Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata1 II. Cladogramme und die Entfaltung der Chelicerata

Phylogeny of the Neotropical Hypoctonine Whip-Scorpions (Thelyphonida, Thelyphonidae), with Descriptions of Two New Genera and Species

Thelyphonida Blanchard, 1852, also known as vinegaroons or whip-scorpions, is a small arachnid order with 140 described species contained in a single family, Thelyphonidae Lucas, 1835. Despite being conspicuous and widely distributed across the tropics and subtropics on four continents, knowledge of the order has been slow to advance. Hypoctoninae Pocock, 1899, one of four subfamilies currently recognized and one of two represented in the New World, comprises five genera. Since its inception, Thelyphonellus Pocock, 1894 has remained the only hypoctonine genus occurring in South America, with only four species described prior to the present contribution. The first detailed morphological study and phylogenetic analysis of Thelyphonellus is presented herein. The morphological phylogenetic analysis-the first for Thelyphonida-includes all except one of the previously described species of Thelyphonellus in addition to two new species described herein; the species of Ravilops Víquez and Armas, 2005 (from the Caribbean island of Hispaniola); and the monotypic Old World genus Etienneus Heurtault, 1984 (from West Africa) scored for 45 morphological characters. A single, most parsimonious phylogenetic hypothesis revealed that Thelyphonellus is paraphyletic with respect to Ravilops. The New World Hypoctoninae comprises four clades with disjunct distributions and well supported by a combination of morphological characteristics, on the basis of which four genera, two of which are new, are recognized: Ravilops, with two species, endemic to Hispaniola; Thelyphonellus, herein restricted to Thelyphonellus amazonicus (Butler, 1872) and Thelyphonellus ruschii Weygoldt, 1979, occurring in Guyana, Suriname, French Guiana, and northern Brazil; Wounaan, gen. n., containing Wounaan vanegasae (Giupponi and Vasconcelos, 2008), comb. n. and Wounaan yarigui, sp. n. from Colombia; and Yekuana, gen. n., containing Yekuana venezolensis (Haupt, 2009), comb. n. and Yekuana wanadi, sp. n. from Venezuela. The two new species are described and illustrated. A key to the identification of the Neotropical genera of Hypoctoninae and a map plotting the known distribution of its species are also presented.

Comparative microscopic anatomy of Schizomida - 1. Segmental axial musculature and body organization

Schizomida is an enigmatic group of arachnids that is traditionally considered the dwarfed sister to Thelyphonida. Schizomids are of interest for evolutionary morphology, because they show a number of features like a tripartite prosoma dorsal shield (pro-, meso-, metapeltidium), formation of three sterna, a complex prosoma-opisthosoma transition and a metasoma. By analyzing the body organization of Schizomida and comparing it to Thelyphonida and other arachnids, this article provides evidence for independent evolution of some of these features in Schizomida. This supports the idea that, among arachnids, multiple and independent evolutionary pathways have resulted in similar morphologies, that conventionally have been considered shared similarities. - The analysis of serial microscopic sections and μCT-imaging of segmental indicator muscles of the prosoma evidences that the propeltidium covers prosoma segments 0-4, and the metapeltidium covers segments 5 and 6. The mesopeltidium is a dorsolateral sclerotization of the pleural membrane, not assigned to a segment, and therefore not a tergite. The topographic association of segmental musculature and sclerites of the tripartite dorsum of the prosoma differs from other taxa with such external body organization, e.g., Palpigradi and Solifugae, suggesting independent evolutionary origin. - The prosoma-opisthosoma transition integrates the first opisthosoma segment into the prosoma. The sternite of the first opisthosoma segment forms the metasternum between the coxae of the fourth pair of walking legs. The morphology of the prosoma-opisthosoma transition is similar to Uropygi and Amblypygi, but is less complex. - The morphology of the metasoma (opisthosoma segments X-XII) of Schizomida and Thelyphonida differs from that of all other arachnids carrying a metasoma, thus providing support for multiple independent evolutionary origins of metasomata.

Comparative microscopic anatomy of Schizomida - 2. The rostrosoma and the pharyngeal suction pump

This paper tests hypotheses of independent parallel evolution of the rostrosoma among euchelicerate taxa by analyzing the microscopic anatomy and histology of the rostrosoma of Uropygi (Schizomida and Thelyphonida) and comparing it with the morphology of the snout region in other euchelicerates. The study employs analysis of multiple histological serial sections, μCT-imaging, and graphical as well as computer-based 3D reconstruction. Results of the study are that Thelyphonida and Schizomida share the same morphology of the rostrosoma. The rostrosoma of both groups contains a unique arrangement of musculature that is functionally interpreted as pre-oral suction pump. This is followed by a pharyngeal suction pump. The muscles of the pharyngeal suction pump attach to the epistome and the epipharyngeal sclerite. Neither Schizomida nor Thelyphonida possess a postcerebral suction pump as reported earlier. The microscopic anatomy of the rostrosoma of both taxa is unique and does not compare with any of the other euchelicerates, thus supporting the idea of independent evolutionary origin of the rostrosoma. Thelyphonida, Amblypygi and Scorpiones share the occurrence of a large epipharyngeal/epistomal sclerite with associated musculature, which is a feature that lines up with the Arachnopulmonata concept. A comparison with all Euchelicerata taxa shows that the snout region is formed by homologous morphological elements but the specific arrangement, additions and reductions shape the formation of the rostrosoma, so that parallel evolution of homologous parts of the arachnid ground pattern can be assumed that has formed those elements into convergent morphologies.

A plurality of morphological characters need not equate with phylogenetic accuracy: A rare genomic change refutes the placement of Solifugae and Pseudoscorpiones in Haplocnemata

Recent advances in higher-level invertebrate phylogeny have leveraged shared features of genomic architecture to resolve contentious nodes across the tree of life. Yet, the interordinal relationships within Chelicerata have remained recalcitrant given competing topologies in recent molecular analyses. As such, relationships between topologically unstable orders remain supported primarily by morphological cladistic analyses. Solifugae, one such unstable chelicerate order, has long been thought to be the sister group of Pseudoscorpiones, forming the clade Haplocnemata, on the basis of eight putative morphological synapomorphies. The discovery, however, of a shared whole genome duplication placing Pseudoscorpiones in Arachnopulmonata provides the opportunity for a simple litmus test evaluating the validity of Haplocnemata. Here, we present the first developmental transcriptome of a solifuge (Titanopuga salinarum) and survey copy numbers of the homeobox genes for evidence of systemic duplication. We find that over 70% of the identified homeobox genes in T. salinarum are retained in a single copy, while representatives of the arachnopulmonates retain orthologs of those genes as two or more copies. Our results refute the placement of Solifugae in Haplocnemata. Subsequent reevaluation of putative interordinal morphological synapomorphies among chelicerates reveals a high incidence of homoplasy, reversals, and inaccurate coding within Haplocnemata and other small clades, as well as Arachnida more broadly, suggesting existing morphological character matrices are insufficient to resolve chelicerate phylogeny.

