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Recent arachnid phylogenies support an Arachnopulmonata clade including scorpions, pseudoscorpions and the tetrapulmonate arachnids (i.e. spiders and their closest relatives). The position of the other arachnids is less certain, with molecular data suggesting that Arachnida may be paraphyletic with respect to horseshoe crabs. Here we explore the potential role fossil data can play in this debate. We outline the beneficial aspects of including fossils in phylogenies -- fossils calibrate trees to time -- as well as the challenges integrating these data. We tabulate the oldest occurrences of all major groups and superimpose these on recent phylogenetic hypotheses. Given that a key question is when (and how often) arachnids moved from water onto land, we review the early plant fossil record as a framework for when arthropod life on land may have been viable. In light of the aquatic ecology of horseshoe crabs, we then discuss the implications of placing this group within the arachnids, especially since some extinct lineages differ substantially from living species. In this context we re-assess what a horseshoe crab is from a palaeontological perspective, and speculate that some traditional Xiphosura fossils may actually lie on other parts of the euchelicerate tree. The oldest unequivocal horseshoe crabs are Ordovician in age (ca. 480 Ma), and probably predate complex terrestrial ecosystems. We conclude that recent phylogenetic results are best reconciled with fossils by inferring multiple terrestrialization events, possibly involving quite different approaches to breathing air. The lung-bearing (arachnopulmonate) branch of the tree is well resolved. Future work should focus on the apulmonate arachnids, and integrate the various early horseshoe-crab-like fossils into chelicerate phylogeny. [ABSTRACT FROM AUTHOR]

Pterygotid eurypterids (Chelicerata) were all regarded as active apex nektonic predators, particularly in the Silurian. The chelicerae and lateral eyes of Acutiramus cummingsi were then interpreted as being inconsistent with a predatory lifestyle. Jaekelopterus, Pterygotus and Erettopterus were interpreted as having more acute vision than Acutiramus, but their chelicerae implied they had different ecologies, with not all taxa as top predators. The chelicerae of Acutiramus are here interpreted as being much more robust than was previously assumed, and their eyes as adapted to hunting at depth, at night or in murky water, so neither precludes a predatory lifestyle. Previous models of the mode of life of pterygotid eurypterids are assessed, based on a review of their trace fossils, functional morphology and a new analysis of their biotic associations. All pterygotid taxa are here interpreted as slow swimming vagrant and ambush predators, using their chelicerae to rapidly capture prey, and their more robust coxal gnathobases and metastoma to process it. Slimonia, and Acutiramus with more gracile cutting chelicerae, tend to associate with, and likely specialised on, lightly-armoured phyllocarid crustaceans. Erettopterus and Pterygotus, with more robust chelicerae, tend to associate with more heavily armoured thelodonts and osteostracans, respectively. Jaekelopterus tend to associate with osteicthyans, placoderms and pteraspids. Praedichnia (predation traces) on the pteraspids Lechriaspis and Larnovaspis, eurypterids Eurypterus and Acutiramus and trilobite Spinisscutellum, and their coprolites, provide further evidence for such predatory interactions. The cheliceral morphology, visual acuity, associations and fossil record all support the hypothesis that Acutiramus is actually more basal to Jaekelopterus and Pterygotus, the largest ever arthropods, with Jaekelopterus now estimated at 2.59 m long. [ABSTRACT FROM AUTHOR]

The first Palaeozoic spider (Arachnida: Araneae) from Germany is described as Arthrolycosa wolterbeeki sp. nov. and comes from the late Carboniferous (Moscovian) strata of Piesberg near Osnabrück in Lower Saxony. Characteristic for the genus, the new fossil reveals a posteriorly tuberculate dorsal opisthosoma, and relatively elongate and setose legs in which the first leg is longer than the second and third legs. Spinnerets are also preserved, confirming its status as a genuine spider rather than being part of an extinct spider-like arachnid lineage. Carboniferous spiders in general are rather rare. While modern spiders are a megadiverse group, the published fossil record suggests they were only moderately diverse in the late Carboniferous compared to other Coal Measures arachnids. Part of the explanation must be that major radiations within Araneae occurred later, probably during the Mesozoic. If the lifestyle of the Carboniferous fossils was similar to that of modern mesothele spiders their rarity as fossils might also relate to them spending much of their lives in burrows, or similar retreats, with limited opportunities for preservation. [ABSTRACT FROM AUTHOR]

The mitochondrial genome of a golden orb-web spider Trichonephila clavata (L. Koch, 1878) from South Korea is determined and characterized in detail, which is the second mitochondrial genome reported from this species: the first was published from the Chinese sample by Pan et al. (). It was 14,436 bp in length being composed of 13 protein-coding genes (PCGs), 22 transfer RNA genes, two ribosomal RNA genes, and one control region (CR). It has a base composition of 35.99% for ‘A,’ 14.88% for ‘G,’ 9.09% for ‘C,’ and 40.04% for ‘T.’ Comparing the South Korean and Chinese mitochondrial genomes, we observed 8% nucleotide sequence differences between their CRs, caused by the different numbers and sorts of possessed tandem repeats, suggesting a promising molecular marker to distinguish South Korean individuals from Chinese ones. The phylogenetic trees using the maximum likelihood (ML) method were reconstructed with nucleotides (without 3rd codon position) and amino acids from 13 PCGs, respectively, which consistently confirmed that T. clavata (Subfamily Nephilinae) from South Korea and China are clustered together, distinctly separated from the other subfamily Araneinae in the monophyletic family Araneidae.

