The effect of model choice on phylogenetic inference using mitochondrial sequence data: lessons from the scorpions

Chelicerates are a diverse group of arthropods, with around 65,000 described species occupying a wide range of habitats. Many phylogenies describing the relationships between the various chelicerate orders have been proposed. While some relationships are widely accepted, others remain contentious. To increase the taxonomic sampling of species available for phylogenetic study based on mitochondrial genomes we produced the nearly complete sequence of the mitochondrial genome of the scorpion Mesobuthus gibbosus. Mitochondrial gene order in M. gibbosus largely mirrors that in Limulus polyphemus but tRNA secondary structures are truncated. A recent analysis argued that independent reversal of mitochondrial genome strand-bias in several groups of arthropods, including spiders and scorpions, could compromise phylogenetic reconstruction and proposed an evolutionary model that excludes mutational events caused by strand-bias (Neutral Transitions Excluded, NTE). An arthropod dataset of six mitochondrial genes, when analyzed under NTE, yields strong support for scorpions as sister taxon to the rest of Chelicerata. We investigated the robustness of this result by exploring the effect of adding additional chelicerate genes and taxa and comparing the phylogenies obtained under different models. We find evidence that (1) placement of scorpions arising at the base of the Chelicerata is an artifact of model mis-specification and scorpions are strongly supported as basal arachnids and (2) an expanded chelicerate dataset finds support for several proposed interordinal relationships (ticks plus mites [Acari] and spiders plus whip spiders plus whip scorpions [Araneae+Pedipalpi]). Mitochondrial sequence data are subject to systematic bias that is positively misleading for evolutionary inference and thus extreme methodological care must be taken when using them to infer phylogenies.     

Functionally Different Pads on the Same Foot Allow Control of Attachment: Stick Insects Have Load-Sensitive “Heel” Pads for Friction and Shear-Sensitive “Toe” Pads for Adhesion

Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal “heel” pads (euplantulae) and a pre-tarsal “toe” pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects'' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised “friction pads” that produce traction when pressed against the substrate, while arolia are “true” adhesive pads that stick to the substrate when activated by pulling forces.     

Tactile learning by a whip spider, <Emphasis Type="Italic">Phrynus marginemaculatus</Emphasis> C.L. Koch (Arachnida,Amblypygi)

The ability of animals to learn and remember underpins many behavioural actions and can be crucial for survival in certain contexts, for example in finding and recognising a habitual refuge. The sensory cues that an animal learns in such situations are to an extent determined by its own sensory specialisations. Whip spiders (Arachnida, Amblypygi) are nocturnal and possess uniquely specialised sensory systems that include elongated ‘antenniform’ forelegs specialised for use as chemo- and mechanosensory feelers. We tested the tactile learning abilities of the whip spider Phrynus marginemaculatus in a maze learning task with two tactile cues of different texture—one associated with an accessible refuge, and the other with an inaccessible refuge. Over ten training trials, whip spiders got faster and more accurate at finding the accessible refuge. During a subsequent test trial where both refuges were inaccessible, whip spiders searched for significantly longer at the tactile cue previously associated with the accessible refuge. Using high-speed cinematography, we describe three distinct antenniform leg movements used by whip spiders during tactile examination. We discuss the potential importance of tactile learning in whip spider behaviour and a possible role for their unique giant sensory neurons in accessing tactile information.     

Structure and function of the arolium of Mantophasmatodea (Insecta)

All species of the insect order Mantophasmatodea characteristically keep the 5th tarsomere and pretarsus (arolium plus two claws) turned upwards and off the substrate. The unusually large arolium was studied in two species of Mantophasmatodea using bright field light microscopy, reflection microscopy, fluorescence microscopy, TEM, SEM, and Cryo‐SEM. It contains an epithelial gland, numerous tracheoles, and nerves. The gland consists of enlarged epithelial cells with large nuclei, mitochondria, RER, golgi complexes, microtubules, and numerous secretion vesicles. Evidence for exocytosis of the vesicles into the gland reservoir between the epithelial gland and the thick cuticle could be observed. Cryo‐SEM revealed that the ventral side of the arolium and distal part of its dorsal side are covered with a liquid film. Fluid footprints of arolia of individuals walking on a glass plate also indicate the presence of secretory fluid on the arolium surface. Behavioral experiments using animals with ablated arolia showed that representatives of Mantophasmatodea do not need their arolia to detect and respond to vibratory communication signals nor to catch small to medium‐sized prey. Individuals with ablated arolia were not able to move upside down on a smooth glass plate. We conclude that Mantophasmatodea use their arolia for attachment when additional adhesion force is required (e.g. windy conditions, handling large prey, mating). They can bring their arolia in contact with the surface in a very fast reflex (18.0 ± 9.9 ms). The secretory fluid found on the surface is produced by the gland and transported to the outside, presumably through small pore channels, to enhance adhesion to the substrate. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Evidence for air movement signals in the agonistic behaviour of a nocturnal arachnid (order Amblypygi)

