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

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.

Evidence for wing development in the Late Palaeozoic Palaeodictyoptera revisited

The numerous fossil specimens described as consecutive series of different larval stages of two species, Tchirkovaea guttata and Paimbia fenestrata (Palaeodictyoptera: Tchirkovaeidae), were reinvestigated with emphasis on comparing the development and growth of their wings with that of the wings of a recent mayfly, Cloeon dipterum. This unique fossil material was for a long time considered as undisputed evidence for an unusual type of wing development in Palaeozoic insects. The original idea was that the larvae of Palaeodictyopterida had wings, which were articulated and fully movable in their early stages of postembryonic development and that these gradually enlarging wings changed their position from longitudinal to perpendicular to the body axis. Moreover, the development of wings was supposed to include two or more subimaginal instars, implying that the fully winged instars moulted several times during their postembryonic development. The results of the present study revealed that there is no evidence that this series of nymphal, subimaginal and imaginal wings provide support for the original idea of wing development in Palaeozoic insects. On the contrary, our results indicate, that the supposed palaeodictyopteran larval wings are in fact wing pads with a wing developing inside the cuticular sheath as in recent hemimetabolous insects. Moreover, this study newly reinterpreted the wing pad base of Parathesoneura carpenteri and confirmed the presence of nygma like structures on wings and wing pads of palaeodictyopteran Tchirkovaeidae.

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.

Ecomorphological diversification of the Late Palaeozoic Palaeodictyopterida reveals different larval strategies and amphibious lifestyle in adults

The Late Palaeozoic insect superorder Palaeodictyopterida exhibits a remarkable disparity of larval ecomorphotypes, enabling these animals to occupy diverse ecological niches. The widely accepted hypothesis presumed that their immature stages only occupied terrestrial habitats, although authors more than a century ago hypothesized they had specializations for amphibious or even aquatic life histories. Here, we show that different species had a disparity of semiaquatic or aquatic specializations in larvae and even the supposed retention of abdominal tracheal gills by some adults. While a majority of mature larvae in Palaeodictyoptera lack unambiguous lateral tracheal gills, some recently discovered early instars had terminal appendages with prominent lateral lamellae like in living damselflies, allowing support in locomotion along with respiratory function. These results demonstrate that some species of Palaeodictyopterida had aquatic or semiaquatic larvae during at least a brief period of their post-embryonic development. The retention of functional gills or gill sockets by adults indicates their amphibious lifestyle and habitats tightly connected with a water environment as is analogously known for some modern Ephemeroptera or Plecoptera. Our study refutes an entirely terrestrial lifestyle for all representatives of the early diverging pterygote group of Palaeodictyopterida, a greatly varied and diverse lineage which probably encompassed many different biologies and life histories.

Palaeodictyopterida

Jakub Prokop and Michael Engel introduce palaeodictyopterids, a very diverse extinct lineage of insects.