Neglected no longer: Phylogenomic resolution of higher-level relationships in Solifugae

Advanced sequencing technologies have expedited resolution of higher-level arthropod relationships. Yet, dark branches persist, principally among groups occurring in cryptic habitats. Among chelicerates, Solifugae ("camel spiders") is the last order lacking a higher-level phylogeny and have thus been historically characterized as "neglected [arachnid] cousins". Though renowned for aggression, remarkable running speed, and xeric adaptation, inferring solifuge relationships has been hindered by inaccessibility of diagnostic morphological characters, whereas molecular investigations have been limited to one of 12 recognized families. Our phylogenomic dataset via capture of ultraconserved elements sampling all extant families recovered a well-resolved phylogeny, with two distinct groups of New World taxa nested within a broader Paleotropical radiation. Divergence times using fossil calibrations inferred that Solifugae radiated by the Permian, and most families diverged prior to the Paleogene-Cretaceous extinction, likely driven by continental breakup. We establish Boreosolifugae new suborder uniting five Laurasian families, and Australosolifugae new suborder uniting seven Gondwanan families using morphological and biogeographic signal.

The Impact of Whole Genome Duplication on the Evolution of the Arachnids

The proliferation of genomic resources for Chelicerata in the past 10 years has revealed that the evolution of chelicerate genomes is more dynamic than previously thought, with multiple waves of ancient whole genome duplications affecting separate lineages. Such duplication events are fascinating from the perspective of evolutionary history because the burst of new gene copies associated with genome duplications facilitates the acquisition of new gene functions (neofunctionalization), which may in turn lead to morphological novelties and spur net diversification. While neofunctionalization has been invoked in several contexts with respect to the success and diversity of spiders, the overall impact of whole genome duplications on chelicerate evolution and development remains imperfectly understood. The purpose of this review is to examine critically the role of whole genome duplication on the diversification of the extant arachnid orders, as well as assess functional datasets for evidence of subfunctionalization or neofunctionalization in chelicerates. This examination focuses on functional data from two focal model taxa: the spider Parasteatoda tepidariorum, which exhibits evidence for an ancient duplication, and the harvestman Phalangium opilio, which exhibits an unduplicated genome. I show that there is no evidence that taxa with genome duplications are more successful than taxa with unduplicated genomes. I contend that evidence for sub- or neofunctionalization of duplicated developmental patterning genes in spiders is indirect or fragmentary at present, despite the appeal of this postulate for explaining the success of groups like spiders. Available expression data suggest that the condition of duplicated Hox modules may have played a role in promoting body plan disparity in the posterior tagma of some orders, such as spiders and scorpions, but functional data substantiating this postulate are critically missing. Spatiotemporal dynamics of duplicated transcription factors in spiders may represent cases of developmental system drift, rather than neofunctionalization. Developmental system drift may represent an important, but overlooked, null hypothesis for studies of paralogs in chelicerate developmental biology. To distinguish between subfunctionalization, neofunctionalization, and developmental system drift, concomitant establishment of comparative functional datasets from taxa exhibiting the genome duplication, as well as those that lack the paralogy, is sorely needed.

Evolutionary morphology of coxal musculature in Pseudoscorpiones (Arachnida)

Pseudoscorpions are an ancient and globally distributed lineage of arachnids with more than 4000 species. Despite being present in virtually all terrestrial habitats, their morphology and anatomy has rarely been studied to date, which hampers homology statements both within and between other arachnid orders. All pseudoscorpions share a morphological peculiarity, the fixation of the coxae of all the walking legs. The same morphological condition is seen in certain other arachnid taxa, such as Solifugae or Scorpiones - potential sistergroups of Pseudoscorpiones. To investigate the musculature apparatus of this unusual feature, we reconstructed the musculature in the coxae of walking legs in three species of pseudoscorpions that represent the three major clades within this order. Using micro-computed tomography (μCT), we show that pseudoscorpions have the highest number of coxal muscles amongst the arachnid orders (12 vs. fewer than 10 in others), and that the muscular composition of the first two legs differs from that in the hind legs, correlating with the difference in function, i.e. pulling in the front legs and pushing in the hind legs. Pseudoscorpions are also unique amongst the arachnids in lacking endoskeletal structures (coxal apodeme or costa coxalis) inside the coxae. We observed that within pseudoscorpions, there is a trend towards a reduction of the number of coxal muscles, with the most basal-branching taxon having the highest number and more derived taxa exhibiting lower counts. We hypothesize the muscular ground pattern for Pseudoscorpiones and discuss the evolution of this system by comparing it to the (scanty) data on other arachnids available in the literature.

Highly diversified mitochondrial genomes provide new evidence for interordinal relationships in the Arachnida

Arachnida is an exceptionally diverse class in the Arthropoda, consisting of 20 orders and playing crucial roles in the terrestrial ecosystems. However, their interordinal relationships have been debated for over a century. Rearranged or highly rearranged mitochondrial genomes (mitogenomes) were consistently found in this class, but their various extent in different lineages and efficiency for resolving arachnid phylogenies are unclear. Here, we reconstructed phylogenetic trees using mitogenome sequences of 290 arachnid species to decipher interordinal relationships as well as diversification through time. Our results recovered monophyly of ten orders (i.e. Amblypygi, Araneae, Ixodida, Mesostigmata, Opiliones, Pseudoscorpiones, Ricinulei, Sarcoptiformes, Scorpiones and Solifugae), while rejecting monophyly of the Trombidiformes due to the unstable position of the Eriophyoidea. The monophyly of Acari (subclass) was rejected, possibly due to the long-branch attraction of the Pseudoscorpiones. The monophyly of Arachnida was further rejected because the Xiphosura nested within arachnid orders with unstable positions. Mitogenomes that are highly rearranged in mites but less rearranged or conserved in the remaining lineages point to their exceptional diversification in mite orders; however, shared derived mitochondrial (mt) gene clusters were found within superfamilies rather than interorders, confusing phylogenetic signals in arachnid interordinal relationships. Molecular dating results show that arachnid orders have ancient origins, ranging from the Ordovician to the Carboniferous, yet have significantly diversified since the Cretaceous in orders Araneae, Mesostigmata, Sarcoptiformes, and Trombidiformes. By summarizing previously resolved key positions of some orders, we propose a plausible arachnid tree of life. Our results underline a more precise framework for interordinal phylogeny in the Arachnida and provide new insights into their ancient evolution.

