The larval head morphology of Xyela sp. (Xyelidae, Hymenoptera) and its phylogenetic implications

Comparative morphology and evolution of the cranial musculature in bees (Hymenoptera: Apoidea)

Comparative morphological studies in bees are mostly restricted to the skeleton, and the musculature of bees has not been explored much from this perspective. Here we investigate the head extrinsic musculature under an evolutionary perspective. The musculature of 34 bee species belonging to six major lineages and 26 tribes plus two apoid wasps is described, illustrated, and compared. A standardized terminology for the extrinsic musculature is proposed and aligned with the Hymenoptera Anatomy Ontology (HAO). A total of 12 characters derived from the analysis were optimized onto a summary phylogenetic tree. The musculature was found to be conserved among the main bee lineages, and most variation was found in the proboscis; no modification was found in antennal muscles. Four characters are interpreted as synapomorphies for bees, one for the long-tongued bees, three for Halictinae, and one for Megachilinae. We also found that the Apinae clade of cleptoparasites is supported by some character state transformations and that stingless bees have a unique posterior fronto-pharyngeal muscle. Both sexes display similar morphology, except for having two fixed attachment points for the anterior cranio-mandibular muscle in the males of Andreninae. This study provides a foundation for future investigations on bee head musculature on various taxonomic levels.

The cephalic anatomy of workers of the ant species Wasmannia affinis (Formicidae, Hymenoptera, Insecta) and its evolutionary implications

Despite the ecological significance of ants and the intensive research attention they have received, thorough treatments of the anatomy and functional morphology are still scarce. In this study we document the head morphology of workers of the myrmicine Wasmannia affinis with optical microscopy, μ-computed tomography, scanning electron microscopy, and 3D reconstruction, providing the first complete anatomical treatment of an ant head with a broad array of modern techniques. We discuss the potential of the applied methods to generate detailed and well-documented morphological data sets with increased efficiency. We also address homology problems, particularly in the context of the cephalic digestive tract. According to our analyses the "pharynx" of previous ant studies is homologous to the prepharynx of other insects. We also discuss the phylogenetic potential and functional significance of the observed characters, with internal features such as tentorium and musculature discussed for the first time. Our investigation underlines that detailed anatomical data for Formicidae are still very fragmentary, which in turn limits our understanding of the major design elements underlying the ant bauplan. We attempt to provide a template for further anatomical studies, which will help to understand the evolution of this fascinating group on the phenotypic level.

The evolution of head structures in lower Diptera

Abstract The head of adult dipterans is mainly characterized by modifications and more or less far-reaching reductions of the mouthparts (e.g., mandibles and maxillae), linked with the specialization on liquid food and the reduced necessity to process substrates mechanically. In contrast, the compound eyes and the antennae, sense organs used for orientation and for finding a suitable mating partner and oviposition site, are well developed. Some evolutionary novelties are specific adaptations to feeding on liquefied substrates, such as labellae with furrows or pseudotracheae on their surface, and the strongly developed pre– and postcerebral pumping apparatuses. In some dipteran groups specialized on blood, the mandibles are still present as piercing stylets. They are completely reduced in the vast majority of families. Within the group far-reaching modifications of the antennae take place, with a strongly reduced number of segments and a specific configuration in Brachycera. The feeding habits and mouthparts of dipteran larvae are much more diverse than in the adults. The larval head is prognathous and fully exposed in the dipteran groundplan and most groups of lower Diptera. In Tipuloidea and Brachycera it is partly or largely retracted, and the sclerotized elements of the external head capsule are partly or fully reduced. The larval head of Cyclorrhapha is largely reduced. A complex and unique feature of this group is the cephaloskeleton. The movability of the larvae is limited due to the lack of thoracic legs. This can be partly compensated by the mouthparts, which are involved in locomotion in different groups. The mouth hooks associated with the cyclorrhaphan cephaloskeleton provide anchorage in the substrate.