Many arthropods possess filiform hair sensilla (termed trichobothria in arachnids), which are extremely sensitive detectors of medium particle displacement. Electrophysiological evidence in some taxa suggests that these sensilla can detect air particle displacements resulting from intraspecific communication signals. However, it has not yet been shown for any species that the air particle displacements detected by the filiform hairs are themselves perceived as a 'signal' (i.e. that individuals make behavioural decisions based upon the responses of these organs to the displays of conspecifics). We investigate the agonistic behaviour of the whip spider Phrynus marginemaculatus and the role of its trichobothria in receiving agonistic signals. Whip spiders have extremely elongated 'antenniform' first legs, which they vibrate close to their opponents during agonistic interactions, inducing air movements that excite their opponents' trichobothria. We find that ablation of the trichobothria causes significant increases in: (I) contest duration, and (II) the probability of contest escalation past aggressive displays to physical fighting. Therefore, in the absence of air movement-sensitive sensilla, contest assessment is impaired. This suggests that whip spiders exploit true air movement signals during agonistic interactions, and that these are received by the trichobothria. Furthermore, these results indicate that, in whip spiders, such signals help mitigate the cost of agonistic interaction.     

Scorpions,spiders and solpugids: predation and competition among distantly related taxa

Summary Scorpions, spiders and solpugids are generalist predators on the same types of arthropod prey. However, these potential competitors also frequently eat one another (=intraguild predation). In a 29 mo. experiment, >6,000 scorpions were removed from 300 (10x10m) quadrats. Significantly more spiders (but not solpugids) occurred in removal versus control quadrats. Two alternate hypotheses potentially explain this result: exploitation competition for jointly exploited prey or intraguild predation. There was no evidence of exploitation competition: no differences existed between removal and control quadrats in prey abundance or spider size and reproductive characteristics. It appears that the release from predation pressure in areas from which scorpions were removed produced the observed increase in spider abundance. Current ecological theory does not fully apply to situations whereby species at the same trophic level interact as both predators and potential competitors.     

Maternal care in the soft tick Antricola marginatus

Among spiders, scorpions, and whip spiders, a common type of maternal care consists of females carrying newly hatched offspring on their body for a few days until they are able to live independently. While this maternal care has been suggested to occur in different argasid tick species, it has been recorded only once, for Antricola marginatus in Cuba; however, this earlier record only superficially mentioned the occurrence of this behavior, with no further details. Here we report the occurrence of maternal care in the argasid tick A. marginatus under natural conditions in a cave at Yucatan, Mexico, where 8 A. marginatus females, while walking on bat guano, had their body entirely covered by a mean number of 305 ± 112 conspecific unfed larvae (range: 105-466). Larvae covered the entire idiosoma of the female tick, where they were motionless or displayed just slight movement. This result substantially expands the number of unique characters that have been found only in Antricola spp. ticks, when compared to the other tick genera. Our findings also indicate that maternal care evolved independently in different taxa of Arachnida, since it has been reported for species of Araneae, Scorpiones, and Amblypygi, and here for an Acari species.     

Amblypygid-fungal interactions: The whip spider exoskeleton as a substrate for fungal growth

Fungi and arthropods represent some of the most diverse organisms on our planet, yet the ecological relationships between them remain largely unknown. In animals, fungal growth on body surfaces is often hazardous and is known to cause mortality. In contrast, here we report the presence of an apparently non-harmful mycobiome on the cuticle of whip spiders (Arachnida: Amblypygi). The associations are not species-specific and involve a diversity of fungal species, including cosmopolitan and local decomposers as well as entomopathogens. We discuss the ecology of the detected fungal species and hypothesize that the thick epicuticular secretion coat of whip spiders (the cerotegument) promotes fungal growth. It is possible that this relationship is beneficial towards the host if it leads to parasite control or chemical camouflage. Our findings, which are the first from this arthropod lineage, indicate that non-pathogenic interactions between arthropods and fungi may be much more widespread than predicted and call for more studies in this area.     