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

BACKGROUND

Phylogenomic studies over the past two decades have consolidated the major branches of the arthropod tree of life. However, especially within the Chelicerata (spiders, scorpions, and kin), interrelationships of the constituent taxa remain controversial. While sea spiders (Pycnogonida) are firmly established as sister group of all other extant representatives (Euchelicerata), euchelicerate phylogeny itself is still contested. One key issue concerns the marine horseshoe crabs (Xiphosura), which recent studies recover either as sister group of terrestrial Arachnida or nested within the latter, with significant impact on postulated terrestrialization scenarios and long-standing paradigms of ancestral chelicerate traits. In potential support of a nested placement, previous neuroanatomical studies highlighted similarities in the visual pathway of xiphosurans and some arachnopulmonates (scorpions, whip scorpions, whip spiders). However, contradictory descriptions of the pycnogonid visual system hamper outgroup comparison and thus character polarization.

RESULTS

To advance the understanding of the pycnogonid brain and its sense organs with the aim of elucidating chelicerate visual system evolution, a wide range of families were studied using a combination of micro-computed X-ray tomography, histology, dye tracing, and immunolabeling of tubulin, the neuropil marker synapsin, and several neuroactive substances (including histamine, serotonin, tyrosine hydroxylase, and orcokinin). Contrary to previous descriptions, the visual system displays a serial layout with only one first-order visual neuropil connected to a bilayered arcuate body by catecholaminergic interneurons. Fluorescent dye tracing reveals a previously reported second visual neuropil as the target of axons from the lateral sense organ instead of the eyes.

CONCLUSIONS

Ground pattern reconstruction reveals remarkable neuroanatomical stasis in the pycnogonid visual system since the Ordovician or even earlier. Its conserved layout exhibits similarities to the median eye pathway in euchelicerates, especially in xiphosurans, with which pycnogonids share two median eye pairs that differentiate consecutively during development and target one visual neuropil upstream of the arcuate body. Given multiple losses of median and/or lateral eyes in chelicerates, and the tightly linked reduction of visual processing centers, interconnections between median and lateral visual neuropils in xiphosurans and arachnopulmonates are critically discussed, representing a plausible ancestral condition of taxa that have retained both eye types.

Comparative anatomy of the rostrosoma of Solifugae, Pseudoscorpiones and Acari

We compare the microscopic anatomy of the mouthparts of representative species of Solifugae, Pseudoscorpiones and Parasitiformes (Acari). Specifically, we focus on the epistome, the labrum, the lateral lips (= endites of the pedipalpal coxae) and the musculature of the pharyngeal suction pump. We provide evidence that the labrum is reduced in Solifugae, but present and functional in Pseudoscorpiones and Acari. The epistome constitutes the entire dorsal face of the rostrosoma in Solifugae, but is internalized into the prosoma in Pseudoscorpiones. In Acari, the epistome shows an ancestral morphology, probably close to the ground pattern of chelicerates. The lateral lips of Solifugae contribute to the ventral face of the rostrosoma and the two lips of the mouth opening. In Solifugae, the ventral rostrosoma also includes a sclerite that might derive from a tritosternum. In Pseudoscorpiones, the lateral lips remain independent of the rostrosoma, they interlock ventral to the rostrosoma forming a perioral space. Here, the rostrosoma has an unpaired ventral lip of unresolved morphological origin, which is, however, clearly distinct from the lateral lips of Solifugae. The pharyngeal suction pump differs in all three clades in attachment, number of muscles and origin of muscles. We interpret the data as evidence for independent, parallel evolution of elements of the ground pattern of the (eu)chelicerate mouth parts. Based on the morphological elements of a common euchelicerate ground plan, the rostrosoma evolved independently in the three clades. We reject earlier hypotheses that consider the rostrosoma a character to support a phylogenetic relationship of the three clades.

How many long branch orders occur in Chelicerata? Opposing effects of Palpigradi and Opilioacariformes on phylogenetic stability

Excepting a handful of nodes, phylogenetic relationships between chelicerate orders remains poorly resolved, due to both the incidence of long branch attraction artifacts and the limited sampling of key lineages. It has recently been shown that increasing representation of basal nodes plays an outsized role in resolving the higher-level placement of long branch chelicerate orders. Two lineages have been consistently undersampled in chelicerate phylogeny. First, sampling of the miniaturized order Palpigradi has been restricted to a fragmentary transcriptome of a single species. Second, sampling of Opilioacariformes, a rarely encountered and key group of Parasitiformes, has been restricted to a single exemplar. These two lineages exhibit dissimilar properties with respect to branch length; Opilioacariformes shows relatively low evolutionary rate compared to other Parasitiformes, whereas Palpigradi possibly acts as another long branch order (an effect that may be conflated with the degree of missing data). To assess these properties and their effects on tree stability, we constructed a phylogenomic dataset of Chelicerata wherein both lineages were sampled with three terminals, increasing the representation of these lineages per locus. We examined the effect of subsampling phylogenomic matrices using (1) taxon occupancy, (2) evolutionary rate, and (3) a principal components-based approach. We further explored the impact of taxon deletion experiments that mitigate the effect of long branches. Here, we show that Palpigradi constitutes a fourth long branch chelicerate order (together with Acariformes, Parasitiformes, and Pseudoscorpiones), which further destabilizes the chelicerate backbone topology. By contrast, the slow-evolving Opilioacariformes were consistently recovered within Parasitiformes, with certain subsampling practices recovering their placement as the sister group to the remaining Parasitiformes. Whereas the inclusion of Opilioacariformes always resulted in the non-monophyly of Acari with support, deletion of Opilioacariformes from datasets consistently incurred the monophyly of Acari except in matrices constructed on the basis of evolutionary rate. Our results strongly suggest that Acari is an artifact of long-branch attraction.

What Is an “Arachnid”? Consensus, Consilience, and Confirmation Bias in the Phylogenetics of Chelicerata

The basal phylogeny of Chelicerata is one of the opaquest parts of the animal Tree of Life, defying resolution despite application of thousands of loci and millions of sites. At the forefront of the debate over chelicerate relationships is the monophyly of Arachnida, which has been refuted by most analyses of molecular sequence data. A number of phylogenomic datasets have suggested that Xiphosura (horseshoe crabs) are derived arachnids, refuting the traditional understanding of arachnid monophyly. This result is regarded as controversial, not least by paleontologists and morphologists, due to the widespread perception that arachnid monophyly is unambiguously supported by morphological data. Moreover, some molecular datasets have been able to recover arachnid monophyly, galvanizing the belief that any result that challenges arachnid monophyly is artefactual. Here, we explore the problems of distinguishing phylogenetic signal from noise through a series of in silico experiments, focusing on datasets that have recently supported arachnid monophyly. We assess the claim that filtering by saturation rate is a valid criterion for recovering Arachnida. We demonstrate that neither saturation rate, nor the ability to assemble a molecular phylogenetic dataset supporting a given outcome with maximal nodal support, is a guarantor of phylogenetic accuracy. Separately, we review empirical morphological phylogenetic datasets to examine characters supporting Arachnida and the downstream implication of a single colonization of terrestrial habitats. We show that morphological support of arachnid monophyly is contingent upon a small number of ambiguous or incorrectly coded characters, most of these tautologically linked to adaptation to terrestrial habitats.

