Microanatomy of Early Devonian book lungs

The book lungs of an exceptionally preserved fossil arachnid (Trigonotarbida) from the Early Devonian (approx. 410 Myr ago) Rhynie cherts of Scotland were studied using a non-destructive imaging technique. Our three-dimensional modelling of fine structures, based on assembling successive images made at different focal planes through the translucent chert matrix, revealed for the first time fossil trabeculae: tiny cuticular pillars separating adjacent lung lamellae and creating a permanent air space. Trabeculae thus show unequivocally that trigonotarbids were fully terrestrial and that the microanatomy of the earliest known lungs is indistinguishable from that in modern Arachnida. A recurrent controversy in arachnid evolution is whether the similarity between the book lungs of Pantetrapulmonata (i.e. spiders, trigonotarbids, etc.) and those of scorpions is a result of convergence. Drawing on comparative studies of extant taxa, we have identified explicit characters (trabeculae, spines on the lamellar edge) shared by living and fossil arachnid respiratory organs, which support the hypothesis that book lungs were derived from a single, common, presumably terrestrial, ancestor.     

Arachnid relationships based on mitochondrial genomes: asymmetric nucleotide and amino acid bias affects phylogenetic analyses

Phylogenetic analyses based on mitochondrial DNA have yielded widely differing relationships among members of the arthropod lineage Arachnida, depending on the nucleotide coding schemes and models of evolution used. We enhanced taxonomic coverage within the Arachnida greatly by sequencing seven new arachnid mitochondrial genomes from five orders. We then used all 13 mitochondrial protein-coding genes from these genomes to evaluate patterns of nucleotide and amino acid biases. Our data show that two of the six orders of arachnids (spiders and scorpions) have experienced shifts in both nucleotide and amino acid usage in all their protein-coding genes, and that these biases mislead phylogeny reconstruction. These biases are most striking for the hydrophobic amino acids isoleucine and valine, which appear to have evolved asymmetrical exchanges in response to shifts in nucleotide composition. To improve phylogenetic accuracy based on amino acid differences, we tested two recoding methods: (1) removing all isoleucine and valine sites and (2) recoding amino acids based on their physiochemical properties. We find that these methods yield phylogenetic trees that are consistent in their support of ancient intraordinal divergences within the major arachnid lineages. Further refinement of amino acid recoding methods may help us better delineate interordinal relationships among these diverse organisms.     

Habits, functional morphology and the evolution of pycnogonids

Recent work on functional morphology has revealed not only how a wide range of animals work, but shows the significance of their shapes in great detail. Also, sound evidence of evolution is provided. A new approach towards the understanding of Pycnogonida comes from an appreciation of the significance of their general habits and shapes and from the structure and mode of action of their legs. Recent fossil evidence shows that the arachnids had at least two terrestrial landings, occurring millions of years apart in time. At least two, but probably more, separate arachnid lines lived in the sea. It is concluded that pycnogonids evolved from one such aquatic group which never became terrestrial.     

The affinities of mites and ticks: a review

Mites and ticks can be divided into two well-defined clades, Anactinotrichida and Actinotrichida, for which a recent work formalized a suite of putative autapomorphies and reciprocal differences. Whether they are sister-taxa – forming a monophyletic Acari – is more controversial. Earlier supporters of two independent origins for mites largely failed to demonstrate convincing synapomorphies between either of the two lineages and other arachnid orders; although recent work on reproductive biology revealed explicit characters of this nature. Furthermore, some of the characters proposed in support of a monophyletic Acari do not stand up to detailed scrutiny when compared with Arachnida in general. Effective morphological comparisons between mites and other arachnids are hindered by incompatible nomenclature and long-standing, mite-specific characters which are difficult to score for other arachnids. Furthermore, taxon-specific characters restricted to individual mite groups have sometimes been treated erroneously as 'typical' for all Acari. Here, previous hypotheses of mite affinities are reviewed. Historically, authors have debated whether mites are basal arachnids or highly derived. Excluding weakly supported early hypotheses, mites have been resolved – in whole or in part – as sister-group of all other Arachnida (based on tagmosis), closely related to Opiliones (based mostly on genital morphology), Palpigradi (based on controversial interpretations of limb morphology), Solifugae (based mostly on the mouthparts, but now perhaps also reproductive characters) and Ricinulei (based on hexapodal larvae and perhaps mouthparts). We cannot provide a final resolution here, but we aim to highlight important character sets which should be included in subsequent phylogenetic analyses, as well as useful areas for future investigations: particularly tagmosis and the nature of the gnathosoma.     

