Muscular anatomy of a whipspider, Phrynus longipes (Pocock) (Arachnida: Amblypygi), and its evolutionary significance

Skeletal muscles in the whipspider Phrynus longipes are surveyed and compared with those of other chelicerates to clarify the evolutionary morphology and phylogenetic relationships of the arachnids. Representatives of 115 muscle groups are described and illustrated, and their possible functions are proposed. Principal results of this analysis include new functional models for the operation of the pharyngeal and sternocoxal mechanisms in Amblypygi and a greatly expanded list of apparently unique synapomorphies supporting the monophyly of Pedipalpi (= Amblypygi, Schizomida, Thelyphonida).     

The accoutrements of spiders' eggs (Araneae) with an exploration of their functional importance

Analysis is presented of the characteristics of spheres coating the chorion surface of the eggs from 41 species belonging to 15 families of spiders. Examples of eggs from the arachnid orders Opiliones, Amblypygi and Uropygi are illustrated for the first time. The spheres are unique, among arachnids, to the Araneae. Their form and size class distributions overlap considerably between families and the spheres are present in all 22 families of Araneae that have been examined; they represent a very conservative morphological feature in the evolution of the Araneae. Evidence of specialized regions of the egg surface (micropyllar areas and plastrons) was found only in an opilionid. As spiders' eggs are laid in silk egg sacs, mostly in clutches, the possible effects of these characteristics are discussed from evidence in the literature and from a functional theoretic stance.     

The fine structure of spermiogenesis in the Amblypygi and the Uropygi (Arachnida)

Summary Spermiogenesis in one species from each of the arachnid groups Amblypygi and Uropygi is described by electron microscopy: The whip spider,Tarantula marginemaculata (Amblypygi), and the whip-scorpion,Mastigoproctus giganteus, (Uropygi). In both species the earliest spermatid has a spherical nucleus and soon acquires an anterior acrosome and a posterior flagellar tail. The flagellun is peculiar in having a 9 + 3 axonemal pattern. By the mid-spermatid stage, the nucleus becomes conspicuously elongated, possibly through the agency of a manchette of microtubules. In the late spermatid, the elongated nucleus begins to coil posteriorly; simultaneously the middle piece and the tail flagellum begin to retract into the cell body to form a coiled intracellular axonema. Membranous profiles appear in the peripheral cytoplasm, possibly to accommodate a decrease in the total area of plasma membrane. The mature sperm is a spherical cell, which includes the following organelles in twisted and fully coiled configuration: an elongated nucleus, an acrosome and an acrosomal filament, a long middle piece with helically arranged mitochondria and an intracellular axonema.     

Spermatophorenbau und samenübertragung bei uropygen (Mastigoproctus brasilianus C. L. Koch) und amblypygen (Charinus brasilianus Weygoldt und Admetus pumilio C. L. Koch) (Chelicerata,Arachnida)

The spermatophores of Amblypygi and Uropygi are complicated structurs and vary in different species. In Mastigoproctus, the size and shape of the spermatophores prevent bastardation between M. giganteus and M. brasilianus. Taxonomic and phylogenetical implications are discussed.

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Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

REVIEW Evolution and systematics of the Chelicerata

After approximately 40 years of discussion about the question of whether the Arthropoda are a monophyletic or a paraphyletic group or even a polyphyletic assemblage of unrelated taxa, most morphologists, palaeontologists and molecular taxonomists agree that the Arthropoda are a monophylum. The Euarthropoda are composed of the Arachnomorpha and Mandibulata. Myriapods are usually considered to be mandibulates; however, new molecular data as well as some morphological characters show similarities which the Myriapoda share with the Chelicerata, suggesting that there is no taxon Antennata or Atelocerata. Chelicerata are usually considered to be the sister group of Trilobita or, more correctly, Trilobita branch off from the chelicerate stem line. The first adaptive radiation of the Chelicerata took place in the Cambrian. All extant and some extinct orders were present during the Carboniferous. Two systems are compared. It is suggested that the Chelicerata contain the Pantopoda and Euchelicerata. The Euchelicerata are divided into Xiphosura and terrestrial Arachnida. Scorpiones are considered to be the sister group of all other arachnids, the Lipoctena and these are further divided into the Megoperculata (Uropygi, Amblypygi, and Araneae) and Apulmonata (all other groups). The Acari are tentatively considered to be a monophylum and the sister group of the Ricinulei. However, the Actinotrichida and Anactinotrichida diverged early and therefore have had a long history of independent evolution.     