A report of the complete mitochondrial genome of Bisetocreagris titanium (Arachnida: Pseudoscorpiones: Neobisiidae) from Yunnan Province, China

A complete mitogenome of a cave dwelling pseudoscorpion Bisetocreagris titanium is reported here. The mitogenome is a circular DNA molecule with a length of 14,756 base pairs (bp), and it contains 13 protein coding genes (PCGs), 22 transfer RNAs (tRNAs), 2 ribosomal RNAs (rRNAs), and 1 putative control region. Phylogenetic analysis of 30 Arachnida species was performed based on the amino acid datasets of 13 PCGs, and the result indicated Pseudoscorpiones is the sister lineage of Acariformes. This result is congruent with the former phylogenetic results of mitogenomes, but incongruent with the results of morphological characters and/or ribosomal DNA data that indicated Pseudoscorpiones are positioned in a clade with the Solifugae.

A revision of Prolimulus woodwardi Fritsch, 1899 with comparison to other highly paedomorphic belinurids

Xiphosurida is an ingroup of marine Euchelicerata often referred to as "living fossils". However, this oxymoronic term is inapplicable for Paleozoic and early Mesozoic forms, as during these periods the group experienced notable evolutionary radiations; particularly the diverse late Palaeozoic clade Belinurina. Despite the iconic nature of the group, select species in this clade have been left undescribed in the light of recent geometric morphometric and phylogenetic considerations and methodologies. To this end, we re-describe Prolimulus woodwardi Fritsch, 1899 using new and type specimens to reveal more details on appendage anatomy and possible ecology. Furthermore, we present geometric morphometric and phylogenetic analyses that uncover relationships between P. woodwardi and other belinurids without genal spines. Both approaches highlight that a clade containing Prolimulus Fritsch, 1899, Liomesaspis Raymond, 1944, Alanops Racheboeuf, Vannier & Anderson, 2002 and Stilpnocephalus Selden, Simonetto & Marsiglio, 2019 may exist. While we do not erect a new group to contain these genera, we note that these genera exemplify the extreme limits of the Belinurina radiation and a peak in horseshoe crab diversity and disparity. This evidence also illustrates how changes in heterochronic timing are a key evolutionary phenomenon that can drive radiations among animals.

The tracheal system of scutigeromorph centipedes and the evolution of respiratory systems of myriapods

The tracheal system of scutigeromorph centipedes (Chilopoda) is special, as it consists of dorsally arranged unpaired spiracles. In this study, we investigate the tracheal systems of five different scutigeromorph species. They are strikingly similar to each other but depict unique characters compared to the tracheal systems of pleurostigmophoran centipedes, which has engendered an ongoing debate over a single versus independent origin of tracheal systems in Chilopoda. Up to now, only the respiratory system of Scutigera coleoptrata was investigated intensively using LM-, TEM-, and SEM-techniques. We supplement this with data for species from all three families of Scutigeromorpha. These reveal interspecific differences in atrial width and the shape and branching pattern of the tracheal tubules. Further, we investigated the tracheal system of Scutigera coleoptrata with three additional techniques: light sheet microscopy, microCT and synchrotron radiation based microCT analysis. This set of techniques allows a comparison between fresh versus fixed and dried material. The question of a unique vs. multiple origin of tracheal systems in centipedes and in Myriapoda as a whole is discussed with regard to their structural similarities and differences and the presence of hemocyanin as an oxygen carrier. We used morphological and molecular data and the fossil record to evaluate the alternative hypotheses.

Outsourcing a visual neuropil - The central visual system of the median eyes of Galeodes granti Pocock, 1903 (Arachnida: Solifugae)

Only a few studies have examined the central visual system of Solifugae until now. To get new insights suitable for phylogenetic analysis we studied the R-cell (or retinula cell) projections and visual neuropils of Galeodes granti using various methods. G. granti possesses large median eyes and rudimentary lateral eyes. In this study, only the R-cells and neuropils of the median eyes were successfully stained. The R-cells terminate in two distinct visual neuropils. The first neuropil is located externally to the protocerebrum directly below the retina, the second neuropil lies in the cell body rind of the protocerebrum, and immediately adjacent is the arcuate body. This layout of the median eye visual system differs from Arachnopulmonata (Scorpiones + Tetrapulmonata). However, there are several similarities with Opiliones. In both, (1) the R-cells are connected to a first and second visual neuropil and not to any other region of the brain, (2) the first neuropil is not embedded in the cell body rind of the protocerebrum, it is rather external to the protocerebrum, (3) the second visual neuropil is embedded in the cell body rind, and (4) the second neuropil abuts the arcuate body. These findings may provide important new characters for the discussion on arachnid phylogeny.

Arachnid monophyly: Morphological, palaeontological and molecular support for a single terrestrialization within Chelicerata

The majority of extant arachnids are terrestrial, but other chelicerates are generally aquatic, including horseshoe crabs, sea spiders, and the extinct eurypterids. It is necessary to determine whether arachnids are exclusively descended from a single common ancestor (monophyly), because only that relationship is compatible with one land colonisation in chelicerate evolutionary history. Some studies have cast doubt on arachnid monophyly and recast the origins of their terrestrialization. These include some phylogenomic analyses placing horseshoe crabs within Arachnida, and from aquatic Palaeozoic stem-group scorpions. Here, we evaluate the possibility of arachnid monophyly by considering morphology, fossils and molecules holistically. We argue arachnid monophyly obviates the need to posit reacquisition/retention of aquatic characters such as gnathobasic feeding and book gills without trabeculae from terrestrial ancestors in horseshoe crabs, and that the scorpion total-group contains few aquatic taxa. We built a matrix composed of 200 slowly-evolving genes and re-analysed two published molecular datasets. We retrieved arachnid monophyly where other studies did not - highlighting the difficulty of resolving chelicerate relationships from current molecular data. As such, we consider arachnid monophyly the best-supported hypothesis. Finally, we inferred that arachnids terrestrialized during the Cambrian-Ordovician using the slow-evolving molecular matrix, in agreement with recent analyses.