Morphology of arolia in Auchenorrhyncha (Insecta,Hemiptera)

The pretarsal arolium serves as an attachment device in many groups of insects, enabling them to walk efficiently on smooth surfaces, where claws alone do not provide sufficient foothold. The arolia of representatives of all major lineages of Auchenorrhyncha are described and illustrated, mainly using scanning electron microscopy and histology. Glands inside the lumen of the arolia are described for the first time in this group. It is shown that the morphology of arolia within Auchenorrhyncha differs considerably. Some of them are even distinctly bilobed. The cuticle of the contact zone is thickened and formed of branching chitinous rods. In some cases, two layers of rods oriented in different directions were found. An extended definition of “arolium” is proposed. J. Morphol. 275:1217–1225, 2014. © 2014 Wiley Periodicals, Inc.     

An integrative study of insect adhesion: mechanics and wet adhesion of pretarsal pads in ants

Many animals that locomote by legs possess adhesive pads. Suchorgans are rapidly releasable and adhesive forces can be controlledduring walking and running. This capacity results from the interactionof adhesive with complex mechanical systems. Here we presentan integrative study of the mechanics and adhesion of smoothattachment pads (arolia) in Asian Weaver ants (Oecophylla smaragdina).Arolia can be unfolded and folded back with each step. Theyare extended either actively by contraction of the claw flexormuscle or passively when legs are pulled toward the body. Regulationof arolium use and surface attachment includes purely mechanicalcontrol inherent in the arrangement of the claw flexor system. Predictions derived from a ‘wet’ adhesion mechanismwere tested by measuring attachment forces on a smooth surfaceusing a centrifuge technique. Consistent with the behavior ofa viscid secretion, frictional forces per unit contact arealinearly increased with sliding velocity and the increment stronglydecreased with temperature. We studied the nature and dimensions of the adhesive liquidfilm using Interference Reflection Microscopy (IRM). Analysisof ‘footprint’ droplets showed that they are hydrophobicand form low contact angles. In vivo IRM of insect pads in contactwith glass, however, revealed that the adhesive liquid filmnot only consists of a hydrophobic fluid, but also of a volatile,hydrophilic phase. IRM allows estimation of the height of theliquid film and its viscosity. Preliminary data indicate thatthe adhesive secretion alone is insufficient to explain theobserved friction and that rubbery deformation of the pad cuticleis involved.     

Size-dependent survivorship in the web-building spiderAgelena limbata

Summary Stage-specific mortality rates and mortality factors for the web-building spiderAgelena limbata, which is suggested to be food-limited, were studied, and the relationship between body size of spiders and survivorship for instar 3 to adults was examined. The mortality rate of the egg sac stage including eggs, deutova (prenymphal stage), and overwintering instar 1 nymphs was low. The low mortality of this stage was partly due to maternal care that reduced the mortality caused by predation and/or abiotic factors. From emergence of instar 1 nymphs from egg sacs to reproduction, the stagespecific mortality rates were almost constant, 32–47%, and the time-specific mortality rates were also constant. These results suggest a Deevey (1947) type II survivorship curve inA. limbata, in contrast to other reports on the wandering or burrowing spiders which suggested type III curves. Important mortality factors for nymphs and adults were parasitism by an ichneumonid wasp and predation by spiders. There were great variations in body size (carapace width) ofA. limbata in the field. Smaller individuals survived at a lower rate to the next stage than larger individuals. This tendency was clearer for the population living under poorer prey availability.A. limbata was unlikely to starve to death in the field because every stage ofA. limbata could survive starvation for a long time in the laboratory, 22–65 days on average. I suggest that the size-dependent survivorship of this spider is associated with vulnerability of smaller individuals to parasitism and predation.     

Terrestrial locomotion in arachnids

In this review, we assess the current state of knowledge on terrestrial locomotion in Arachnida. Arachnids represent a single diverse (>100,000 species) clade containing well-defined subgroups (at both the order and subordinal levels) that vary morphologically around a basic body plan, yet exhibit highly disparate limb usage, running performance, and tarsal attachment mechanisms. Spiders (Araneae), scorpions (Scorpiones), and harvestmen (Opiliones) have received the most attention in the literature, while some orders have never been subject to rigorous mechanical characterization. Most well-characterized taxa move with gaits analogous to the alternating tripod gaits that characterize fast-moving Insecta - alternating tetrapods or alternating tripods (when one pair of legs is lifted from the ground for some other function). However, between taxa, there is considerable variation in the regularity of phasing between legs. Both large and small spiders appear to show a large amount of variation in the distribution of foot-ground contact, even between consecutive step-cycles of a single run. Mechanisms for attachment to vertical surfaces also vary, and may depend on tufts of adhesive hairs, fluid adhesives, silks, or a combination of these. We conclude that Arachnida, particularly with improvements in microelectronic force sensing technology, can serve as a powerful study system for understanding the kinematics, dynamics, and ecological correlates of sprawled-posture locomotion.     

