unknown for Bardohymenidae. Sylvohymen marginatussp. nov. is described from the early Permian of Tshekarda based on unique traits in the wing venation. The genera Paleohymen and Taigahymen are both removed from Bardohymenidae and the latter is transferred to Vorkutiidae. Alexahymen aestatis (Brauckmann, 1991) comb. nov. from Pennsylvanian at Piesberg is transferred from Aspidothoracidae to Bardohymenidae. Piesbergbrodiagen. nov. is designated for Piesbergbrodia tristrata (Brauckmann and Herd, 2003) comb. nov. as a member of Brodiidae and the first known record of this family from Piesberg quarry. The placement of Sylvohymen peckae in the Bardohymenidae is considered doubtful due to lack of significant characters in its venation. Furthermore, our study is focused on the form of the apical cell and the pattern of wing pigmentation. Peculiarities of the integumental outgrowths and external genitalia of representatives of Aspidothoracidae and Bardohymenidae, and other close relatives, are highlighted.

Genital morphology and copulatory behavior in triatomine bugs (Reduviidae: Triatominae)

Triatomines (Heteroptera: Reduviidae) include around 139 species, widely known as vectors of Chagas disease. Our aim is to review the existing knowledge of the genital morphology and sexual behavior and provide some functional analysis of these traits in triatomines. A complex set of traits comprise genitalia and these are highly variable among species. The components of the phallus and seminal products (secreted by action of testes and two accessory glands) interact to allow successful sperm transfer to the female spermathecae (usually a pair of blind tubes that emerge from the common oviduct). Seminal products may inhibit female physiology and extend mating duration. Mating behavior in triatomines is best characterized as scramble competition. We suggest that males may evaluate female condition prior to copulation, given that female fitness is largely affected by food (blood) source. Although rearing several triatomine species may be difficult and discourage from undertaking studies on this group, any further investigation on sexual behavior and mating interactions may provide data for applicative studies including Chagas disease vectors control.

The wing base of the palaeodictyopteran genus Dunbaria Tillyard: Where are we now?

The structure of insect wing articulation is considered as reliable source of high level characters for phylogenetic analyses. However, the correct identification of homologous structures among the main groups of Pterygota is a hotly debated issue. Therefore, the reconstruction of the wing bases in Paleozoic extinct relatives is of great interest, but at the same time it should be treated with extreme caution due to distortions caused by taphonomic effects. The present study is focused on the wing base in Dunbaria (Spilapteridae). The articulation in Dunbaria quinquefasciata is mainly formed by a prominent upright axillary plate while the humeral plate is markedly reduced. Due to unique preservation of surface relief of the axillary plate, its composition shows a detailed pattern of three fused axillary sclerites and presumable position of the sclerite 3Ax. The obtained structures were compared among Spilapteridae and to other palaeodictyopterans Ostrava nigra (Homoiopteridae) and Namuroningxia elegans (Namuroningxiidae). The comparative study uncovered two patterns of 3Ax in Dunbaria and Namuroningxia, which correspond to their different suprafamilial classification. In contrast to previous studies these new results reveal the homologous structural elements in the wing base between Paleozoic Palaeodictyoptera and their extant relatives of Ephemeroptera, Odonata and Neoptera.