Evolutionary and functional substitution of extrinsic musculature in Solifugae (Arachnida)

The locomotory system of Solifugae is distinct from that of other Arachnida in several ways. Only three pairs of legs are involved in locomotion, while the first pair function as sensory appendages. Morphologically, the proximal region of the locomotory system in Solifugae is characterized by fused coxae. Within the prosoma of Solifugae, an endosternite is missing: in other Arachnida, this endosternite serves as the proximal attachment site for a portion of the extrinsic musculature. How then do these skeletal modifications influence the muscular anatomy in the proximal region of the locomotory system? To answer this question, we studied the skeletomuscular anatomy of Galeodes granti at the interface between the prosoma and legs, reinvestigating the complex muscular anatomy of this body region for the first time in over 80 years and-for the first time-using detailed micro-computed tomography scans to analyze the skeletomuscular morphology. Specimens of three further species were checked for comparison. The analysis revealed differences in the number and composition of coxa-trochanter muscles in each of the four pairs of legs. These are compared in the light of serial homology. The comparison between the proximal locomotory system of Solifugae and that of other Arachnida unveils a series of analogies. Primarily, the coxa-trochanter joint is the most proximal joint to move the leg relative to the prosoma. Therefore, we argue that from a morpho-functional point of view, the coxa-trochanter muscles in Solifugae should be considered secondary extrinsic musculature. Thus, the legs gain a stable, articulated joint in the most proximal region of the leg to the prosoma, which might be advantageous for agile locomotion.

The evolution of feeding within Euchelicerata: data from the fossil groups Eurypterida and Trigonotarbida illustrate possible evolutionary pathways

When the evolution of Euarthropoda is discussed, often the lineage of Chelicerata s. str. is assumed to be the more ‘primitive’ or ‘basal’ part of the tree, especially when compared to the other major lineage, Mandibulata. This claimed primitiveness is (at least partly) based on the assumption that different morphological structures are still in an ancestral state and did not evolve any further. One of these sets of structures is the feeding apparatus, which has been stated to be highly advanced in Mandibulata, but not ‘properly’ developed, or at least not to such a high degree, within Chelicerata s. str. In this study, I reinvestigate the feeding apparatus of different ingroups of Euchelicerata, with a focus on assumed ‘primitive’ groups such as Eurypterida and Trigonotarbida. The basis of this study is a large amount of material from different museum collections, with fossils with the entire feeding apparatuses being exceptionally well preserved. Based on high-resolution micro-photography and three-dimensional imaging, it is possible to resolve fine details of the feeding apparatuses. The results make clear that the feeding apparatuses of different ingroups of Euchelicerata are highly specialised and often possess morphological structures comparable to those of the feeding apparatuses of representatives of Mandibulata, apparently convergently evolved. Though the reconstruction of the evolution of the feeding apparatus within Euchelicerata is to a certain degree hampered by unclear phylogenetic relationships, there was clearly a shortening of the feeding apparatus from posterior (i.e. only the anterior appendages being involved in the feeding apparatus), probably linked to the colonisation of land in Arachnida.

A Cambrian-Ordovician Terrestrialization of Arachnids

Understanding the temporal context of terrestrialization in chelicerates depends on whether terrestrial groups, the traditional Arachnida, have a single origin and whether or not horseshoe crabs are primitively or secondarily marine. Molecular dating on a phylogenomic tree that recovers arachnid monophyly, constrained by 27 rigorously vetted fossil calibrations, estimates that Arachnida originated during the Cambrian or Ordovician. After the common ancestor colonized the land, the main lineages appear to have rapidly radiated in the Cambrian-Ordovician boundary interval, coinciding with high rates of molecular evolution. The highest rates of arachnid diversification are detected between the Permian and Early Cretaceous. A pattern of ancient divergence estimates for terrestrial arthropod groups in the Cambrian while the oldest fossils are Silurian (seen in both myriapods and arachnids) is mirrored in the molecular and fossil records of land plants. We suggest the discrepancy between molecular and fossil evidence for terrestrialization is likely driven by the extreme sparseness of terrestrial sediments in the rock record before the late Silurian.

The innervation of the male copulatory organ of spiders (Araneae) - a comparative analysis

Background

Nervous tissue is an inherent component of the many specialized genital structures for transferring sperm directly into the female’s body. However, the male copulatory organ of spiders was considered a puzzling exception. Based on the recent discovery of nervous tissue in the pedipalps of two distantly related spider species, we investigated representatives of all major groups across the spider tree of life for the presence of palpal nerves. We used a correlative approach that combined histology, micro-computed tomography and electron microscopy.

Results

We show that the copulatory organ is innervated in all species investigated. There is a sensory organ at the base of the sperm transferring sclerite in several taxa and nervous tissue occurs close to the glandular tissue of the spermophor, where sperm are stored before transfer.

Conclusions

The innervation of the copulatory organ by the bulb nerve and associated efferent fibers is part of the ground pattern of spiders. Our findings pave the way for unraveling the sensory interaction of genitalia during mating and for the still enigmatic mode of uptake and release of sperm from the male copulatory organ.

The visual system of Thelyphonida (whip scorpions): Support for Arachnopulmonata

Only a few studies have examined the central visual system of Thelyphonida (whip scorpions) until now. To obtain new insights suitable for phylogenetic analysis we studied the axonal trajectories and neuropil architecture of the central visual systems of two whip scorpion species (Mastigoproctus giganteus and Typopeltis dalyi) with different neuroanatomical techniques (Cobalt fills, Wigglesworth stains, and μCT). The central visual system of whip scorpion comprises one pair of median eyes and one pair of lateral eye triplets. The R-cells (or retinula cells) of both eye types each terminate in a first and a second visual neuropil. Furthermore, a few R-cell fibres from the median eyes leave the second median eye visual neuropil and terminate in the second and the first lateral eye neuropil. This means R-cell terminals from the lateral eyes and the median eyes overlap here. Additionally, the arcuate body and mushroom bodies are described. A detailed comparison of our findings with previously studied chelicerate central visual systems seems to support a monophyly of Arachnopulmonata, i.e. a clade comprising Tetrapulmonata (Thelyphonida, Schizomida, Amblypygi, and Araneae) and Scorpions. Furthermore, the architecture of the central visual systems hints at a close evolutionary relationship of Arachnopulmonata and Xiphosura.

Increasing species sampling in chelicerate genomic-scale datasets provides support for monophyly of Acari and Arachnida

Chelicerates are a diverse group of arthropods, represented by such forms as predatory spiders and scorpions, parasitic ticks, humic detritivores, and marine sea spiders (pycnogonids) and horseshoe crabs. Conflicting phylogenetic relationships have been proposed for chelicerates based on both morphological and molecular data, the latter usually not recovering arachnids as a clade and instead finding horseshoe crabs nested inside terrestrial Arachnida. Here, using genomic-scale datasets and analyses optimised for countering systematic error, we find strong support for monophyletic Acari (ticks and mites), which when considered as a single group represent the most biodiverse chelicerate lineage. In addition, our analysis recovers marine forms (sea spiders and horseshoe crabs) as the successive sister groups of a monophyletic lineage of terrestrial arachnids, suggesting a single colonisation of land within Chelicerata and the absence of wholly secondarily marine arachnid orders.