The larval head of Exechia (Mycetophilidae) and Bibio (Bibionidae) (Diptera)

Exechia and Bibio have retained several plesiomorphic groundplan features of Diptera and Bibionomorpha, including a fully exposed and sclerotized head capsule, the transverse undivided labrum, the absence of movable premandibles, and undivided mandibles without combs. The fusion of the hypostomal bridge with the head capsule and largely reduced antennae are derived features shared by both taxa. The absence of teeth at the anterior hypostomal margin is a potential autapomorphy of Bibionomorpha. A basal position of Anisopodidae is suggested by a number of plesiomorphies retained in this family. Apomorphies of Bibionomorpha excluding Anisopodidae are the reduction of tentorial elements, the partial fusion of the labrum and clypeus, one-segmented antennae, the absence of a separate submental sclerite, the loss of the labial palpus, and the reduction of the pharyngeal filter apparatus. Head structures of Bibio are largely unmodified. The subprognathous orientation is one of few autapomorphic features. In contrast, the mouthparts of Exechia are highly modified in correlation with the specialized food uptake. The rasping counterrotating movements of maxillae and mandibles with teeth oriented in opposite directions are carried out by strongly developed extensors and flexors of the paired mouthparts. The modified labium mechanically supports the "drill head" formed by the mandibles und maxillae. The necessary stability of the head capsule is provided by the hypostomal bridge which also compensates the far-reaching reduction of the tentorium.

The evolution of head structures in lower Diptera

The head of adult dipterans is mainly characterized by modifications and more or less far reaching reductions of the mouthparts (e.g., mandibles, maxillae), linked with the specialization on liquid food and the reduced necessity to process substrates mechanically. In contrast, the compound eyes and the antennae, sense organs used for orientation and for finding a suitable mating partner and oviposition site, are well developed. Some evolutionary novelties are specific adaptations to feeding on less liquefied substrates, such as labellae with furrows or pseudotracheae on their surface, and the strongly developed pre- and postcerebral pumping apparatuses. In some dipteran groups specialized on blood the mandibles are still present as piercing stylets. They are completely reduced in the vast majority of families. Within the group far-reaching modifications of the antennae take place, with a strongly reduced number of segments and a specific configuration in Brachycera. The feeding habits and mouthparts of dipteran larvae are much more diverse than in the adults. The larval head is prognathous and fully exposed in the dipteran groundplan and most groups of lower Diptera. In Tipuloidea and Brachycera the head is partly or largely retracted and the sclerotized elements of the external head capsule are partly or fully reduced. The head of Cyclorrhapha is largely reduced. A complex and unique feature of this group is the cephaloskeleton. The movability of the larvae is limited due to the lack of thoracic legs. This can be partly compensated by the mouthparts, which are involved in locomotion in different groups. The mouth hooks associated with the cyclorrhaphan cephaloskeleton provide anchorage in the substrate.

Resolving insect phylogeny: The significance of cephalic structures of the Nannomecoptera in understanding endopterygote relationships

The Nannomecoptera are among the most enigmatic and controversial taxa of endopterygote insects, the phylogenetic resolution of which is crucial to understanding the evolution of neopteran insects. Once considered a subordinate lineage within the Mecoptera, renewed interest in nannochoristids has been prompted by evidence that the Nannomecoptera are not admissible to the clade of extant scorpion flies but are more likely to belong to the clade Siphonaptera + Nannomecoptera + Diptera. The overarching purpose of the present account is to provide novel and extensive morphological character traits in addition to those already existing for adult structures. The aim is to determine if these traits support molecular data sets that have been suggested elsewhere to clarify the phyletic position of Nannochoristidae. This account focuses on nannomecopteran larvae, which unlike those of other mecopterans have received little attention. Thus, the thrust of this investigation is to provide detailed anatomical data on nannochoristid larvae for a targeted inquiry into their phylogenetic affinities. The described characters are discussed and presented in a data matrix comprising representatives of all endopterygote orders. While the study is based primarily on the New Zealand species Nannochorista philpotti, it is proposed that all nannomecopteran larvae will prove to be similar to this taxon in most if not all structural features of significance to a higher-level phylogenetic context.