Simple Summary: Pain is an unpleasant emotional state that produces behavioural changes to minimize future tissue damage and promote recovery and survival. These behavioural changes have been demonstrated in crustaceans, insects, and, to a lesser extent, spiders. Other arthropod groups have received little attention with respect to pain. The examination of situations in which individuals might attempt to cause pain in order to manipulate others might offer new opportunities for research into pain in arthropods. For example, defensive venom, traumatic mating, and fighting might inflict pain. This might benefit the animal causing the pain and result in a cost to the animal in pain. Pain in response to tissue damage functions to change behaviour so that further damage is minimised whereas healing and survival are promoted. This paper focuses on the behavioural criteria that match the function to ask if pain is likely in the main taxa of arthropods. There is evidence consistent with the idea of pain in crustaceans, insects and, to a lesser extent, spiders. There is little evidence of pain in millipedes, centipedes, scorpions, and horseshoe crabs but there have been few investigations of these groups. Alternative approaches in the study of pain are explored and it is suggested that studies on traumatic mating, agonistic interactions, and defensive venoms might provide clues about pain. The evolution of high cognitive ability, sensory systems, and flexible decision-making is discussed as well as how these might influence the evolution of pain-like states. [ABSTRACT FROM AUTHOR]

Abstract Background The process of early development varies across the species-rich phylum Arthropoda. Owing to the limited research strategies for dissecting lineage-specific processes of development in arthropods, little is known about the variations in early arthropod development at molecular resolution. The Theridiidae spider, Parasteatoda tepidariorum, has its genome sequenced and could potentially contribute to dissecting early embryonic processes. Results We present genome-wide identification of candidate genes that exhibit locally restricted expression in germ disc forming stage embryos of P. tepidariorum, based on comparative transcriptomes of isolated cells from different regions of the embryo. A subsequent pilot screen by parental RNA interference identifies three genes required for body axis formation. One of them is a GATA-like gene that has been fast evolving after duplication and divergence from a canonical GATA family gene. This gene is designated fuchi nashi (fuchi) after its knockdown phenotypes, where the cell movement toward the formation of a germ disc was reversed. fuchi expression occurs in cells outside a forming germ disc and persists in the endoderm. Transcriptome and chromatin accessibility analyses of fuchi pRNAi embryos suggest that early fuchi activity regulates chromatin state and zygotic gene activation to promote endoderm specification and pattern formation. We also show that there are many uncharacterized genes regulated by fuchi. Conclusions Our genome-based research using an arthropod phylogenetically distant from Drosophila identifies a lineage-specific, fast-evolving gene with key developmental roles in one of the earliest, genome-wide regulatory events, and allows for molecular exploration of the developmental variations in early arthropod embryos.

Hibbertopterus lamsdelli sp. nov., from the Late Carboniferous Kinney Quarry Lagerstätte of New Mexico (USA), is a large (ca. 1.1 m long) stylonurid eurypterid (sea-scorpion; Chelicerata), similar to H. scouleri from Scotland but with less serrate segment margins, a wider pretelson, shorter telson (tail-spine), and more parallel ventral keels. It is only the fourth, yet most reliable record of an American hibbertopterid. A taxonomic reassessment of Hibbertopterus regards Dunsopterus and Vernonopterus (but not Cyrtoctenus) as synonyms. Hibbertopterids were aquatic (benthic) scavengers and microphagous sweep-feeders, but their trackways indicate that they were capable of brief terrestrial, seasonal nuptial walks, despite their large size; Hibbertopterus had walking legs with spinose extensions at the base (Laden) to spread their load, and the ventral keels on their telson functioned like sled rails to reduce body drag. Hibbertopterids were interpreted as moving into freshwater during the Late Palaeozoic, but a trackway from the middle Permian Collingham Formation (Ecca Group) of South Africa may be from a marine setting, though further analysis is needed to fully evaluate this possibility. [ABSTRACT FROM AUTHOR]

We describe a new sea spider species, Pantopipetta hosodai sp. nov., based on one juvenile female collected from a submarine cave ('Akumanoyakata' Cave) in Shimojijima Island, Miyako Island Group, Ryukyu Islands, southwestern Japan. It was collected from the second slope zone of the cave, 80–100 m from the entrance, no light, low salinity and with rocky substrate. This is the first record globally of a Pantopipetta species from a submarine cave and anchialine environment. Pantopipetta hosodai sp. nov. resembles Pantopipetta auxiliata , Pantopipetta lenis and Pantopipetta oculata in having auxiliary claws, but differs from them in having a palp with three short distal articles, lateral processes without dorsodistal tubercles, coxae 1 and 3 of legs 1–3 each with one long dorsal tubercle and one dorsodistal tubercle bearing a seta on each femur. Features of the palp appear to delineate two species groups in Pantopipetta , i.e. (1) those having four small distal articles, and a small, basal palp article between the lateral cephalon process and longest palp article (eight-articulate palp) and (2) those having three small distal articles, and lacking the small basal article (six-articulate palp), but further detail examination of the described species is needed. We discuss the diagnostic characters separating Pantopipetta and Austrodecus and the generic affiliation of Austrodecus aconae. Few pycnogonids from marine or anchialine caves have been identified to species, and it is generally unknown whether cave-dwelling pycnogonids tend to be troglobites. [ABSTRACT FROM AUTHOR]

Background : The process of early development varies across the species-rich phylum Arthropoda. Owing to the limited research strategies for dissecting lineage-specific processes of development in arthropods, little is known about the variations in early arthropod development at molecular resolution. The Theridiidae spider, Parasteatoda tepidariorum, has its genome sequenced and could potentially contribute to dissecting early embryonic processes. Results: We present genome-wide identification of candidate genes that exhibit locally restricted expression in germ disc forming stage embryos of P. tepidariorum, based on comparative transcriptomes of isolated cells from different regions of the embryo. A subsequent pilot screen by parental RNA interference identifies three genes required for body axis formation. One of them is a GATA-like gene that has been fast evolving after duplication and divergence from a canonical GATA family gene. This gene is designated fuchi nashi (fuchi) after its knockdown phenotypes, where the cell movement toward the formation of a germ disc was reversed. fuchi expression occurs in cells outside a forming germ disc and persists in the endoderm. Transcriptome and chromatin accessibility analyses of fuchi pRNAi embryos suggest that early fuchi activity regulates chromatin state and zygotic gene activation to promote endoderm specification and pattern formation. We also show that there are many uncharacterized genes regulated by fuchi. Conclusions: Our genome-based research using an arthropod phylogenetically distant from Drosophila identifies a lineage-specific, fast-evolving gene with key developmental roles in one of the earliest, genome-wide regulatory events, and allows for molecular exploration of the developmental variations in early arthropod embryos. [ABSTRACT FROM AUTHOR]