A Critical Appraisal of the Placement of Xiphosura (Chelicerata) with Account of Known Sources of Phylogenetic Error

Horseshoe crabs (Xiphosura) are traditionally regarded as sister group to the clade of terrestrial chelicerates (Arachnida). This hypothesis has been challenged by recent phylogenomic analyses, but the non-monophyly of Arachnida has consistently been disregarded as artifactual. We re-evaluated the placement of Xiphosura among chelicerates using the most complete phylogenetic data set to date, expanding outgroup sampling, and including data from whole genome sequencing projects. In spite of uncertainty in the placement of some arachnid clades, all analyses show Xiphosura consistently nested within Arachnida as the sister group to Ricinulei (hooded tick spiders). It is apparent that the radiation of arachnids is an old one and occurred over a brief period of time, resulting in several consecutive short internodes, and thus is a potential case for the confounding effects of incomplete lineage sorting (ILS). We simulated coalescent gene trees to explore the effects of increasing levels of ILS on the placement of horseshoe crabs. In addition, common sources of systematic error were evaluated, as well as the effects of fast-evolving partitions and the dynamics of problematic long branch orders. Our results indicated that the placement of horseshoe crabs cannot be explained by missing data, compositional biases, saturation, or ILS. Interrogation of the phylogenetic signal showed that the majority of loci favor the derived placement of Xiphosura over a monophyletic Arachnida. Our analyses support the inference that horseshoe crabs represent a group of aquatic arachnids, comparable to aquatic mites, breaking a long-standing paradigm in chelicerate evolution and altering previous interpretations of the ancestral transition to the terrestrial habitat. Future studies testing chelicerate relationships should approach the task with a sampling strategy where the monophyly of Arachnida is not held as the premise.

Also looking like Limulus? - retinula axons and visual neuropils of Amblypygi (whip spiders)

BACKGROUND

Only a few studies have examined the visual systems of Amblypygi (whip spiders) until now. To get new insights suitable for phylogenetic analysis we studied the axonal trajectories and neuropil architecture of the visual systems of several whip spider species (Heterophrynus elaphus, Damon medius, Phrynus pseudoparvulus, and P. marginemaculatus) with different neuroanatomical techniques. The R-cell axon terminals were identified with Cobalt fills. To describe the morphology of the visual neuropils and of the protocerebrum generally we used Wigglesworth stains and μCT.

RESULTS

The visual system of whip spiders comprises one pair of median and three pairs of lateral eyes. The R-cells of both eye types terminate each in a first and a second visual neuropil. Furthermore, a few R-cell fibres from the median eyes leave the second median eye visual neuropil and terminate in the second lateral eye neuropil. This means R-cell terminals from the lateral eyes and the median eyes overlap. Additionally, the arcuate body and the mushroom bodies are described.

CONCLUSIONS

A detailed comparison of our findings with previously studied chelicerate visual systems (i.e., Xiphosura, Scorpiones, Pseudoscorpiones, Opiliones, and Araneae) seem to support the idea of close evolutionary relationships between Xiphosura, Scorpiones, and Amblypygi.

A Sea Scorpion's Strike: New Evidence of Extreme Lateral Flexibility in the Opisthosoma of Eurypterids

Among the largest and most abundant aquatic predators during much of the early evolution of vertebrates, eurypterids have long been an iconic and intensely studied group of Paleozoic arthropods. We report a new specimen of the eurypterid Slimonia acuminata, which includes a fully articulated series of tail (postabdominal and telsonal) segments preserved in a tight lateral curve. Such a high degree of apparent lateral tail flexibility has not been previously recognized in eurypterids. From the perspective of hydrodynamics, the dorsoventrally flattened body plan of eurypterids would have limited the effectiveness of lateral tail motion as a means of propulsion. However, the long and serrated terminal tail spine of S. acuminata (and other eurypterids) would have made lateral tail strikes-which would have met a minimum of hydraulic resistance-an effective means of predatory attack and self-defense. Thus, many eurypterids are reinterpreted as substantially better-armed predators than previously supposed.

The phylogenetic position of eriophyoid mites (superfamily Eriophyoidea) in Acariformes inferred from the sequences of mitochondrial genomes and nuclear small subunit (18S) rRNA gene

Eriophyoid mites (superfamily Eriophyoidea) comprise >4400 species worldwide. Despite over a century of study, the phylogenetic position of these mites within Acariformes is still poorly resolved. Currently, Eriophyoidea is placed in the order Trombidiformes. We inferred the high-level phylogeny of Acari with the mitochondrial (mt) genome sequences of 110 species including four eriophyoid species, and the nuclear small subunit (18S) rRNA gene sequences of 226 species including 25 eriophyoid species. Maximum likelihood (ML), Bayesian inference (BI) and Maximum parsimony (MP) methods were used to analyze the sequence data. Divergence times were estimated for major lineages of Acari using Bayesian approaches. Our analyses consistently recovered the monophyly of Eriophyoidea but rejected the monophyly of Trombidiformes. The eriophyoid mites were grouped with the sarcoptiform mites, or were the sister group of sarcoptiform mites+non-eriophyoid trombidiform mites, depending on data partition strategies. Eriophyoid mites diverged from other mites in the Devonian (384Mya, 95% HPD, 352-410Mya). The origin of eriophyoid mites was dated to the Permian (262Mya, 95% HPD 230-307Mya), mostly prior to the radiation of gymnosperms (Triassic-Jurassic) and angiosperms (early Cretaceous). We propose that the placement of Eriophyoidea in the order Trombidiformes under the current classification system should be reviewed.

The water-repellent cerotegument of whip-spiders (Arachnida: Amblypygi)

The cuticle of arthropods is usually composed of layers of a chitin-protein-microcomposite, a proteinaceous epicuticle and a thin lipid coating. However, in some instances a thick cement layer (cerotegument) covers the cuticle and may produce elaborate microstructures. This has previously been described for millipedes and mites. Here we report the previously unknown presence of a superhydrophobic cerotegument in whip-spiders (Ambypygi) and reveal its variation in ultrastructure and water-repellence between species. We discuss the relevance of found micro-morphological and physical characters for taxonomy and phylogenetics of this group, and the potential biological functions.

The visual system of harvestmen (Opiliones, Arachnida, Chelicerata) - a re-examination

Background

The visual systems in chelicerates are poorly understood, even though they show strong variation in eye and visual neuropil architecture, thus may provide valuable insights for the understanding of chelicerate phylogeny and eye evolution. Comparable morphological characters are desperately sought for reconstructions of the phylogeny of Chelicerata, especially with respect to Arachnida. So far, reliable data exist only for Pycnogonida, Xiphosura, Scorpiones, and Araneae. The few earlier studies of the organisation of the visual system in harvestmen are contradictory concerning the number, morphology, and position of the visual neuropils.

