Search

Xenos vesparum of the highly specialized Strepsiptera is a new insect model in the context of host-parasite relationships. The endoparasitic female and male secondary larvae were studied using µCT, 3D-reconstructions, histology, and photomicrography. The infectious primary larva is followed by a trophic and endoparasitic secondary larval stage. In contrast to immature stages of other holometabolous groups, the second instar increases dramatically in size. Compound eyes and external wing anlagen are present in male larvae before the pupal stage. In contrast to the females, the brain of males bears well-developed optic neuropils and retinula cells are present. The cephalothorax is comparatively simple in the female larvae, yet distinctly more complex than in the adult, where most muscles are reduced. Large testes are present in male larvae and numerous oocytes in the females, but they are still immature. The larval features are discussed in the context of holome­tabolous development and heterochronic processes. Unique features of Strepsiptera are the early differentiation of the sexes and the occurrence of compound eyes and external wing anlagen in male secondary larvae. The phylogenetic position of Strepsiptera suggests that this is a secondary feature and thus an autapomorphy. To address mature females of Stylopidia as neotenic adults is an oversimplification. They display a mosaic pattern of paedomorphic characters such as features of the nervous system and the presence of stemmata, but also non-paedomorphic structures, such as the spiracles.

We document external and internal thoracic structures of the free-living pselaphine beetle Pselaphus heisei (Pselaphitae) using a set of traditional and modern techniques. Like in the specialized myrmecophile Claviger testaceus (Clavigeritae), the skeletal elements of the pro- and pterothorax are highly compact, with largely reduced inter- and intrasegmental sutures. Features previously listed as synapomorphies of staphylinid subgroups, to which Pselaphinae belong, are confirmed for P. heisei. The only previously proposed thoracic synapomorphy of Pselaphinae, the mesoventral foveae, is likely transformed in P. heisei: we assume that the concavities are directed mesad and internally fused, thus forming a broad channel ("perforation") extending through the keel-like median region of the mesoventrite. The prothoracic foveal system is strongly reduced, with only one pair of pits present in front of the procoxae. Their internal invaginations form a transverse ventral endoskeletal bar that stabilizes the prothorax. The condition observed in the free-living P. heisei is in contrast with previous hypotheses linking the reduction of the foveal system with myrmecophily. Moreover, traces of the mesoventral foveae are even preserved in the highly specialized inquiline C. testaceus. Gland cells are associated with areas of hyaline squamose setae on different body regions, suggesting release of secretions on the ventral side of the head, pro- and mesothorax, and abdominal base. Similar specialized setal patches are common in Pselaphini and related groups within Pselaphitae. The prothoracic musculature in P. heisei is more complex than that in the myrmecophilous C. testaceus and the free-living, unspecialized Creophilus maxillosus (Staphylininae). The metathoracic muscle system is strongly simplified, demonstrating that P. heisei cannot fly, even though wings, some skeletal elements of the flight apparatus, and some small direct flight muscles are preserved. It cannot be fully excluded that indirect flight muscles and thus a functional flight apparatus is preserved in a certain percentage of individuals.

The fossil record allows a unique glimpse into the evolutionary history of organisms living on Earth today. We discovered a specimen of the stem group ant †Gerontoformica gracilis (Barden and Grimaldi, 2014) in Kachin amber with near-complete preservation of internal head structures, which we document employing µ-computed-tomography-based 3D reconstructions. We compare †Gerontoformica to four outgroup taxa and four extant ant species, employing parsimony and Bayesian ancestral state reconstruction to identify morphological differences and similarities between stem and crown ants and thus improve our understanding of ant evolution through the lens of head anatomy. Of 149 morphological characters, 87 are new in this study, and almost all applicable to the fossil. †Gerontoformica gracilis shares shortened dorsal tentorial arms, basally angled pedicels, and the pharyngeal gland as apomorphies with other total clade Formicidae. Retained plesiomorphies include mandible shape and features of the prepharynx. Implications of the reconstructed transitions especially for the ant groundplan are critically discussed based on our restricted taxon sampling, emphasizing the crucial information derived from internal anatomy which is applied to deep time for the first time. Based on the falcate mandible in †Gerontoformica and other Aculeata, we present hypotheses for how the shovel-shaped mandibles in crown Formicidae could have evolved. Our results support the notion of †Gerontoformica as ‘generalized’ above-ground predator missing crucial novelties of crown ants which may have helped the latter survive the end-Cretaceous extinction. Our study is an important step for anatomical research on Cretaceous insects and a glimpse into the early evolution of ant heads.

The mesosoma is the power core of the ant, containing critical structural and muscular elements for the movement of the head, legs, and metasoma. It has been hypothesized that adaptation to ground locomotion and the loss of flight led to the substantial rearrangements in the mesosoma in worker ants and that it is likely the ant mesosoma has undergone functional modifications as ants diversified into different ecological and behavioral niches. Despite this importance, studies on the anatomy of the ant mesosoma are still scarce, and there is limited understanding of important variation of internal structures across the ant phylogeny. Recent advances in imaging techniques have made it possible to digitally dissect small insects, to document the anatomy efficiently and in detail, and to visualize these data in 3D. Here we document the mesosomal skeletomuscular system of workers of the red wood ant, Formica rufa Linnaeus, 1761, and use it to establish a 3D atlas of mesosomal anatomy that will serve as reference work for further studies. We discuss and illustrate the configuration of the skeletomuscular components and the function of the muscles in interaction with the skeletal elements. This anatomical evaluation of a ‘generalized’ ant provides a template for future studies of the mesosoma across the radiation of Formicidae, with the ultimate objective of synthesizing structural, functional, and transformational information to understand the evolution of a crucial body region of ants.

Sperm of the dictyopteran key taxon Cryptocercus punctulatus was examined. It has largely maintained a blattodean groundplan condition, with a three-layered acrosome, an elongate nucleus, a single centriole, a conspicuous centriole adjunct material, two connecting bands (=accessory bodies), and a long functional flagellum with a 9+9+2 axoneme provided with accessory tubules with 16 protofilaments and intertubular material. These sperm characters are shared with several other polyneopterans. The sperm of C. punctulatus is very similar to what is found in Periplaneta americana and species of other groups of roaches, including the sperm of Loboptera decipiens described here for the first time. The general sperm organization here described can be assumed for the groundplan of Insecta and Pterygota. The following evolutionary path can be suggested: after the split between Cryptocercidae and the common ancestor of Isoptera, the typical pattern of sperm formation was altered very distinctly, resulting in a duplication or multiplication (Mastotermitidae) of the centrioles. Mastotermes has maintained a certain sperm motility, but with a very unusual apparatus of multiple flagella with a 9+0 axoneme pattern. After the split into Mastotermitidae and the remaining Isoptera, sperm motility was completely abandoned, and different modifications of sperm components occurred, and even the loss of the sperm flagellum.