Head morphology of selected Staphylinoidea (Coleoptera: Staphyliniformia) with an evaluation of possible groundplan features in Staphylinidae

A unique case of commensalism: The beaver beetle Platypsyllus castoris (Leiodidae, Coleoptera) and its morphological adaptations

Platypsyllus castoris is closely associated with beavers and displays a unique set of structural specializations. We document the morphology of adults with modern techniques, and interpret evolutionary changes linked with the specific life style. The small subfamily Platypsyllinae has evolved an entire suite of features correlated with a more or less close association with mammals, for instance a flattened body, a dorsal cephalic shield, flightlessness, eye reduction, and depigmentation. Within this small group, Platypsyllus displays numerous autapomorphic features, correlated with a close association with the beaver. Essential is a combination of mechanical stabilization and firm anchorage on the host, and efficient forward movement in the fur. Exo- and endoskeletal structures of the head and thorax are reinforced by vertical cuticular columns and by an array of internal ridges. The antennae are shortened and strongly modified, the mandibles distinctly reduced and flattened, unsuitable for cutting, scraping or grinding. The musculature of the mouthparts is simplified, whereas an enhanced set of prepharyngeal and pharyngeal dilators forms an efficient sucking pump. The prothoracic musculature is strongly developed. In contrast, the pterothoracic muscle system is distinctly simplified, even though leg muscles are strongly developed. Using the legs, the flattened beetles move sideways through the dense fur of the beaver, using posteriorly directed groups of setae and ctenidia to prevent being pushed backwards by the densely arranged hairs. In contrast to the anterior body, the cuticle of the abdomen is thin, and the entire tagma flexible, with thin layers of segmental muscles. The hind gut is not connected with the mid gut. The beetles probably consume liquid, possibly with emulgated minute skin debris. As the morphology of the mouthparts excludes damage to the skin of the host, the association should not be addressed as ectoparasitic but as commensalism.

In the twilight zone-The head morphology of Bergrothia saulcyi (Pselaphinae, Staphylinidae, Coleoptera), a beetle with adaptations to endogean life but living in leaf litter

The pselaphine Bergrothia saulcyi shows features seemingly linked with life in deep soil layers, such as greatly reduced and non-functional compound eyes, a sensorium of long tactile setae, long appendages, and flightlessness. However, the tiny beetles occur in forest leaf litter, together with a community of beetles with wings and well-developed eyes. We hypothesize that B. saulcyi moves into deep soil under dry conditions, and returns to upper layers when humidity increases again. Despite the evolutionary cost of a reduced dispersal capacity, this life strategy may be more efficient and less hazardous than moving to different habitats using flight and the visual sense in an environment periodically drying out. We also discuss cephalic features with potential phylogenetic relevance. Plesiomorphies of B. saulcyi include the presence of anterior tentorial arms, well-developed labral retractors, and a full set of extrinsic maxillary and premental muscles. Apomorphic cephalic features support clades Protopselaphinae + Pselaphinae, and Pselaphinae. A conspicuous derived condition, the clypeo-ocular carina, is a possible synapomorphy of Batrisitae and genera assigned to Goniaceritae. A complex triple set of cephalic glands found in B. saulcyi is similar to a complex identified in the strict myrmecophile Claviger testaceus (Clavigeritae). It is conceivable that glands linked with food uptake in free-living pselaphines were genetically re-programmed in ancestors of inquilines, to enable them to appease the host ants. We suggest that behavioral studies are necessary to understand the poorly known life habits of B. saulcyi. Additional information is required to explain why a species with irreversibly reduced visual sense and other adaptations typical of endogean or cave-dwelling beetles was only collected from the upper leaf litter layer.

The specialized thoracic skeletomuscular system of the myrmecophile Claviger testaceus (Pselaphinae, Staphylinidae, Coleoptera)

External and internal structures of the thorax of the myrmecophile beetle Claviger testaceus (Clavigeritae, Pselaphinae) were examined and documented with state-of-the-art visualization techniques. Following a general trend in the omaliine lineage (Staphylinidae), the skeletal elements of the pro- and pterothorax in Claviger reach a maximum degree of compactness, with largely reduced inter- and intrasegmental sutures and skeletal elements linked with the flight apparatus. The musculature, especially metathoracic direct and indirect flight muscles, also shows a high degree of reduction. Two forms of wings were found among individuals of C. testaceus, both non-functional and representing an advanced stage of reduction. However, that wing vestiges are still present and the metanotum, only slightly reduced, suggests that loss of flight in this species is likely the result of a young evolutionary process. Several structures are linked with myrmecophilous habits: small body size facilitates transportation of beetles by ant workers and makes it easier to move inside nest tunnels; the remarkably compact body and mechanically robust appendages make the beetles less vulnerable to attacks by ant mandibles; the improved elytral interlocking mechanism and unusually expanded epipleura enhance the protection of vulnerable dorsal parts of the pterothorax and anterior abdomen; and glands associated with trichomes on the posterolateral elytral angle produce secretions attractive for ants. Various modifications of the thorax and anterior abdomen lead to an optimization of intimate associations with ants. The morphological syndrome enabling these beetles to cope with life in ant colonies evolved in several steps. This is suggested by an increasing solidification of the thoracic skeleton in related non-myrmecophilous groups and also by less modified related clavigerites;for instance, ant-associated tropical species are still able to fly.

The evolution of "deformed" brains in ant-like stone beetles (Scydmaeninae, Staphylinidae)

We present the first study of the central nervous system of adult representatives of Scydmaeninae. Histological staining, scanning electron microscopy and computer-based 3D reconstruction techniques were used to document the shape and configuration of the major cephalic elements of the central nervous system and to explain its anomalies compared to other Coleoptera. For the first time we report the presence of cephalic glands in ant-like stone beetles: in Scydmaenus (Cholerus) hellwigii openings of voluminous glands are located near the occipital constriction and their secretion accumulates in a large cavity of the dorsal head region. In Scydmaenus (Cholerus) perrisi the proto-, deuto-, tritocerebrum and the suboesophageal ganglion together form a large and compact ganglionic mass around the anterior foregut in the retracted neck region of the head. We exclude miniaturization as the driving force of the observed modifications. Comparative study of the head anatomy of S. perrisi, S. hellwigii, Scydmaenus (s. str.) tarsatus, Scydmaenus (Parallomicrus) rufus and Neuraphes elongatulus suggests a possible evolutionary scenario. We propose an evolutionary reversal hypothesis, involving a) the displacement and concentration of the cephalic central nervous system induced by the development of glandular cavities of the head, followed by b) a reduction of the glandular structures, without a secondary relocation of the cephalic CNS. The interpretation of head modifications in Scydmaeninae in the light of such a scenario may turn out as important for the reconstruction of the phylogeny and evolution of this highly successful group of beetles.

Phase contrast X-ray synchrotron imaging for assessing external and internal morphology of Rhodnius prolixus

PhC-SR-μCT is a nondestructive technique that allows the microanatomical investigations and 3D images reconstructions. This technique is performed in blood sucker, Rhodnius prolixus--one of the most important insect vectors of Trypanosoma cruzi, ethiologic agent of Chagas' disease in Latin America--was able to provide excellent information about the microanatomy of the thorax and head allowing a new tool for further studies of development and physiology of triatomine by a non-invasive method of observation.