Double spermatogenesis in Chelicerata

Sperm dimorphism is a rare phenomenon in Chelicerata. Until now, it was known only from three species of the opilionid genus Siro (Sironidae, Cyphophthalmi). Fertilizing (eusperm) and nonfertilizing spermatozoa (parasperm) develop in the same cyst and are thus sister cells. The fine structure of the spermatozoa of two species has been examined and is compared here. In contrast to Siro rubens, S. duricorius spermatozoa lack an acrosomal complex. Both sperm types produce a transitional process, a more or less modified flagellum, which is later retracted. Hence, the spermatozoa are aflagellate. Eusperm and parasperm of all three species form highly ordered sperm balls that are stored in the deferent duct. Reviewing and adding new results about the sperm dimorphism in this arachnid taxon provides the basis for some considerations of another enigmatic morphological character found in Uropygi and Amblypygi, i.e., the tubular accessory genital glands that show holocrine extrusion. These glands are suggested to represent modified, infertile derivatives of the testis anlage. Their secretion is produced in a way reminiscent of a strongly degenerated spermatogenesis. Consequently, these products may be regarded as strongly degenerated germ cells representing a line of germ cell development, which has been separated very early in spermatogenesis from the usual line leading to fertilizing sperm cells. This further, although less evident, case of probable dichotomous germ cell development is discussed with respect to the controversial phylogenetic-systematic relationships between Uropygi (Thelyphonida and Schizomida), Amblypygi, and Araneae.     

Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications

The morphological diversity of locomotor appendages in Arachnida is surveyed lo reconstruct phylogenetic relationships and discover evolutionary trends in form and function. The appendicular skeleton and musculature of representatives from the ten living arachnid orders ate described, and a system of homology is proposed. Character polarities are established through comparison with an outgroup. Limulus polyphemus Xiphosura). Cladistic analysis suggests that Arachnida is monophyletic and that absence of extensor muscles is a primitive condition. Extensors are primitively absent in Araneae. Amblypygi, Uropygi, Palpigradi, Ricinulei and Acari. Most similarities in the appendages of these orders are symplesiomorphic so that phylogenetic relationships among the 'extensorless' groups cannot be resolved solely on the basis of appendicular characters. Extensor muscles appear to have evolved once, and their presence is considered a synapomorphic feature of Opiliones, Scorpiones, Pseudoscorpiones and Solifugae. Solifugae lack extensors, but a parsimonious interpretation of other characters indicates that this is a secondary, derived condition. The phylogenetic relationships among these four orders are clarified by modifications of the patellotibial joint. Cladistic analysis indicates that Opiliones may be the sister group of the other three orders and that Scorpiones is the sister group of Pseudoscorpiones and Solifugae. Conclusions concerning phylogenetic relationships and evolutionary morphology presented here differ substantially from those of earlier studies on the locomotor appendages of Arachnida.     

Species richness and biogeography of non-acarine arachnids in Namibia

A total of 821 species, 296 genera, and 69 families of non-acarine arachnids (Araneae, Solifugae, Scorpiones, Pseudoscorpiones, Opiliones and Amblypygi) are presently known from Namibia. Patterns of spider, solifuge and scorpion species richness and endemism are summarized relative to the 14 major Namibian types of vegetation. Spiders are most speciose in the higher rainfall areas, solifuges in the dry areas, and scorpions in rocky areas. Namibia probably has the world's greatest diversity of solifuges. Many arachnids endemic to Namibia are associated with the Namib Desert sand dunes.     

Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications

The morphological diversity of locomotor appendages in Arachnida is surveyed lo reconstruct phylogenetic relationships and discover evolutionary trends in form and function. The appendicular skeleton and musculature of representatives from the ten living arachnid orders ate described, and a system of homology is proposed. Character polarities are established through comparison with an outgroup. Limulus polyphemus Xiphosura). Cladistic analysis suggests that Arachnida is monophyletic and that absence of extensor muscles is a primitive condition. Extensors are primitively absent in Araneae. Amblypygi, Uropygi, Palpigradi, Ricinulei and Acari. Most similarities in the appendages of these orders are symplesiomorphic so that phylogenetic relationships among the ‘extensorless’ groups cannot be resolved solely on the basis of appendicular characters. Extensor muscles appear to have evolved once, and their presence is considered a synapomorphic feature of Opiliones, Scorpiones, Pseudoscorpiones and Solifugae. Solifugae lack extensors, but a parsimonious interpretation of other characters indicates that this is a secondary, derived condition. The phylogenetic relationships among these four orders are clarified by modifications of the patellotibial joint. Cladistic analysis indicates that Opiliones may be the sister group of the other three orders and that Scorpiones is the sister group of Pseudoscorpiones and Solifugae. Conclusions concerning phylogenetic relationships and evolutionary morphology presented here differ substantially from those of earlier studies on the locomotor appendages of Arachnida.     

A phylogenetic analysis of the arachnid orders based on morphological characters

Morphological evidence for resolving relationships among arachnid orders was surveyed and assembled in a matrix comprising 59 euchelicerate genera (41 extant, 18 fossil) and 202 binary and unordered multistate characters. Parsimony analysis of extant genera recovered a monophyletic Arachnida with the topology (Palpigradi (Acaromorpha (Tetrapulmonata (Haplocnemata, Stomothecata nom. nov. )))), with Acaromorpha containing Ricinulei and Acari, Tetrapulmonata containing Araneae and Pedipalpi (Amblypygi, Uropygi), Haplocnemata (Pseudoscorpiones, Solifugae) and Stomothecata (Scorpiones, Opiliones). However, nodal support and results from exploratory implied weights analysis indicated that relationships among the five clades were effectively unresolved. Analysis of extant and fossil genera recovered a clade, Pantetrapulmonata nom nov. , with the topology (Trigonotarbida (Araneae (Haptopoda (Pedipalpi)))). Arachnida was recovered as monophyletic with the internal relationships (Stomothecata (Palpigradi, Acaromorpha (Haplocnemata, Pantetrapulmonata))). Nodal support and exploratory implied weights indicated that relationships among these five clades were effectively unresolved. Thus, some interordinal relationships were strongly and/or consistently supported by morphology, but arachnid phylogeny is unresolved at its deepest levels. Alternative hypotheses proposed in the recent literature were evaluated by constraining analyses to recover hypothesized clades, an exercise that often resulted in the collapse of otherwise well-supported clades. These results suggest that attempts to resolve specific nodes based on individual characters, lists of similarities, evolutionary scenarios, etc., are problematic, as they ignore broader impacts on homoplasy and analytical effects on non-target nodes.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 150 , 221–265.     

The idiosomatic euneotaxy and epineotaxy in gamasids (Arachnida, Parasitiformes)

From the systematic point of view, the nearest relatives of the Parasitiformes seem to be small holopeltidial latigastric Palaeozoic arachnids. It is pointed out that GRAND JEAN'S theory “L'Evolution selon l'ǎge” and Ghilarov 's theory of the division of ecological work are complementary. This evolutionary picture, as well as the evolutionary vocabulary introduced by Grandjean , are used to describe the different conditions of two homoeotypical cuticular systems encountered in gamasids “mites” (crobylophores and hairs). Most of the known evolutionary peculiarities of the crobylophores of these arachnids are reported and the problem of the functions of these glands is discussed.     