Results

We undertook a descriptive and comparative analysis of the neuroanatomy of the visual system in several phalangid harvestmen species. Various traditional and modern methods were used that allow comparisons with previous results (cobalt fills, DiI/DiO labelling, osmium ethyl gallate procedure, and TEM). The R-cells (photoreceptor and arhabdomeric cells) in the eyes of Opiliones are linked to a first and a second visual neuropil. The first visual neuropil receives input from all R-cell axons, in the second only few R-cells terminate in the distal part. Hence, the second visual neuropil is subdivided in a part with direct R-cell input and a part without. The arcuate body is located in a subsequent position with direct contact to the second visual neuropil.

Conclusions

This re-examination comes to conclusions different from those of all previous studies. The visual system of phalangid Opiliones occupies an intermediate position between Pycnogonida, Xiphosura, and Scorpiones on the one side, and Araneae on the other side. The projection of the R-cells is similar to that in the former grouping, the general neuropil arrangement to that in the latter taxon. However, more research on the visual systems in other chelicerate orders is needed in order to draw inferences on phylogeny or eye evolution.

Morphology of the tracheal system of camel spiders (Chelicerata: Solifugae) based on micro-CT and 3D-reconstruction in exemplar species from three families

We studied the tracheal system of exemplar species representing three families of Solifugae Sundevall, 1833, i.e., Galeodes granti Pocock, 1903, Ammotrechula wasbaueri Muma, 1962 and Eremobates sp., using μCT-imaging and 3D-reconstruction. This is the first comparative study of the tracheal system of Solifugae in 85 years and the first using high-resolution nondestructive methods. The tracheal system was found to be structurally similar in all three species, with broad major tracheae predominantly in the prosoma as well as anastomoses (i.e., connections between tracheal branches from different stigmata) in the prosoma and opisthosoma. Differences among the three species were observed in the presence or absence of cheliceral air sacs, the number of tracheae supplying the heart, and the ramification of major tracheae in the opisthosoma. The structure of the tracheal system with its extensive branches and some anastomoses is assumed to aid rapid and efficient gas exchange in the respiratory tissues of these active predators. The large diameter of cheliceral tracheae (air sacs) of taxa with disproportionally heavier chelicerae suggests a role in weight reduction, enabling solifuges to reach greater speeds during predation. The air sacs may also permit more rapid and efficient gaseous exchange, necessary to operate the musculature of these structures, thereby improving their use for predation in an environment where prey is scarce.

Histological and ultrastructural analysis of the respiratory tracheae of Galeodes granti (Chelicerata: Solifugae)

Solifuges lack oxygen-carrying proteins like hemocyanins found in other chelicerates. For conduction of gases, therefore, we hypothesize that the tracheal system is divided into convective and respiratory parts, the latter having intimate association with respiring cells, tissues and organs. This hypothesis is supported by studies of tracheae in other arthropods. We used light, scanning, and transmission electron microscopy to examine the tracheae of Galeodes granti (Chelicerata, Solifugae). We studied tracheae in cheliceral and leg muscles, midgut and midgut diverticula, the heart and the supra- and subesophageal ganglia of the central nervous system. In all tissues examined, the tracheae penetrate into tissues and can be found between cells. Terminal branches with diameter less than one micron were found embedded into cell bodies of respiring tissue, thus reducing the diffusion distance. We suggest that the terminal branches are specialized for respiratory function due to their thin intima, with reduced endocuticle and exocuticle. Comparison with other tracheate chelicerates suggests parallel evolution of a common tracheal pattern based on homologous cellular building materials within the chelicerates and arthropods.

New Proposal of Setal Homology in Schizomida and Revision of Surazomus (Hubbardiidae) from Ecuador

The homology of three somatic systems in Schizomida is studied yielding the following results: (1) proposal of homology and chaetotaxy of abdominal setae in Surazomus; (2) revision of the cheliceral chaetotaxy in Schizomida, with suggestion of new homology scheme between Hubbardiidae and Protoschizomidae, description of a new group of setae in Hubbardiinae (G7), and division of setae group 5 in two subgroups, G5A and G5B; (3) proposal of segmental homology between trimerous and tetramerous female flagellum in Hubbardiinae with association of segment III of tri-segmented species to segments III + IV of tetra-segmented species. Considerations about the dorsal microsetae on the male flagellum are made. The genus Surazomus in Ecuador is revised. The sympatric species Surazomus palenque sp. nov. and S. kitu sp. nov. (Ecuador, Pichincha) are described and illustrated. The female of S. cuenca (Rowland and Reddell, 1979) is described, with two new distributional records for the species. Surazomus cumbalensis (Kraus, 1957) is recorded for the first time from Ecuador (Pichincha).

Origin and higher-level diversification of acariform mites - evidence from nuclear ribosomal genes, extensive taxon sampling, and secondary structure alignment

BACKGROUND

Acariformes is the most species-rich and morphologically diverse radiation of chelicerate arthropods, known from the oldest terrestrial ecosystems. It is also a key lineage in understanding the evolution of this group, with the most vexing question whether mites, or Acari (Parasitiformes and Acariformes) is monophyletic. Previous molecular studies recovered Acari either as monophyletic or non-monophyletic, albeit with a limited taxon sampling. Similarly, relationships between basal acariform groups (include little-known, deep-soil 'endeostigmatan' mites) and major lineages of Acariformes (Sarcoptiformes, Prostigmata) are virtually unknown. We infer phylogeny of chelicerate arthropods, using a large and representative dataset, comprising all main in- and outgroups (228 taxa). Basal diversity of Acariformes is particularly well sampled. With this dataset, we conduct a series of phylogenetically explicit tests of chelicerate and acariform relationships and present a phylogenetic framework for internal relationships of acariform mites.

RESULTS

Our molecular data strongly support a diphyletic Acari, with Acariformes as the sister group to Solifugae (PP =1.0; BP = 100), the so called Poecilophysidea. Among Acariformes, some representatives of the basal group Endeostigmata (mainly deep-soil mites) were recovered as sister-groups to the remaining Acariformes (i. e., Trombidiformes + and most of Sarcoptiformes). Desmonomatan oribatid mites (soil and litter mites) were recovered as the monophyletic sister group of Astigmata (e. g., stored product mites, house dust mites, mange mites, feather and fur mites). Trombidiformes (Sphaerolichida + Prostigmata) is strongly supported (PP =1.0; BP = 98-100). Labidostommatina was inferred as the basal lineage of Prostigmata. Eleutherengona (e. g., spider mites) and Parasitengona (e. g., chiggers, fresh water mites) were recovered as monophyletic. By contrast, Eupodina (e. g., snout mites and relatives) was not. Marine mites (Halacaridae) were traditionally regarded as the sister-group to Bdelloidea (Eupodina), but our analyses show their close relationships to Parasitengona.