Agonistic signals received by an arthropod filiform hair allude to the prevalence of near-field sound communication

Arthropod filiform hairs respond to air particle movements and are among the most sensitive animal sensory organs. In many species, they are tuned to detect predators or prey and trigger escape or prey capture behaviours. Here we show for the first time that these hairs also receive intraspecific near-field sound signals in an arachnid. During agonistic encounters, whip spiders (Arachnida, Amblypygi) perform antenniform leg vibration (ALV) displays that have significantly longer duration in contest winners than losers. During an ALV display: (i) the vibrating antenniform leg of the displaying whip spider is positioned close to the trichobothria (filiform hairs) on its opponent's walking legs, (ii) the vibrating antenniform leg can excite these trichobothria via air movements and without direct contact, (iii) the antenniform leg of the displaying whip spider vibrates at a frequency that causes particularly strong, sustained excitation and little adaptation in the trichobothria, and (iv) the duration of an ALV display can be extracted from the response of a trichobothrium. Since filiform hairs are widespread among arthropods, communication via such hairs could be extremely prevalent.     

Burrow decorations as antipredatory devices

Animal decorations are normally interpreted as signals of quality.In spiders, however, decorations may have different functions,including the attraction of prey to the web or making the spidercryptic to predators. To date, there is scant evidence for thelatter hypothesis. Here we use the burrow-decorating wolf spiderLycosa tarantula to test whether turrets around the burrow serveto prevent burrow invasion and predation from the Occitan scorpionButhus occitanus. We located spiders and scorpions in fieldenclosures and manipulated the presence or absence of decorationsor turrets. We found that the presence of the turret decreasesthe rate of burrow invasion and improves spider survival, possiblybecause the turret makes the burrow cryptic to scorpions. Inaddition, a field survey showed that burrows with larger decorationshad a lower chance of being invaded by scorpions. These resultsprovide evidence that the decoration has an antipredatory functionin nature.     

Haltung und Zucht der Geißelspinne (Damon variegatus) (Phrynichidae) (Chelicerata: Amblypygi)

In the temporary carboniferous hall of „GONDWANA – Das Prähistorium“ in Germany, whip spiders (Damon variegatus) were kept and bred. 3,3 were purchased from a dealer in summer 2007. The night active spiders were kept together in a terrarium. No aggresions were observed. The animals were fed little locusts, young hissing cockroaches, and crickets. In 2008 courtship display and mating could be observed. The biggest male was the only active one. After 7 days after the intake of the spermatophores all females carried eggsacs under the ophistosoma. Each egg sack contained approximately 25–30 eggs. During the carrying phase only one female feasted on a cricket. 98 days after the egg sack was produced the young spiderlings hatched. The coloration of the spiderlings was completely different from the coloration of the adults. The spiderlings were carried by the female for the next 9 days. Then, the spiderlings moulted and left the female immediately. The adults were now separated from their young. The three groups of spiderlings stayed together in a smaller terrarium. However, each group from each female stayed separate from the others for weeks. The second moulting took place 41 days after the first one. No cannibalism was observed. Obviously Damon variegatus seems to be a spider with a social touch. If the terrarium and the climate conditions inside are adapted to the needs of the whip spiders keeping and breeding is possible. The species is highly attractive to visitors.     

Contractile tissues in the cirri of ancient crinoids: criteria for recognition

Stalked isocrinid and 'stalkless' comatulid crinoids are able to relocate by crawling on or swimming with their muscular arms. Reattachment is achieved using cirri containing contractile tissues which produce aboral flexure. The following cirral adaptations for active attachment were observed during a SEM study of two comasterid comatulids, Davidaster rubiginosa (Pourtalès) and D. discoidea (Carpenter): synarthrial articulations; fulcral ridges corresponding to short axes of ossicles; cirri flattened laterally; each cirrus with a claw at the tip and an aboral attachment pad; cirri serrated aborally and distally. Epizoans are only able to encrust single ossicles in actively motile cirri. These adaptations are associated with crinoids that have muscular arms which are used in relocation. Such a suite of characters is unknown in Palaeozoic crinoids; the ability to relocate only evolved in crinoids during the Mesozoic.□ Crinoids, cirri, comatulids, evoiution, functional morphology .     