Lebenszyklus und postembryonale entwicklung der geibelspinne Tarantula marginemaculata C. L. Koch (Chelicerata,Amblypygi) im laboratorium

In the laboratory, Tarantula marginemaculata breeds during the summer months. Egg development takes 98 days. Development from hatching to maturity takes 8 to 10 months and involves 5 to 8 moults. Mature animals continue to moult and to grow and normally produce 1 to 2 broods per year. Moulting frequency, growth, development of sexual organs and of trichobothriotaxy, and regeneration are described and compared to those of other arachnids.

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Lebenszyklus und postembryonale entwicklung der geibelspinne Tarantula marginemaculata C. L. Koch (Chelicerata, Amblypygi) im laboratorium

     

On the Emsian (Lower Devonian) arthropods of the Rhenish Schiefergebirge: 1.Xenarachne, an enigmatic arachnid from Willwerath, Germany

A new fossil arachnid,Xenarachne willwerathensis n. gen., n. sp. is described from the Lower Devonian (upper Lower Emsian) Klerf-beds of Willwerath, Germany. This intriguing fossil has a pedicel and pygidium, indicating that it belongs to the tetrapulmonate arachnids, but cannot be referred with confidence to any particular order. It is placed as Tetrapulmonata incertae sedis, though has similarities to whip spiders (Amblypygi) and spiders (Araneae).Xenarachne could even represent a very early spider, though the Devonian arachnid fauna could have included taxa which did not belong in any currently recognised order.Xenarachne may be an example of just such a fossil.     

The evolution of pedipalps and glandular hairs as predatory devices in harvestmen (Arachnida,Opiliones)

Pedipalps are the most versatile appendages of arachnids. They can be equipped with spines (Amblypygi), chelae (Scorpiones), or adhesive pads (Solifugae), all of which are modifications to grasp and handle fast‐moving prey. Harvestmen (Opiliones) show a high diversity of pedipalpal morphologies. Some are obviously related to prey capture, like the enlargement and heavy spination of Laniatores pedipalps. Many Dyspnoi, by contrast, exhibit thin, thread‐like pedipalps that are covered with complex glandular setae (clavate setae). These extrude viscoelastic glue that is used to immobilize prey items. Comparable setae (plumose setae) have previously been found in representatives of both Eupnoi and Dyspnoi, yet comprehensive data on their distribution are lacking. This study examined the distribution and ultrastructure of glandular setae in harvestmen and related them to pedipalpal morphology. Pedipalpal and setal characters were analysed in a phylogenetic framework. We found that glandular setae are synapomorphic for and widespread in the Palpatores clade (Eupnoi plus Dyspnoi). Their occurrence correlates with pedipalp morphology and feeding habit. Remnants of arthropod cuticular structures or secretions, frequently found attached to glandular setae, and behavioural observations, underlined the importance of the setae for capturing and securing prey. We hypothesize that glandular setae evolved as an adaptation to capture small and agile prey, which are hard to catch with a capture basket. Details of ultrastructure indicate that the setae are derived sensilla chaetica, with both a secretory and sensory function. Derived ultrastructural characters of the glandular setae, such as slit‐like channel openings and a globular arrangement of the microtrichia, may increase their effectiveness. The functional role of further pedipalpal modifications, such as apophyses, stalked and hyperbendable joints, and curved segments, as well as sexual dimorphism and ontogenetic polymorphism, are discussed. Some implications of the results obtained for the taxonomic treatment of Phalangiidae are also discussed. These results shed new light on the biology and evolutionary history of this fascinating group of arthropods.     