CONCLUSIONS

Non-trivial relationships recovered by our analyses with high support (i.e., basal arrangement of endeostigmatid lineages, the position of marine mites, polyphyly of Eupodina) had been proposed by previous underappreciated morphological studies. Thus, we update currently the accepted taxonomic classification to reflect these results: the superfamily Halacaroidea Murray, 1877 is moved from the infraorder Eupodina Krantz, 1978 to Anystina van der Hammen, 1972; and the subfamily Erythracarinae Oudemans, 1936 (formerly in Anystidae Oudemans, 1902) is elevated to family rank, Erythracaridae stat. ressur., leaving Anystidae only with the nominal subfamily. Our study also shows that a clade comprising early derivative Endeostigmata (Alycidae, Nanorchestidae, Nematalycidae, and maybe Alicorhagiidae) should be treated as a taxon with the same rank as Sarcoptiformes and Trombidiformes, and the scope of the superfamily Bdelloidea should be changed. Before turning those findings into nomenclatural changes, however, we consider that our study calls for (i) finding shared apomorphies of the early derivative Endeostigmata clade and the clade including the remaining Acariformes; (ii) a well-supported hypothesis for Alicorhagiidae placement; (iii) sampling the families Proterorhagiidae, Proteonematalycidae and Grandjeanicidae not yet included in molecular analyses; (iv) undertake a denser sampling of clades traditionally placed in Eupodina, Anystina (Trombidiformes) and Palaeosomata (Sarcoptiformes), since consensus networks and Internode certainty (IC) and IC All (ICA) indices indicate high levels of conflict in these tree regions. Our study shows that regions of ambiguous alignment may provide useful phylogenetic signal when secondary structure information is used to guide the alignment procedure and provides an R implementation to the Bayesian Relative Rates test.

Fine structure of the male genital system of the predatory mite Rhagidia halophila (Rhagidiidae, Prostigmata, Actinotrichida)

The male genital system of the actinotrichid mite Rhagidia halophila is described and compared with other mites and arachnids. The large testes are composed of germinal and glandular parts and produce numerous small sperm cells. The glandular parts are connected via a testicular bridge. Spermiogenesis occurs in cysts containing spermatids in equal stages of development. Cysts of spermatids are embedded in huge somatic cells. The nuclei of the spermatids loose their envelope. Mature sperm cells are simple exhibiting a ring-shaped chromatin body and lacking an acrosomal complex. They are most similar to the sperm cells of the related mite Linopodes motatorius. The spermatopositor contains the ejaculatory duct divided into a dorsal channel and a ventral channel that are connected via a narrow passage. At its distal end, the spermatopositor is divided into three eugenital lips. The function of the spermatopositor during deposition of the peculiar thread-like spermatophores is discussed. Details of the sensilla of the spermatopositor and the progenital lips are reported. The genital papillae located on the inner side of the progenital lips exhibit characteristics of cells performing transport of ions and/or water. The results confirm the overall similarity of actinotrichid genital systems, which is profoundly different from that of anactinotrichid mites. With reference to other Arachnida it is corroborated that testes and sperm structure of Actinotrichida are most similar to that of Solifugae. However, synapomorphies between sperm cells of Rhagidia and Solifugae that could suggest a closer relationship between these two taxa as was suggested in earlier studies were not recognizable. On the contrary, the sperm cells of Rh. halophila being devoid of an acrosomal complex appeared to be more apomorphic than those of many other actinotrichid mites as well as Solifugae.

An "ancient" complexity? Evolutionary morphology of the circulatory system in Xiphosura

Horseshoe crabs (Xiphosura) have been an object of zoological research for almost 200 years. Although some morphological work on the circulatory system has been done, the three-dimensional structure of this complex organ system has never been shown satisfactorily and some crucial questions remain unanswered. Here, the circulatory systems of juveniles of the horseshoe crab taxa Limulus polyphemus and Carcinoscorpius rotundicauda were investigated using a combination of an injection method and micro-computed tomography. Data were processed and 3D-visualized using reconstruction software. Furthermore, the heart was examined using scanning electron microscopy. Additionally, the histology of some structures was investigated via light microscopy and transmission electron microscopy. The results show the high degree of complexity of the arterial and lacunar systems of Xiphosura and provide insights into their three-dimensional structure and relationship to other organ systems such as the central nervous system. We show that the major lacunae, previously described as vessel-like - though indeed highly ramified - can clearly be distinguished from arteries in histological sections because they have no distinct walls. Similarities and differences between the xiphosuran species and arachnids are highlighted and possible phylogenetic implications and evolutionary scenarios discussed.

Evolutionary biology of harvestmen (Arachnida, Opiliones)

Opiliones are one of the largest arachnid orders, with more than 6,500 species in 50 families. Many of these families have been erected or reorganized in the last few years since the publication of The Biology of Opiliones. Recent years have also seen an explosion in phylogenetic work on Opiliones, as well as in studies using Opiliones as test cases to address biogeographic and evolutionary questions more broadly. Accelerated activity in the study of Opiliones evolution has been facilitated by the discovery of several key fossils, including the oldest known Opiliones fossil, which represents a new, extinct suborder. Study of the group's biology has also benefited from rapid accrual of genomic resources, particularly with respect to transcriptomes and functional genetic tools. The rapid emergence and utility of Phalangium opilio as a model for evolutionary developmental biology of arthropods serve as demonstrative evidence of a new area of study in Opiliones biology, made possible through transcriptomic data.

Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders

Arachnids are an important group of arthropods. They are: diverse and abundant; a major constituent of many terrestrial ecosystems; and possess a deep and extensive fossil record. In recent years a number of exceptionally preserved arachnid fossils have been investigated using tomography and associated techniques, providing valuable insights into their morphology. Here we use X-ray microtomography to reconstruct members of two extinct arachnid orders. In the Haptopoda, we demonstrate the presence of 'clasp-knife' chelicerae, and our novel redescription of a member of the Phalangiotarbida highlights leg details, but fails to resolve chelicerae in the group due to their small size. As a result of these reconstructions, tomographic studies of three-dimensionally preserved fossils now exist for three of the four extinct orders, and for fossil representatives of several extant ones. Such studies constitute a valuable source of high fidelity data for constructing phylogenies. To illustrate this, here we present a cladistic analysis of the chelicerates to accompany these reconstructions. This is based on a previously published matrix, expanded to include fossil taxa and relevant characters, and allows us to: cladistically place the extinct arachnid orders; explicitly test some earlier hypotheses from the literature; and demonstrate that the addition of fossils to phylogenetic analyses can have broad implications. Phylogenies based on chelicerate morphology-in contrast to molecular studies-have achieved elements of consensus in recent years. Our work suggests that these results are not robust to the addition of novel characters or fossil taxa. Hypotheses surrounding chelicerate phylogeny remain in a state of flux.