Arachnid lipoproteins: comparative aspects

Findings on hemolymph lipoproteins in the class Arachnida are reviewed in relation to their lipid and protein compositions, hydrated densities, the capacity of apoproteins to bind lipids, and the influence of xenobiotics on their structures and functionality. The occurrence of hemolymphatic lipoproteins in arachnids has been reported in species belonging to the orders Araneida, Scorpionida, Solpugida and Acarina. However, lipoproteins were properly characterized in only three species, Eurypelma californicum, Polybetes pythagoricus and Latrodectus mirabilis. Like insect and crustaceans the arachnids examined contain high density lipoproteins (HDLs) as predominant circulating lipoproteins. Although in most arachnids these particles resemble those of insect HDLs called "lipophorins", in two arachnid species they differ from lipophorins in their apoproteins, total mass and lipid composition. The hemolymph of P. pythagoricus and L. mirabilis contains another HDL of higher density, while P. pythagoricus and E. californicum hemolymph contain a third lipoprotein of very high density (VHDL). Composition of arachnid lipoproteins regarding apoprotein classes as well as lipid classes differ among species. Hemocyanin, in addition to the classical role of this protein as respiratory pigment, is presented here performing the function of apolipoprotein in some arachnid species. Reports on experiments demonstrating the capacity of hemocyanin to bind neutral and polar lipid classes, including ecdysteroids, are commented. Recent works about the changes evoked by a phosphorous pesticide on the structures and functionality of spider lipoproteins are also reviewed.     

How to stay on mummy's back: Morphological and functional changes of the pretarsus in arachnid postembryonic stages

A specific type of maternal care occurs in several groups of Arachnida: mothers carry their offspring on their back (pulli-carrying behaviour). In scorpions, whip scorpions and whip spiders it is the prenymphal stage that settles on the mother. The prenymph is not yet fully developed for a free life and very limited in its mobility, but its feet are equipped with special adhesive organs (arolia) that become lost at the nymphal stage. Here we study the morphology, ultrastructure and mechanical function of the arolia. In scorpions (Scorpiones) the contact area between arolia and substrate and thus adhesion of the pad is controlled by the antagonistic work of hydrostatic pressure and muscular retraction. Arolia of whip scorpions (Thelyphonida) do not require muscular action for strong attachment. Arrays of long, branching fibres in the mesocuticle lead to high compliancy of the pad. In whip spiders (Amblypygi) the prenymphal pretarsus is already equipped with sclerites and claws. Its arolium is retained in nymphs and adults in some taxa, but acquires a more complex structure. These results contribute to our knowledge on the postembryonic development of arachnids and to the understanding of attachment pad evolution among arthropods. Some of the described developmental, structural, and mechanical phenomena are not known from other animals and might be of potential interest for further biomimetic developments.     

Sealing junctions in a number of arachnid tissues

The junctions present in the central nervous system (CNS), midgut, silk gland and venom gland of arachnids have been investigated. Special care was taken to try to locate tight junctions in tissue other than CNS but they were not found in any of the other tissues. The detailed structure of the junctions present are discussed. The tight junctions present in CNS are somewhat different in appearance and fracturing behaviour to most vertebrate tight junctions and closely resemble only those found in Urochordates (a non-vertebrate chordate). The two types of septate junctions found in the other tissues belong to the pleated septate and smooth septate classes but show some interesting differences. It appears probable that the septate junctions in Arachnida, Merostomata and Myriopoda have different fracturing properties from those found in other arthropods. The finding that only septate junctions are present in most arachnid tissues, although tight junctions are present in CNS, is discussed in the context of the sealing function of septate junctions in invertebrate tissues.     

海南岛的蝎子地理分布研究

蝎目是蛛形动物中较早登陆且又比较原始的类群,在蛛形动物进化历程中占重要地位,是已知最古老的陆生节肢动物之一,具有较高的药用价值和食用价值.本文对海南岛蝎子的生态地理分布进行了研究,从理论上丰富了热带地区蝎目动物的地理分布和物种丰富度等方面的内容;在实践上,拓宽了应用的领域,有利于对不同蝎目动物的资源开发和利用,同时也为保护这类古老的动物提供了依据.经调查,共发现蝎目动物2科3属3种,均为海南已有记录种类,其中钳蝎科2种,分别为等蝎属的斑等蝎和狼蝎属的细尖狼蝎;瘦尾蝎科1种,系链尾蝎属的澳链尾蝎.     