Spermatozoa and spermiogenesis of Liphistius cf. phuketensis (Mesothelae, Araneae, Arachnida) with notes on phylogenetic implications

The present study deals with the spermatozoa and spermiogenesis of Liphistius cf. phuketensis, a representative of the most primitive and enigmatic spider group Mesothelae. The general organization of the spermatozoa is very similar to the condition known from Amblypygi supporting a sister-group relationship between Araneae and Amblypygi. Besides plesiomorphic characters such as, e.g., an elongated and corkscrew shaped nucleus, the sperm cells are characterized by several apomorphic characters, e.g., the giant body and conspicuous membranous areas which are formed at the end of spermiogenesis. As the transfer form, coenospermia are formed at the end of spermiogenesis, which strongly supports the idea that this type of sperm aggregation is the primitive transfer form within spiders. A very remarkable character of the spermatozoa of some groups of arachnids is the coiling of the main cell organelles at the end of spermiogenesis. Previously, the Mesothelae were believed to be the only spider group which does not show a complete coiling of the main cell organelles. With the present study the first evidence of a complete coiling of spermatozoa within this primitive spider group could be documented, indicating that this character is part of the ground pattern of spider spermatozoa. Consequently, the incomplete coiling seems to be a synapomorphy of certain species of Mesothelae, which sheds new light on the discussion of the phylogenetic relationships of this group.     

Ultrastructural observations of spermatozoa and spermiogenesis in Wandella orana Gray, 1994 (Araneae: Filistatidae) with notes on their phylogenetic implications

Spermatozoa and spermiogenesis of the prithine filistatid spider Wandella orana are described. The spider produces coenospermia, i.e. sperm aggregations that include several single sperm cells commonly surrounded by a secretion sheath. One sectioned coenospermium in W. orana contains at least five spermatozoa. During copulation many coenospermia are transferred into the female. Coenospermia are regarded as a peculiar transfer form of sperm which occurs in early derivative spiders such as Liphistiomorphae and Mygalomorphae. The only exception which was found in Araneomorphae until now was the filistatine spider Filistata insidiatrix. Our observation is the second case and supports the view that Filistatidae represent an early derivative taxon. Furthermore, the individual sperm cells show characteristics which also may be regarded as being plesiomorphic. There is a cone-shaped acrosomal vacuole, a very long acrosomal filament, a rather stout nucleus and a small implantation fossa. The axoneme shows the 9x2+3 pattern of microtubules which is synapomorphic in Megoperculata (Uropygi, Amblypygi and Araneae). The finding of coenospermia in two distant taxa of Filistatidae may have consequences for phylogenetic and systematic considerations.     

Die Cheliceren der Araneae,Amblypygi und Uropygi mit den Skleriten,den Plagulae (Chelicerata,Arachnomorpha)

Zusammenfassung 1. Die Arachnomorpha — Uropygi, Amblypygi, Araneae — besitzen in den Cheliceren ein oder zwei Sklerite an der Basis der Klaue, die Plagula ventralis und die Plagula dorsalis. Diese sind Bildungen der Haut und werden mit der Exuvie abgeworfen.2. Die Plagula ventralis ist im allgemeinen ein plattenförmiger oder stäbchenförmiger Sklerit. Proximal setzen an ihr die Beugemuskeln für die Klaue an. Distal ist sie durch ein biegsames Stück mit der Klaue verbunden. Da die Plagula ventralis vor dem Gelenk der Klaue ansetzt, verlängert sie den Hebelarm für die Beugemuskeln.3. Die Plagula ventralis ist im allgemeinen einfach, sie ist weiter entwickelt bei den Mygalomorphae der Araneae. Außen ist sie von einer dünnen Schicht Epicuticula begrenzt, darunter folgen dickere Schichten von Exo- und Mesocuticula. Der biegsame Teil am Ansatz der Klaue besteht nur aus Mesocuticula unter der dünnen Epicuticula.4. Die Plagula dorsalis findet sich nur bei Mygalomorphae der Araneae. Nur die Masteriinae besitzen sie nicht. Die Plagula dorsalis liegt als ein schmales Band quer vor der Basis der Klaue im dorsalen Fenster. Sie ist an drei Stellen verdickt und hier von feinen Kanälen durchzogen. Proximal und distal setzen die Sehnen der Streckmuskeln für die Klaue an. Eine besondere Funktion konnte nicht ermittelt werden.5. Die Nahrungsaufnahme, Kauen oder Saugen, ist an der Struktur der Cheliceren zu erkennen. Kauer besitzen zwei Reihen von Zähnen, manchmal in großer Anzahl. An der Basis des Grundgliedes befindet sich außen ein großer Condylus als Führungsschiene. Bei den Saugern ist die Anzahl der Zähne manchmal bis auf einen reduziert. Der Condylus an der Basis des Grundgliedes fehlt oder ist nur als Vestigium vorhanden.6. Die Plagula ventralis ist eine Autapomorphie des Taxon Arachnomorpha. Die Plagula dorsalis ist eine Autapomorphie der Mygalomorphae innerhalb der Araneae.