Haltung und Zucht der Geißelspinne (Damon variegatus) (Phrynichidae) (Chelicerata: Amblypygi)

In the temporary carboniferous hall of „GONDWANA - Das Prähistorium“ in Germany, whip spiders (Damon variegatus) were kept and bred. 3,3 were purchased from a dealer in summer 2007. The night active spiders were kept together in a terrarium. No aggresions were observed. The animals were fed little locusts, young hissing cockroaches, and crickets. In 2008 courtship display and mating could be observed. The biggest male was the only active one. After 7 days after the intake of the spermatophores all females carried eggsacs under the ophistosoma. Each egg sack contained approximately 25-30 eggs. During the carrying phase only one female feasted on a cricket. 98 days after the egg sack was produced the young spiderlings hatched. The coloration of the spiderlings was completely different from the coloration of the adults. The spiderlings were carried by the female for the next 9 days. Then, the spiderlings moulted and left the female immediately. The adults were now separated from their young. The three groups of spiderlings stayed together in a smaller terrarium. However, each group from each female stayed separate from the others for weeks. The second moulting took place 41 days after the first one. No cannibalism was observed. Obviously Damon variegatus seems to be a spider with a social touch. If the terrarium and the climate conditions inside are adapted to the needs of the whip spiders keeping and breeding is possible. The species is highly attractive to visitors.     

Tarsal morphology and attachment ability of the codling moth Cydia pomonella L. (Lepidoptera, Tortricidae) to smooth surfaces

Despite several studies on the attachment ability of different insect taxa, little is known about this phenomenon in adult Lepidoptera. In this study we combined morphological and experimental analyses of tarsal adhesive devices and the attachment ability of the codling moth Cydia pomonella (L.) (Lepidoptera, Tortricidae) to smooth surfaces. Pretarsi of C. pomonella attach to smooth substrates by means of their smooth, flexible and well developed arolia. Using the centrifugal force measurement technique, friction forces of males and females were assessed on hydrophobic and hydrophilic glass surfaces. Adults of both sexes generated similar forces in spite of the noticeable difference in their body masses. That is why males showed significantly higher safety factors (attachment force divided by body weight) compared to those of females. Hydrophobicity of the substrate had no considerable effect on friction forces. For females, friction forces (sliding parallel to the substrate plane) were compared with adhesive forces (pulling off perpendicularly from the substrate plane) measured on Plexiglas surfaces. It can be concluded that the attachment system of C. pomonella is rather robust against physico-chemical properties of the substrate and is able to achieve a very good attachment on vertical and horizontal substrata.     

The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): evidence for homology and a single terrestrialisation event of a common arachnid ancestor

The question of whether Arachnida (Chelicerata) conquered terrestrial habitats only once or several times is controversial. The key group in this respect is the Scorpiones. Several authors claim that they became terrestrial independently of other arachnid lineages. This argumentation uses two lines of evidence. One is that book lungs of scorpions and other arachnids are considered non-homologous because they occur on different segments. The other line is based on fossil evidence which suggests that early scorpions were aquatic, together with a putative sister group relationship between scorpions and the aquatic Eurypterida. To address this problem we undertook a comparative scanning electron microscopical and histological study of the book lungs of scorpions, amblypygids, uropygids, and mesothelid spiders. In addition, we included the book gills of a xiphosuran. We found several detailed similarities in the book lungs shared by all arachnid taxa studied. Based on these findings we conclude that arachnid book lungs are homologous. Furthermore, we suggest that the apomorphic book lungs of arachnids indicate a single terrestrialisation event in the stem lineage leading to Arachnida.     

Hemprich’s long-eared bat (<Emphasis Type="Italic">Otonycteris hemprichii</Emphasis>) as a predator of scorpions: whispering echolocation,passive gleaning and prey selection

Over 70% of the droppings of the gleaning bat Otonycteris hemprichii can contain scorpion fragments. Yet, some scorpions found in its desert habitat possess venom of the highest known toxicity, rendering them a very dangerous prey. In this study, we describe how O. hemprichii catches and handles scorpions, quantify its flight and echolocation behaviour in the field, investigate what sensory modality it uses to detect scorpions, and test whether it selects scorpions according to their size or toxicity. We confirmed that O. hemprichi is a whispering bat (approx. 80 dB peSPL) with short, multi-harmonic calls. In a flight room we also confirmed that O. hemprichii detects scorpions by their walking noises. Amplitudes of such noises were measured and they reach the flying bat at or below the level of echoes of the loess substrate. Bats dropped straight onto moving scorpions and were stung frequently even straight in their face. Stings did not change the bats’ behaviour and caused no signs of poisoning. Scorpions were eaten including poison gland and stinger. Bats showed no preference neither for any of the scorpion species nor their size suggesting they are generalist predators with regard to scorpions.