The formation of extraembryonic membranes (EEMs) contributes to the proper development of many animals. In arthropods, the formation and function of EEMs have been studied best in insects. Regarding the development of extraembryonic tissue in chelicerates (spiders and relatives), most information is available for spiders (Araneae). Especially two populations of cells have been considered to represent EEMs in spiders. The first of these potential EEMs develops shortly after egg deposition, opposite to a radially symmetrical germ disc that forms in one hemisphere of the egg and encloses the yolk. The second tissue, which has been described as being extraembryonic is the so-called dorsal field, which is required to cover the dorsal part of the developing spider germ rudiment before proper dorsal closure. In this review, we summarize the current knowledge regarding the formation of potential extraembryonic structures in the Chelicerata. We describe the early embryogenesis of spiders and other chelicerates, with a special focus on the formation of the potential extraembryonic tissues. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'. [ABSTRACT FROM AUTHOR]

Abstract Background The comparative embryology of Chelicerata has greatly advanced in recent years with the integration of classical studies and genetics, prominently spearheaded by developmental genetic works in spiders. Nonetheless, the understanding of the evolution of development and polarization of embryological characters in Chelicerata is presently limited, as few non-spider species have been well studied. A promising focal species for chelicerate evo-devo is the daddy-long-legs (harvestman) Phalangium opilio, a member of the order Opiliones. Phalangium opilio, breeds prolifically and is easily accessible in many parts of the world, as well as tractable in a laboratory setting. Resources for this species include developmental transcriptomes, a draft genome, and protocols for RNA interference, but a modern staging system is critically missing for this emerging model system. Results We present a staging system of P. opilio embryogenesis that spans the most important morphogenetic events with respect to segment formation, appendage elongation and head development. Using time-lapse imaging, confocal microscopy, colorimetric in situ hybridization, and immunohistochemistry, we tracked the development of synchronous clutches from egg laying to adulthood. We describe key events in segmentation, myogenesis, neurogenesis, and germ cell formation. Conclusion Considering the phylogenetic position of Opiliones and the unduplicated condition of its genome (in contrast to groups like spiders and scorpions), this species is poised to serve as a linchpin for comparative studies in arthropod development and genome evolution. The staging system presented herein provides a valuable reference for P. opilio that we anticipate being useful to the arthropod evo-devo community, with the goal of revitalizing research in the comparative development of non-spider arachnids.

Abstract Background Pycnogonida (sea spiders) is the sister group of all other extant chelicerates (spiders, scorpions and relatives) and thus represents an important taxon to inform early chelicerate evolution. Notably, phylogenetic analyses have challenged traditional hypotheses on the relationships of the major pycnogonid lineages (families), indicating external morphological traits previously used to deduce inter-familial affinities to be highly homoplastic. This erodes some of the support for phylogenetic information content in external morphology and calls for the study of additional data classes to test and underpin in-group relationships advocated in molecular analyses. In this regard, pycnogonid internal anatomy remains largely unexplored and taxon coverage in the studies available is limited. Results Based on micro-computed X-ray tomography and 3D reconstruction, we created a comprehensive atlas of in-situ representations of the central nervous system and midgut layout in all pycnogonid families. Beyond that, immunolabeling for tubulin and synapsin was used to reveal selected details of ganglionic architecture. The ventral nerve cord consistently features an array of separate ganglia, but some lineages exhibit extended composite ganglia, due to neuromere fusion. Further, inter-ganglionic distances and ganglion positions relative to segment borders vary, with an anterior shift in several families. Intersegmental nerves target longitudinal muscles and are lacking if the latter are reduced. Across families, the midgut displays linear leg diverticula. In Pycnogonidae, however, complex multi-branching diverticula occur, which may be evolutionarily correlated with a reduction of the heart. Conclusions Several gross neuroanatomical features are linked to external morphology, including intersegmental nerve reduction in concert with trunk segment fusion, or antero-posterior ganglion shifts in partial correlation to trunk elongation/compaction. Mapping on a recent phylogenomic phylogeny shows disjunct distributions of these traits. Other characters show no such dependency and help to underpin closer affinities in sub-branches of the pycnogonid tree, as exemplified by the tripartite subesophageal ganglion of Pycnogonidae and Rhynchothoracidae. Building on this gross anatomical atlas, future studies should now aim to leverage the full potential of neuroanatomy for phylogenetic interrogation by deciphering pycnogonid nervous system architecture in more detail, given that pioneering work on neuron subsets revealed complex character sets with unequivocal homologies across some families.