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The plagulae, sclerites at the base of the chelicerae of Araneae, Amblypygi, and Uropygi (Chelicerata, Arachnomorpha)

Summary 1. The Arachnomorpha (Uropygi, Amblypygi, Araneae) have one or two sclerites at the base of the fangs of their chelicerae, here called plagula ventralis and plagula dorsalis. These sclerites are part of the exoskeleton and are thus also visible in exuviae.2. Generally, the plagula ventralis is a plate- or rod-shaped sclerite, with the fang flexor muscles attached at its proximal end. Distally it is connected to the fang by a flexible part. Being attached distad of the fang articulation, the plagula ventralis extends the leverage of the flexor muscles. The plagula ventralis is simple in most Arachnomorpha, but in Mygalomorphae (Araneae) it is more complicated. Externally there is a thin epicuticular layer, with thicker layers of exo- and mesocuticula underneath. The flexible part at its contact with the fang consists exclusively of mesocuticula.3. The plagula dorsalis is found only in Mygalomorphae. It forms a narrow strip across the dorsal base of the fang. Three sections of it are thicker and passed by thin channels, with tendons of the fang extensor muscles attached to their proximal and distal ends. The function of the plagula dorsalis remains uncertain.4. The presence of a plagula ventralis is an autapomorphy of the Arachnomorpha, whereas the plagulae dorsalis are hypothesized to be an autapomorphy of the Mygalomorphae within the Araneae.5. Different modes of food ingestion (chewing and sucking) may be recognized by different cheliceral structures. Chewers have numerous teeth; outside at the base of the paturon a big condylus acts as a leading strip. Those that pump insects out have fewer teeth, in several cases only one tooth. The condylus at the base of the paturon is missing or vestigial.

     

The Phylogeny of the Extant Chelicerate Orders

The phylogeny of the extant chelicerate orders is examined in the light of morphological and molecular evidence. Representatives from each of the chelicerate ‘orders’ and mandibulate and onychophoran outgroups are examined. Molecular (small and large ribosomal subunit DNA) and morphological information is combined in a total evidence regime to determine the most consistent picture of extant chelicerate relationships for these data. Multiple phylogenetic analyses are performed with variable analysis parameters yielding largely consistent results. A normalized incongruence length metric is used to assay the relative merit of the multiple analyses. The combined analysis with lowest character incongruence yields the scheme of relationships (Pycnogonida+ (Xiphosura+((Opiliones+((Solifugae+Pseudoscorpiones)+Scorpiones))+((Ricinulei+Acari)+(Palpigradi+ ((Thelyphonida+Schizomida=Uropygi)+(Amblypygi+ Araneae))))))). This result is fairly robust to variation in analysis parameters, with the placement of solifugids and the status of the pedipalps responsible for most disagreement.