A biomechanical analysis of prognathous and orthognathous insect head capsules: evidence for a many‐to‐one mapping of form to function

Insect head shapes are remarkably variable, but the influences of these changes on biomechanical performance are unclear. Among ‘basal’ winged insects, such as dragonflies, mayflies, earwigs and stoneflies, some of the most prominent anatomical changes are the general mouthpart orientation, eye size and the connection of the endoskeleton to the head. Here, we assess these variations as well as differing ridge and sclerite configurations using modern engineering methods including multibody dynamics modelling and finite element analysis in order to quantify and compare the influence of anatomical changes on strain in particular head regions and the whole head. We show that a range of peculiar structures such as the genal/subgenal, epistomal and circumocular areas are consistently highly loaded in all species, despite drastically differing morphologies in species with forward‐projecting (prognathous) and downward‐projecting (orthognathous) mouthparts. Sensitivity analyses show that the presence of eyes has a negligible influence on head capsule strain if a circumocular ridge is present. In contrast, the connection of the dorsal endoskeletal arms to the head capsule especially affects overall head loading in species with downward‐projecting mouthparts. Analysis of the relative strains between species for each head region reveals that concerted changes in head substructures such as the subgenal area, the endoskeleton and the epistomal area lead to a consistent relative loading for the whole head capsule and vulnerable structures such as the eyes. It appears that biting‐chewing loads are managed by a system of strengthening ridges on the head capsule irrespective of the general mouthpart and head orientation. Concerted changes in ridge and endoskeleton configuration might allow for more radical anatomical changes such as the general mouthpart orientation, which could be an explanation for the variability of this trait among insects. In an evolutionary context, many‐to‐one mapping of strain patterns onto a relatively similar overall head loading indeed could have fostered the dynamic diversification processes seen in insects

The Strepsiptera-Odyssey: the history of the systematic placement of an enigmatic parasitic insect order

The history of the phylogenetic placement of the parasitic insect order Strepsiptera is outlined. The first species was described in 1793 by P. Rossi and assigned to the hymenopteran family Ichneumonidae. A position close to the cucujiform beetle family Rhipiphoridae was suggested by several earlier authors. Others proposed a close relationship with Diptera or even a group <em>Pupariata</em> including Diptera, Strepsiptera and Coccoidea. A subordinate placement within the polyphagan series Cucujiformia close to the wood-associated Lymexylidae was favored by the coleopterist R.A. Crowson. W. Hennig considered a sistergroup relationship with Coleoptera as the most likely hypothesis but emphasized the uncertainty. Cladistic analyses of morphological data sets yielded very different placements, alternatively as sistergroup of Coleoptera, Antliophora, or all other holometabolan orders. Results based on ribosomal genes suggested a sistergroup relationship with Diptera (Halteria concept). A clade Coleopterida (Strepsiptera and Coleoptera) was supported in two studies based on different combinations of protein coding nuclear genes. Analyses of data sets comprising seven or nine genes (7 single copy nuclear genes), respectively, yielded either a subordinate placement within Coleoptera or a sistergroup relationship with Neuropterida. Several early hypotheses based on a typological approach − affinities with Diptera, Coleoptera, a coleopteran subgroup, or Neuropterida − were revived using either a Hennigian approach or formal analyses of morphological characters or different molecular data sets. A phylogenomic approach finally supported a sistergroup relationship with monophyletic Coleoptera

Head anatomy of Xyelidae (Hexapoda: Hymenoptera) and phylogenetic implications

AbstractExternal and internal head structures of Macroxyela ferruginea (Say) and Xyela julii (Brébisson) were examined. A detailed description is provided for Macroxyela. The results are compared to the conditions found in other basal hymenopterans and representatives of other groups of endopterygote insects. Hitherto unnoticed autapomorphies of Hymenoptera are the concavity of the posterior head capsule, the very dense, regular vestiture of hairs, the collar-like, strongly developed posterior tentorium, and a large epipharyngopharyngeal lobe. Microphagous habits and associated features (asymmetric mandibular molae, epipharyngeal brush, infrabuccal pouch) are possibly groundplan features of Hymenoptera and Endopterygota. A switch to more or less liquefied food took place early in the evolution of Hymenoptera. The sitophore plate and a constricted, elongated prepharyngeal tube are likely synapomorphies of Hymenoptera and Mecopterida. Monophyly of Hymenoptera excluding Xyelidae is supported by the reduction of the mandibular molae and epipharyngeal brush. These changes are likely related to modified feeding habits. Widely separated mandibular bases, the loss of the median labral retractor (parallel loss in Xyelidae), and the presence of a hypostomal bridge are potential apomorphies of Hymenoptera excluding Xyelidae and Tenthredinoidea. Monophyly of Xyelinae and Macroxyelinae, respectively, is well supported by the results of our study. There is conflicting evidence as to whether Xyelidae is monophyletic. The presence of a subdivided galea is a putative autapomorphy of the family. The presence of unsclerotised paraglossae with dense fringes of thin hairs and the presence of a muscle connecting the anterior tentorial arm with the posterior edge of the sitophore plate are features shared by Xyelinae and members of non-xyelid families

The morphological evolution of the Adephaga (Coleoptera)

The evolution of the coleopteran suborder Adephaga is discussed based on a robust phylogenetic background. Analyses of morphological characters yield results nearly identical to recent molecular phylogenies, with the highly specialized Gyrinidae placed as sister to the remaining families, which form two large, reciprocally monophyletic subunits, the aquatic Haliplidae + Dytiscoidea (Meruidae, Noteridae, Aspidytidae, Amphizoidae, Hygrobiidae, Dytiscidae) on one hand, and the terrestrial Geadephaga (Trachypachidae + Carabidae) on the other. The ancestral habitat of Adephaga, either terrestrial or aquatic, remains ambiguous. The former option would imply two or three independent invasions of aquatic habitats, with very different structural adaptations in larvae of Gyrinidae, Haliplidae and Dytiscoidea.Deutsche Forschungsgemeinschaft. Grant Number: BE 1789/11‐

Pushing versus pulling: division of labour between tarsal attachment pads in cockroaches

Adhesive organs on the legs of arthropods and vertebrates are strongly direction dependent, making contact only when pulled towards the body but detaching when pushed away from it. Here we show that the two types of attachment pads found in cockroaches (Nauphoeta cinerea), tarsal euplantulae and pretarsal arolium, serve fundamentally different functions. Video recordings of vertical climbing revealed that euplantulae are almost exclusively engaged with the substrate when legs are pushing, whereas arolia make contact when pulling. Thus, upward-climbing cockroaches used front leg arolia and hind leg euplantulae, whereas hind leg arolia and front leg euplantulae were engaged during downward climbing. Single-leg friction force measurements showed that the arolium and euplantulae have an opposite direction dependence. Euplantulae achieved maximum friction when pushed distally, whereas arolium forces were maximal during proximal pulls. This direction dependence was not explained by the variation of shear stress but by different contact areas during pushing or pulling. The changes in contact area result from the arrangement of the flexible tarsal chain, tending to detach the arolium when pushing and to peel off euplantulae when in tension. Our results suggest that the euplantulae in cockroaches are not adhesive organs but ‘friction pads’, mainly providing the necessary traction during locomotion