The mitochondrial genome of a golden orb-web spider Trichonephila clavata (L. Koch, 1878) from South Korea is determined and characterized in detail, which is the second mitochondrial genome reported from this species: the first was published from the Chinese sample by Pan et al. (2016). It was 14,436 bp in length being composed of 13 protein-coding genes (PCGs), 22 transfer RNA genes, two ribosomal RNA genes, and one control region (CR). It has a base composition of 35.99% for 'A,' 14.88% for 'G,' 9.09% for 'C,' and 40.04% for 'T.' Comparing the South Korean and Chinese mitochondrial genomes, we observed 8% nucleotide sequence differences between their CRs, caused by the different numbers and sorts of possessed tandem repeats, suggesting a promising molecular marker to distinguish South Korean individuals from Chinese ones. The phylogenetic trees using the maximum likelihood (ML) method were reconstructed with nucleotides (without 3rd codon position) and amino acids from 13 PCGs, respectively, which consistently confirmed that T. clavata (Subfamily Nephilinae) from South Korea and China are clustered together, distinctly separated from the other subfamily Araneinae in the monophyletic family Araneidae. [ABSTRACT FROM AUTHOR]

Background: The sea spiders (Pycnogonida Latreille, 1810) of the Hunsrück Slate (Lower Devonian, ~400 million years ago) are iconic in their abundance, exquisite pyritic preservation, and in their distinctive body plan compared to extant sea spiders (Pantopoda Gerstäcker, 1863). Consequently, the Hunsrück sea spiders are important in understanding the deep evolutionary history of Pycnogonida, yet they remain poorly characterised, impacting upon attempts to establish a time-calibrated phylogeny of sea spiders., Methods: Here, we investigated previously described and new material representing four of the five Hunsrück pycnogonids: Flagellopantopus blocki Poschmann & Dunlop, 2006; Palaeoisopus problematicus Broili, 1928; Palaeopantopus maucheri Broili, 1929; and Pentapantopus vogteli Kühl, Poschmann & Rust, 2013; as well as a few unidentified specimens. Using X-ray microtomography and Reflectance Transformation Imaging, we describe new fossils, provide evidence for newly revealed anatomical features, and interpret these data in comparison to extant species. We also reinterpret the previously published illustration of the (probably lost) holotype of Palaeothea devonica Bergström, Stürmer & Winter, 1980., Results: We provide the first detailed description of the cephalic appendages of Palaeoisopus problematicus and revise the interpretation of the organisation of its ocular tubercle. Furthermore, we provide new insights into the structure of the legs and the proboscis of Palaeopantopus maucheri , the first description of the body of Flagellopantopus blocki and describe a new specimen of Pentapantopus vogteli , demonstrating that it had eight legs, in contrast to previous interpretations. We argue that, contrary to previous suggestions, Palaeothea devonica probably had a different body plan from extant pantopods. We discuss the ecological traits of the Hunsrück pycnogonids based on their morphological adaptations, and conclude that there is no compelling evidence of Pantopoda in the Devonian. Through comparative interpretation of the legs as well as general morphology, we can divide the Hunsrück pycnogonids into two morphological groups, while Pantopoda constitutes a third morphological group., Competing Interests: The authors declare that they have no competing interests., (© 2024 Sabroux et al.)

Pain in response to tissue damage functions to change behaviour so that further damage is minimised whereas healing and survival are promoted. This paper focuses on the behavioural criteria that match the function to ask if pain is likely in the main taxa of arthropods. There is evidence consistent with the idea of pain in crustaceans, insects and, to a lesser extent, spiders. There is little evidence of pain in millipedes, centipedes, scorpions, and horseshoe crabs but there have been few investigations of these groups. Alternative approaches in the study of pain are explored and it is suggested that studies on traumatic mating, agonistic interactions, and defensive venoms might provide clues about pain. The evolution of high cognitive ability, sensory systems, and flexible decision-making is discussed as well as how these might influence the evolution of pain-like states.

Background: Pycnogonida (sea spiders) is the sister group of all other extant chelicerates (spiders, scorpions and relatives) and thus represents an important taxon to inform early chelicerate evolution. Notably, phylogenetic analyses have challenged traditional hypotheses on the relationships of the major pycnogonid lineages (families), indicating external morphological traits previously used to deduce inter-familial affinities to be highly homoplastic. This erodes some of the support for phylogenetic information content in external morphology and calls for the study of additional data classes to test and underpin in-group relationships advocated in molecular analyses. In this regard, pycnogonid internal anatomy remains largely unexplored and taxon coverage in the studies available is limited. Results: Based on micro-computed X-ray tomography and 3D reconstruction, we created a comprehensive atlas of in-situ representations of the central nervous system and midgut layout in all pycnogonid families. Beyond that, immunolabeling for tubulin and synapsin was used to reveal selected details of ganglionic architecture. The ventral nerve cord consistently features an array of separate ganglia, but some lineages exhibit extended composite ganglia, due to neuromere fusion. Further, inter-ganglionic distances and ganglion positions relative to segment borders vary, with an anterior shift in several families. Intersegmental nerves target longitudinal muscles and are lacking if the latter are reduced. Across families, the midgut displays linear leg diverticula. In Pycnogonidae, however, complex multi-branching diverticula occur, which may be evolutionarily correlated with a reduction of the heart. Conclusions: Several gross neuroanatomical features are linked to external morphology, including intersegmental nerve reduction in concert with trunk segment fusion, or antero-posterior ganglion shifts in partial correlation to trunk elongation/compaction. Mapping on a recent phylogenomic phylogeny shows disjunct distributions of these traits. Other characters show no such dependency and help to underpin closer affinities in sub-branches of the pycnogonid tree, as exemplified by the tripartite subesophageal ganglion of Pycnogonidae and Rhynchothoracidae. Building on this gross anatomical atlas, future studies should now aim to leverage the full potential of neuroanatomy for phylogenetic interrogation by deciphering pycnogonid nervous system architecture in more detail, given that pioneering work on neuron subsets revealed complex character sets with unequivocal homologies across some families. [ABSTRACT FROM AUTHOR]