Developmental changes in the embryo, pronymph, and first molt of the scorpion Centruroides vittatus (scorpiones: buthidae)

For the first time the scanning electron microscope was used to compare developmental changes in scorpion embryos and the first and second stadia. In the buthid species of this study, Centruroides vittatus, and all other scorpions, the newborn climb up on their mother's back and remain there without feeding for several days. At this location, they undergo their first molt and in a few days they disperse, fully capable of foraging in the terrestrial environment. The results here support earlier suggestions that the first stadium (pronymph) is a continuation and extension of embryological development. The first molt results in a nymph with exoskeletal features much like those in the adult. In the first molt the metasoma becomes relatively longer, and the sting (aculeus) becomes sharp and functional. The metasomal segments are modified for dorsal flexion and sting use. The embryos and the pronymphs have spiracles that open into an invagination near the posterior margin of flap-like abdominal plates in segments 4-7 of the ventral mesosoma. The second instars have spiracles that lead to book lungs farther anterior in sternites. Tubular legs with cylindrical segments in embryos and pronymphs become more sculptured and oval in the transverse plane. Each leg in the pronymph has a blunt, cup-shaped tip while distal claws (ungues, dactyl) are present in the second instar and subsequent stages. There are some sharp bristles and primordial sensilla in the pronymphs, but the second stadium has adult-like surface features: rows of knobs or granulations (carinae), serrations on the inner surfaces of cheliceral and pedipalpal claws, filtering hairs at the mouthparts, peg sensilla on the pectines, and mechano- and chemoreceptor sensilla on the body and appendages. Scorpion embryos and pronymphs have some structures like fossil scorpions thought to have been aquatic. There is a gradual development of features that appear to be terrestrial adaptations. Evidence is provided for the formation of the sternum from third and fourth leg coxal primordia and possibly from the first abdominal segment. This study is the first to provide evidence for a forward shift of the gonopore along with other structures in the anterior abdomen.     

Characters in the book lungs of Scorpiones (Chelicerata,Arachnida) revealed by scanning electron microscopy

The fine structure of the book lungs in 29 species representing ten monophyletic taxa of the Scorpiones (Arachnida) was investigated using scanning electron microscopy (SEM). Scorpion lungs are not homogeneous across the group. Here we describe and score three sets of phylogenetically informative characters: (1) the surface ornament of the lung lamellae, (2) the distal margins of the lamellae and (3) the fine structure of the spiracle margin. Provisional results suggest that reticulation on the surface of the lung lamellae is characteristic of the Buthidae. By contrast, non-buthid scorpions maintain the air space between adjacent lamellae using projecting trabeculae. Typically they are simple struts, but the trabeculae are distally branched in all investigated Scorpionidae, plus at least one species belonging to the Liochelidae. Simple thorns on the lamellar margins probably represent the plesiomorphic condition, while more complex, branched, arcuate morphologies appear to be homoplastic, occurring sporadically in numerous scorpion sub-groups. The tightly packed, hexagonal pillars around the posterior margin of the spiracle support a close relationship between Scorpionidae and Liochelidae, to the exclusion of the Urodacidae.     

An enigmatic, solifuge-like fossil arachnid from the Lower Carboniferous of Kamienna Göra (Intra-Sudetic Basin), Poland

A new arachnid (Chelicerata: Arachnida) from the Lower Carboniferous (Upper Viséan) Szczawno Formation of Kamienna Göra, Poland, is described asSchneidarachne saganii n. gen. et n. sp. Early Carboniferous arachnids are generally rare and this new fossil cannot be easily assigned to any of the known arachnid Orders. It shares a number of features with some members of the arachnid order Solifugae (camel spiders, sun spiders): namely large, forward-projecting, chelate chelicerae with dentate fixed and free fingers, a distinct median sulcus on the carapace and an interrupted ridge of tubercles on the dorsal opisthosoma set into a loosely-defïned median field. However, it lacks unequivocal autapomorphies of Solifugae. This fossil may be one of a growing number of stem-group Palaeozoic arachnids which lack the füll set of diagnostic characters seen in crown-group members of the various orders. Thus,Schneidarachne saganii could represent a basai member of the lineage leading up to modem solifuges.     

Differential reliance on aquatic prey subsidies influences mercury exposure in riparian arachnids and songbirds