Background: The comparative embryology of Chelicerata has greatly advanced in recent years with the integration of classical studies and genetics, prominently spearheaded by developmental genetic works in spiders. Nonetheless, the understanding of the evolution of development and polarization of embryological characters in Chelicerata is presently limited, as few non-spider species have been well studied. A promising focal species for chelicerate evo-devo is the daddy-long-legs (harvestman) Phalangium opilio, a member of the order Opiliones. Phalangium opilio, breeds prolifically and is easily accessible in many parts of the world, as well as tractable in a laboratory setting. Resources for this species include developmental transcriptomes, a draft genome, and protocols for RNA interference, but a modern staging system is critically missing for this emerging model system. Results: We present a staging system of P. opilio embryogenesis that spans the most important morphogenetic events with respect to segment formation, appendage elongation and head development. Using time-lapse imaging, confocal microscopy, colorimetric in situ hybridization, and immunohistochemistry, we tracked the development of synchronous clutches from egg laying to adulthood. We describe key events in segmentation, myogenesis, neurogenesis, and germ cell formation. Conclusion: Considering the phylogenetic position of Opiliones and the unduplicated condition of its genome (in contrast to groups like spiders and scorpions), this species is poised to serve as a linchpin for comparative studies in arthropod development and genome evolution. The staging system presented herein provides a valuable reference for P. opilio that we anticipate being useful to the arthropod evo-devo community, with the goal of revitalizing research in the comparative development of non-spider arachnids. [ABSTRACT FROM AUTHOR]

Deciphering the evolutionary relationships of Chelicerata (arachnids, horseshoe crabs, and allied taxa) has proven notoriously difficult, due to their ancient rapid radiation and the incidence of elevated evolutionary rates in several lineages. Although conflicting hypotheses prevail in morphological and molecular data sets alike, the monophyly of Arachnida is nearly universally accepted, despite historical lack of support in molecular data sets. Some phylotranscriptomic analyses have recovered arachnid monophyly, but these did not sample all living orders, whereas analyses including all orders have failed to recover Arachnida. To understand this conflict, we assembled a data set of 506 high-quality genomes and transcriptomes, sampling all living orders of Chelicerata with high occupancy and rigorous approaches to orthology inference. Our analyses consistently recovered the nested placement of horseshoe crabs within a paraphyletic Arachnida. This result was insensitive to variation in evolutionary rates of genes, complexity of the substitution models, and alternative algorithmic approaches to species tree inference. Investigation of sources of systematic bias showed that genes and sites that recover arachnid monophyly are enriched in noise and exhibit low information content. To test the impact of morphological data, we generated a 514-taxon morphological data matrix of extant and fossil Chelicerata, analyzed in tandem with the molecular matrix. Combined analyses recovered the clade Merostomata (the marine orders Xiphosura, Eurypterida, and Chasmataspidida), but merostomates appeared nested within Arachnida. Our results suggest that morphological convergence resulting from adaptations to life in terrestrial habitats has driven the historical perception of arachnid monophyly, paralleling the history of numerous other invertebrate terrestrial groups. [ABSTRACT FROM AUTHOR]

Three morphological characters of eurypterids (Arthropoda, Chelicerata) have been suggested as potentially affecting the ease with which this group experienced ecdysis and, therefore, the likelihood of mortality during moulting: (1) the presence or absence of appendages bearing numerous or long spines; (2) the width of the swimming appendage; and (3) the type of prosoma ventral plate. If ecological processes have evolutionary consequences, the proposed relationship between eurypterid morphology and differential success during ecdysis might be reflected in differential longevity of species with these different morphologies. This hypothesis can be tested using stratigraphical duration data (longevity) of eurypterid species. Results are suggestive but not conclusive: species of the streamlined, non‐spinose stylonurid body type, presumed to facilitate ecdysis, have the longest average duration, species of the spinose megalograptid type are the shortest persisting. Members of these two morphotypes also differ in other ecological characters such as environment and feeding habit, which may also contribute to differences in species longevity. More fine‐grained, species‐level stratigraphical, morphological and palaeoecological data are needed to test the proposed relationships between eurypterid morphology, ecdysial habit and species duration; this initial contribution provides a rationale for future investigation. [ABSTRACT FROM AUTHOR]

Chelicerata, the second largest subphylum of Arthropoda, includes invertebrates with a wide range of body size. Pseudoscorpions are among small or miniature chelicerates which exhibit several morphological, anatomical, and developmental features related to miniaturization, e.g., replacement of book lungs by tracheae, unpaired gonads, and matrotrophic development of the embryos outside the female body, in the brood sac. In this paper, we show the ovary structure of two pseudoscorpion species, Cheiridium museorum and Apocheiridium ferum (Cheiridiidae). Both cheiridiids are one of the smallest pseudoscorpions. The results of our observations conducted in light, transmission electron, and confocal microscopy demonstrate that the ovary of C. museorum and A. ferum, displays a significant structural difference that is unusual for chelicerates. The difference concerns the spatially restricted position of the germarium. We show that such ovary architecture results in a significantly reduced number of growing oocytes and in consequence a reduced number of deposited eggs. A centrally located germarium implies also a modified pattern of ovary development during oocyte growth due to long distance migration of the germline and the accompanying somatic cells. Herein, we postulate that such an ovary structure is related to the pseudoscorpion's small body size and it is a step towards miniaturization in the smaller pseudoscorpions species. [ABSTRACT FROM AUTHOR]

Vestigial organs provide a link between ancient and modern traits and therefore have great potential to resolve the phylogeny of contentious fossils that bear features not seen in extant species. Here we show that extant daddy-longlegs (Arachnida, Opiliones), a group once thought to possess only one pair of eyes, in fact additionally retain a pair of vestigial median eyes and a pair of vestigial lateral eyes. Neuroanatomical gene expression surveys of eye-patterning transcription factors, opsins, and other structural proteins in the daddy-longlegs Phalangium opilio show that the vestigial median and lateral eyes innervate regions of the brain positionally homologous to the median and lateral eye neuropils, respectively, of chelicerate groups like spiders and horseshoe crabs. Gene silencing of eyes absent shows that the vestigial eyes are under the control of the retinal determination gene network. Gene silencing of dachshund disrupts the lateral eyes, but not the median eyes, paralleling loss-of-function phenotypes in insect models. The existence of lateral eyes in extant daddy-longlegs bears upon the placement of the oldest harvestmen fossils, a putative stem group that possessed both a pair of median eyes and a pair of lateral eyes. Phylogenetic analysis of harvestman relationships with an updated understanding of lateral eye incidence resolved the four-eyed fossil group as a member of the extant daddy-longlegs suborder, which in turn resulted in older estimated ages of harvestman diversification. This work underscores that developmental vestiges in extant taxa can influence our understanding of character evolution, placement of fossils, and inference of divergence times. • Neuroanatomical and molecular characterization of eye development in daddy-longlegs • A living daddy-longlegs possesses six eyes, including two vestigial pairs • Innervation patterns to the brain reflect chelicerate ancestral architecture • Lateral eyes in living harvestmen change phylogenetic position of four-eyed fossils Gainett et al. report that living daddy-longlegs, previously thought to have only two eyes, have additional vestigial eyes only seen in four-eyed fossil relatives. This discovery changes the phylogenetic position of the fossils and pushes back age estimates of this ancient group. These findings underscore the role of vestigial organs in evolution. [ABSTRACT FROM AUTHOR]

Abstract Background Arachnids possess highly specialized and unorthodox sense organs, such as the unique pectines of Scorpiones and the malleoli of Solifugae. While the external morphology, numbers, and shapes of sensory organs are widely used in taxonomic studies, little is known about the internal anatomy of these organs and their associated processing neuropils in the central nervous system. Camel spiders (Solifugae) possess pedipalps and first walking legs heavily endowed with sensory structures, as well as conspicuous malleoli located ventrally on the proximal fourth walking legs. Malleoli are fan-shaped organs that contain tens of thousands of presumptive chemoreceptor neurons, but mechanoreceptive structures are absent. Results Here, we examine the organization of the synganglion based on microCT analysis, 3D reconstruction of serial paraffin sections, and backfill preparations to trace the malleolar pathway. The projection area of malleolar afferents is intriguingly located in the most anterior ventral nerve cord, located in between the pedipalpal neuromere hemispheres. However, malleolar axon bundles are separated by a thin soma layer that points to an anteriad projection of the fourth walking leg neuromere. A conspicuous projection neuron tract that may receive additional input from pedipalpal sensory organs connects the malleolar neuropil with the mushroom bodies in the protocerebrum. Conclusion Arthropod chemosensory appendages or organs and primary processing neuropils are typically located in the same segment, which also holds true in Solifugae, although the malleolar neuropil is partially shifted towards the pedipalpal neuromere. A comparison of the malleoli in Solifugae and the pectines in Scorpiones, and of their primary processing neuropils, reveals certain similarities, while striking differences are also evident. Similarities include the ventral arrangement of peg-shaped sensory structures on the respective segmental appendage, exposing dense arrays of chemoreceptive sensilla, and projections to a primary processing neuropil with glomerular subdivision. Differences are, e.g., the lack of mechanoreceptive afferents and an associated processing neuropil.

Abstract 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.

BACKGROUND: The southern cattle tick, Rhipicephalus microplus, is a primary vector of the deadly bovine disease babesiosis. Worldwide populations of ticks have developed resistance to acaricides, underscoring the need for novel target discovery for tick control. The arthropod‐specific R. microplus kinin receptor is such a target, previously validated by silencing, which resulted in female reproductive fitness costs, including a reduced percentage of eggs hatching. RESULTS: In order to identify potent small molecules that bind and activate or inhibit the kinin receptor, a high‐throughput screening (HTS) assay was developed using a CHO‐K1 cell line expressing the recombinant tick kinin receptor (BMLK3). A total of ~20 000 molecules from a random in‐house small molecule library were screened in a 'dual‐addition' calcium fluorescence assay. This was followed by dose–response validation of the hit molecules identified both from HTS and an in silico screen of ~390 000 molecules. We validated 29 antagonists, 11 of them were full antagonists with IC50 values between 0.67 and 8 μmol L–1. To explore the structure–activity relationships (SAR) of the small molecules, we tested the activities of seven analogs of the most potent identified antagonist, additionally discovering three full antagonists and four partial antagonists. These three potent antagonists (IC50 < 3.2 μmol L–1) were validated in vitro using the recombinant mosquito kinin receptor and showed similar antagonistic activities. In vivo, these three compounds also inhibited the mosquito hindgut contraction rate induced by a myotropic kinin agonist analog 1728. CONCLUSION: Antagonists identified in this study could become pesticide leads and are reagents for probing the kinin signaling system. © 2021 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The paper aims to elucidate the physico-chemical characteristics of the shell of mangrove horseshoe crabs (Carcinoscorpius rotundicauda) and determine the compilation matrix for the first time. The shell composition matrix of C. rotundicauda has never been studied in detail before, especially the shape of the foam, the chemical composition, the functional groups and the mechanical-physical and thermal properties of the shell. Based on this study, the shell structure of the mangrove horseshoe crab has the potential to be used as the base structure for developing bio-foam insulator material in the future. Therefore, the shell of mangrove horseshoe crabs has a unique natural structure in the form of foam. Its robust and elastic structure has the potential for further development for new marine biomaterials. The formation and composition of horseshoe crab shells foam are also believed to be multifunctional in mobility, used for defense mechanisms and thermal stability. The horseshoe crab samples were collected from Pacitan coastal waters, East Java, Indonesia. The research was conducted using physico-chemical and mechanical-physical analysis. The scanning electron microscopy was used in order to clarify the physico-chemical characteristics. The measurements of the mechanical-physical characteristics included density, unit cell size, and water absorption. The tensile strength and compressive strength were analyzed based on the American Society for Testing Material. Thermal resistance was measured by thermal gravimetric analysis. The results showed that the horseshoe crab shells have a unique structure, where chitin, protein and some minerals are the main chemical elements. The combination and major constituents of the horseshoe crab shell material provide strong and plastic mechanical properties with a maximum tensile strength of 60.46 kPa and maximum compressive strength of 110.55 kPa, water absorption of 0.01195 ± 0.001% and a density value of 0.1545 ± 0.011 g/cm3 as well as the capability to withstand thermal loads with peak decomposition values of 267.4–823.2°C and thermal stability of 60.59%. Using natural marine biomaterials in the future will be beneficial because it leaves no harmful residues and therefore has environmental advantages and at the same time, it is also more cost-effective.