Cross‐ecosystem subsidies move substantial amounts of nutrients between ecosystems. Emergent aquatic insects are a particularly important prey source for riparian songbirds but may also move aquatic contaminants, such as mercury (Hg), to riparian food webs. While many studies focus on species that eat primarily emergent aquatic insects, we instead study riparian songbirds with flexible foraging strategies, exploiting both aquatic and terrestrial prey sources. The goal in this study is to trace reliance on aquatic prey sources and correlate it to Hg concentrations in common riparian arachnids (Families Tetragnathidae, Opiliones, and Salticidae) and songbirds (Common Yellowthroat Geothlypis trichas, Spotted Towhee Pipilo maculatus, Swainson''s Thrush Catharus ustulatus, Song Sparrow Melospiza melodia, and Yellow Warbler Setophaga petechia). We used stable isotopes of δ¹³C and δ¹⁵N and Bayesian mixing models in MixSIAR to determine the reliance of riparian predators on aquatic prey sources. Using mixed effects models, we found that arachnid families varied in their reliance on aquatic prey sources. While songbird species varied in their reliance on aquatic prey sources, songbirds sampled earlier in the season consistently relied more on aquatic prey sources than those sampled later in the season. For both arachnids and songbirds, we found a positive correlation between the amount of the aquatic prey source in their diet and their Hg concentrations. While the seasonal pulse of aquatic prey to terrestrial ecosystems is an important source of nutrients to riparian species, our results show that aquatic prey sources are linked with higher Hg exposure. For songbirds, reliance on aquatic prey sources early in the breeding season (and subsequent higher Hg exposure) coincides with timing of egg laying and development, both of which may be impacted by Hg exposure.     

A trigonotarbid arachnid from the Lower Devonian of Tredomen, Wales

A new trigonotarbid (Arachnida: Trigonotarbida) Arianrhoda bennetti gen. et sp. nov. is described from the Lower Devonian (Lochkovian) of a quarry near Tredomen, Powys, mid Wales, UK. This relatively complete specimen is the first record of a pre-Carboniferous arachnid from Wales, one of only a handful of early Devonian arachnids, and the second oldest trigonotarbid recorded. Based on the rounded prosomal dorsal shield and the relatively narrow, elongate opisthosoma we refer this new fossil to the family Anthracosironidae. A distinct flange-like ornament on the leg 4 tibia in the new fossil is unique among trigonotarbids and is the primary autapomorphy for the new genus.     

The Lower Devonian scorpionWaeringoscorpio and the respiratory nature of its filamentous structures, with the description of a new species from the Westerwald area, Germany

The fossil scorpionWaeringoscorpio hefteri Størmer, 1970 (Arachnida: Scorpiones) from the Lower Devonian of the Rhenish Massif of Germany is redescribed based on both the original type and newly collected material. A second, more tuberculate species from Siegenian strata near Bürdenbach in the Westerwald (also part of the Rhenish Massif) is described asW. westerwaldensis n. sp. Details of the coxo-sternal region — including the lack of an oral tube — and the number of ventral mesosomal plates are discussed.Waeringoscorpio Størmer, 1970 is best known for its possession of externally-projecting ‘gills’. Our new material reveals that these are indeed pair-wise bundles of rigid, branching filaments which originate laterally, quite possibly from those segments of the mesosoma associated with the book lungs in extant scorpions. Their gross morphology is most consistent with a respiratory organ adapted for use in water. Indeed their closest modern analogues are the tracheal gills of secondarily aquatic insects. We suggest that the morphology and likely palaeoenvironment ofWaeringoscorpio could indicate an aquatic animal, but we draw attention to the uniqueness of its gill-structures, which may not be part of the scorpion ground-pattern. Thus,Waeringoscorpio was perhaps a secondarily aquatic scorpion adapted for benthic life in oxygen-stressed, freshwater-brackish environments.     

Same same but different: a stunning analogy between tracheal and vascular supply in the CNS of different arachnids

Described herein is an as yet unprecedented structural and functional analogy of both the tracheal supply of the prosomal ganglion in opilionids and the arterial supply of the prosomal ganglion in pulmonate arachnids. Within Arachnida, two different modes of respiration can be observed: the so-called book lungs, and the tube-like tracheae. These different respiratory modes always correlate with a specific setup concerning the complexity of the circulatory system. This fact has a particular influence on the supply of certain organ systems, such as the central nervous system. It has recently been shown that pulmonate arachnids possess a highly complex pattern of intraganglionic arteries. Here, we show that Opiliones (harvestmen) possess a complex tracheal system (which supplies the different organ systems with oxygen) and only a relatively simple vascular system, comprising a short heart and an anterior aorta that runs directly to the prosomal ganglion. Using a variety of modern and classical morphological methods, we studied the vascular, tracheal and nervous systems of different representatives from all higher taxa of Opiliones. We show that the prosomal ganglion is extensively supplied with intraganglionic tracheae. What is especially surprising is the high degree of correspondence between the pattern of these ganglionic tracheae in harvestmen and the pattern of arteries in the prosomal ganglion of pulmonate arachnids. We aim to provide mechanistic causal explanations of these analogous patterns by applying the concepts of role analogy and constructional analogy. We also aim to establish the circulatory system as a model organ system and hope that this may, in turn, provide a starting point for future research programmes.