The recent re-interpretation of the Lower Palaeozoic euarthropod group Mollisonia as belonging to Chelicerata has triggered a renewed interest for the poorly known family Mollisoniidae. In this contribution, we revise the anatomy, taxonomic diversity, and systematics of Thelxiope, the sister-taxon of Mollisonia. This mollisoniid genus comprises four species, and is characterized by the presence of one cephalic, seven thoracic (one per tergite), and three pygidial long sagittal spines. The type species, T. palaeothalassia Simonetta & Delle Cave, is a rare taxon in the Wuliuan Burgess Shale Formation of Canada, which can be recognized by the hypertrophy of a single of its sagittal spines, the posteriomost one. T. spinosa (Conway Morris & Robison)–a species originally assigned to a distinct genus ‘Ecnomocaris’ herein synonymised with Thelxiope–is known from a single specimen found in the Drumian Wheeler Formation of the House Range of Utah. It differs from the type-species in the hypertrophy of both the anteriormost (cephalic) and the posteriormost (third pygidial) sagittal spines. The same Wheeler strata have also yielded a single specimen of a new taxon, T. holmani sp. nov., which lacks hypertrophied sagittal spines and features blunt thoracic tergopleural tips. A putative fourth species, referred to Thelxiope sp. nov. A, extends the stratigraphical range of Thelxiope to the Lower Ordovician (Tremadocian), and its palaeographic range to West Gondwana. Currently under study, this relatively common component of the lower Fezouata Shale fauna is only briefly discussed. Features characterizing the genus Thelxiope and its components almost exclusively pertain to the sagittal spines, for the scarcity and inconsistent preservation of the Cambrian materials as-yet available preclude a confident assessment of the variability of other morphological features. The pygidium in Thelxiope and Mollisonia is not composed of four, but three tergites essentially similar to thoracic ones, except for the lack of articulations.

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.

Many arachnid taxa have evolved unique, highly specialized sensory structures such as antenniform legs in Amblypygi (whip spiders), for instance, or mesosomal pectines in scorpions. Knowledge of the neuroanatomy as well as functional aspects of these sensory organs is rather scarce, especially in comparison to other arthropod clades. In pseudoscorpions, no special sensory structures have been discovered so far. Nevertheless, these animals possess dominant, multifunctional pedipalps, which are good candidates for being the primary sensory appendages. However, only little is known about the anatomy of the nervous system and the projection pattern of pedipalpal afferents in this taxon. By using immunofluorescent labeling of neuronal structures as well as lipophilic dye labeling of pedipalpal pathways, we identified the arcuate body, as well as a comparatively small mushroom body, the latter showing some similarities to that of Solifugae (sun spiders and camel spiders). Furthermore, afferents from the pedipalps terminate in a glomerular and a layered neuropil. Due to the innervation pattern and structural appearance, we conclude that these neuropils are the first integration centers of the chemosensory and mechanosensory afferents. Within Arthropoda, but also other invertebrates or even vertebrates, sensory structures show rather similar neuronal arrangement. Thus, these similarities in the sensory systems of different evolutionary origin have to be interpreted as functional prerequisites of the respective modality.

Within arthropods, the investigation of navigational aspects including homing abilities has mainly focused on insect representatives, while other arthropod taxa have largely been ignored. As such, scorpions are rather underrepresented concerning behavioral studies for reasons such as low participation rates and motivational difficulties. Here, we review the sensory abilities of scorpions related to navigation. Furthermore, we present an improved laboratory setup to shed light on navigational abilities in general and homing behavior in particular. We tracked directed movements towards home shelters of the lesser Asian scorpion Mesobuthus eupeus to give a detailed description of their departure and return movements. To do so, we analyzed the departure and return angles as well as measures of directness like directional deviation, lateral displacement, and straightness indices. We compared these parameters under different light conditions and with blinded scorpions. The motivation of scorpions to leave their shelter depends strongly upon the light condition and the starting time of the experiment; highest participation rates were achieved with infrared conditions or blinded scorpions, and close to dusk. Naïve scorpions are capable of returning to a shelter object in a manner that is directionally consistent with the home vector. The first-occurring homing bouts are characterized by paths consisting of turns about 10 cm to either side of the straightest home path and a distance efficiency of roughly three-quarters of the maximum efficiency. Our results show that neither chemosensation nor vision, but rather path integration based on proprioception, plays a superior role in the homing of scorpions. [ABSTRACT FROM AUTHOR]

The putative xiphosuran Kiaeria Størmer, 1934, from the Late Silurian (Ludlow) of Ringerike, Norway, is redescribed from the holotype and only known specimen as a chasmataspidid chelicerate arthropod. Morphological features such as the presence of a fused buckler of three opisthosomal segments clearly indicate a chasmataspidid affinity, while the size of Kiaeria, along with the occurrence of a raised axial region and expanded anterior articulation, suggest a close phylogenetic relationship to the Ordovician Chasmataspis. As such, Kiaeria represents the first indication that the Chasmataspis-type chasmataspidid morphology persisted alongside the radiation of diploaspidids. This is also only the third chasmataspidid species recorded from the Silurian. [ABSTRACT FROM AUTHOR]

We describe a new pycnogonid species, Pantopipetta lenis sp. nov., collected from 141–152 m depth in southern Japan, North Pacific Ocean. Pantopipetta lenis closely resembles Pantopipetta auxiliata and Pantopipetta oculata in having auxiliary claws on the legs, but differs from them in lacking dorsodistal tubercles on the lateral processes, having the first short distal article of palp shorter than combined length of the other three short distal articles, having one slight dorsal tubercle on the coxa 1 of legs, having a short dorsal tubercle on the coxa 3 of legs, and lacking a long dorsodistal tubercle on the femur of legs. The palp base (the short article-like structure proximal to the longest palp article) in P. lenis is articulated with the cephalon, i.e. it is actually an article. Although the palp base has been regarded as a lateral process of the cephalon, it is the first article of the palp in P. lenis. [ABSTRACT FROM AUTHOR]

We hypothesize that the diversification of motile marine arthropods with hard carapaces resulted in a concurrent increase in the diversity of encrusting marine bryozoans whose larvae exploited those substrates through the Mesozoic and Cenozoic. To test this, family‐level data were tabulated from the literature on the post‐Palaeozoic diversity of motile marine arthropod basibionts and sessile marine bryozoan epibionts. We found strong temporal correlation from general to more specific basibiont‐epibiont relationships (i.e. arthropods and bryozoans in general to decapods and encrusting gymnolaemates to robust decapods and encrusting cheilostomes in particular). We compared the diversification of bryozoans to another common group of basibionts (i.e. molluscs) and found weaker correlations. This suggests that the diversification of motile arthropods with hard carapaces (e.g. brachyuran crabs) may have played a role in the diversification of sessile bryozoans (e.g. encrusting cheilostomes) in the post‐Palaeozoic. [ABSTRACT FROM AUTHOR]

The recent re-interpretation of the Lower Palaeozoic euarthropod group Mollisonia as belonging to Chelicerata has triggered a renewed interest for the poorly known family Mollisoniidae. In this contribution, we revise the anatomy, taxonomic diversity, and systematics of Thelxiope, the sister-taxon of Mollisonia. This mollisoniid genus comprises four species, and is characterized by the presence of one cephalic, seven thoracic (one per tergite), and three pygidial long sagittal spines. The type species, T. palaeothalassia Simonetta & Delle Cave, is a rare taxon in the Wuliuan Burgess Shale Formation of Canada, which can be recognized by the hypertrophy of a single of its sagittal spines, the posteriomost one. T. spinosa (Conway Morris & Robison)–a species originally assigned to a distinct genus ‘Ecnomocaris’ herein synonymised with Thelxiope–is known from a single specimen found in the Drumian Wheeler Formation of the House Range of Utah. It differs from the type-species in the hypertrophy of both the anteriormost (cephalic) and the posteriormost (third pygidial) sagittal spines. The same Wheeler strata have also yielded a single specimen of a new taxon, T. holmani sp. nov., which lacks hypertrophied sagittal spines and features blunt thoracic tergopleural tips. A putative fourth species, referred to Thelxiope sp. nov. A, extends the stratigraphical range of Thelxiope to the Lower Ordovician (Tremadocian), and its palaeographic range to West Gondwana. Currently under study, this relatively common component of the lower Fezouata Shale fauna is only briefly discussed. Features characterizing the genus Thelxiope and its components almost exclusively pertain to the sagittal spines, for the scarcity and inconsistent preservation of the Cambrian materials as-yet available preclude a confident assessment of the variability of other morphological features. The pygidium in Thelxiope and Mollisonia is not composed of four, but three tergites essentially similar to thoracic ones, except for the lack of articulations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

With over 48,000 species currently described, spiders (Arthropoda: Chelicerata: Araneae) comprise one of the most diverse groups of animals on our planet, and exhibit an equally wide array of fascinating behaviors. Studies of central nervous systems (CNSs) in spiders, however, are relatively sparse, and no reports have yet characterized catecholaminergic (dopamine [DA]‐ or norepinephrine‐synthesizing) neurons in any spider species. Because these neuromodulators are especially important for sensory and motor processing across animal taxa, we embarked on a study to identify catecholaminergic neurons in the CNS of the wolf spider Hogna lenta (Lycosidae) and the jumping spider Phidippus regius (Salticidae). These neurons were most effectively labeled with an antiserum raised against tyrosine hydroxylase (TH), the rate‐limiting enzyme in catecholamine synthesis. We found extensive catecholamine‐rich neuronal fibers in the first‐ and second‐order optic neuropils of the supraesophageal mass (brain), as well as in the arcuate body, a region of the brain thought to receive visual input and which may be involved in higher order sensorimotor integration. This structure likely shares evolutionary origins with the DA‐enriched central complex of the Mandibulata. In the subesophageal mass, we detected an extensive filigree of TH‐immunoreactive (TH‐ir) arborizations in the appendage neuromeres, as well as three prominent plurisegmental fiber tracts. A vast abundance of TH‐ir somata were located in the opisthosomal neuromeres, the largest of which appeared to project to the brain and decorate the appendage neuromeres. Our study underscores the important roles that the catecholamines likely play in modulating spider vision, higher order sensorimotor processing, and motor patterning. [ABSTRACT FROM AUTHOR]

Abstract Background Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile “panchelicerate” body plan. Results In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates. Conclusions Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheiran Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods—Chelicerata and Mandibulata—and implies that this diversification was facilitated by the expansion of durophagous niches.

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. • Central visual system of two whip scorpion species is studied with different neuroanatomical techniques. • R-cells (or retinula cells) of median eyes terminate in 1st and 2nd median eye neuropil and in addition in 1st and 2nd lateral eye neuropil. • R-cells of lateral eyes terminate in 1st and 2nd lateral eye visual neuropil. • Two regions exist (1st and 2nd lateral eye neuropil), where fibres from median and lateral eye overlap. • The monophyly of Arachnopulmonata is supported: A similar architecture of the central visual system is also found in whip spiders and scorpions. [ABSTRACT FROM AUTHOR]