Defensive stinging by Parabuthus transvaalicus scorpions: risk assessment and venom metering

Evaluating the Use of Chemical Weapons for Capturing Prey by a Venomous Mammal, the Greater Slow Loris (Nycticebus coucang)

Few mammals are venomous, including one group of primates-slow (Nycticebus spp.) and pygmy (Xanthonycticebus spp.) lorises. Hypotheses for the evolutionary function of venom in these primates include defense from predators or ectoparasites, communication or competition with conspecifics, and the capture of prey. We tested the prey capture hypothesis in 75 trials with 22 wild-caught greater slow lorises (N. coucang) housed in a rescue center in Java, Indonesia. We experimentally offered the slow lorises arthropod prey items varying in size, escape potential, and toxicity and recorded venom-related and predatory behaviors using live and video observations. The slow lorises visually targeted arthropod prey, approached it quickly and efficiently, and captured it with a manual grasping motion. They rarely performed venom-related behaviors and seemed to do so in a defensive context. The slow lorises exhibited little variation in pre-capture behavior as a function of prey size or escape potential. In response to noxious prey, the slow lorises performed tongue-flicking and other investigative behaviors that indicate they are using chemosensory input to assess prey characteristics. These data suggest it is unlikely that slow lorises use chemical weapons to subdue arthropod prey and may support, instead, a defensive function for slow loris venom.

Proteomic profile of the venom of three dark-colored Tityus (Scorpiones: Buthidae) from the tropical rainforests of Costa Rica

OBJECTIVE

We aimed to elucidate the potential differences in the venom peptide sequences of three Tityus species from Costa Rican rainforests: T. jaimei, T. championi and T. dedoslargos, compared to T. cf. asthenes from Colombia, which could explain the low level of scorpionism in Costa Rica, evidenced by the lack of epidemiological data.

METHODOLOGY

We applied venom proteomics of peptides purified by RP-HPLC and compared the obtained sequences from venoms of these Tityus species to the sequences previously identified from Tityus inhabiting other Central and South American regions.

RESULTS

Venom proteome analysis evidences that most of the putative peptide toxins identified in Costa Rican dark-colored Tityus are very similar to those present in other T. (Atreus) from the region.

CONCLUSIONS

Our study suggests that, in the case of potential envenomation by Tityus in Costa Rica, the same level of toxicity should be observed, compared to other cases caused by members of the subgenus from other geographical localities. On the other hand, compared to countries with more accelerated urban expansion, Costa Rican Tityus still inhabit secondary rainforests and do not commonly share the same spaces with humans, so the lack of epidemiological evidence of medical emergencies caused by envenoming by this scorpion group could be more related to ecological and demographic factors and less attributed to the characteristics of the venom.

Peptidomics

Peptides are biopolymers, typically consisting of 2-50 amino acids. They are biologically produced by the cellular ribosomal machinery or by non-ribosomal enzymes and, sometimes, other dedicated ligases. Peptides are arranged as linear chains or cycles, and include post-translational modifications, unusual amino acids and stabilizing motifs. Their structure and molecular size render them a unique chemical space, between small molecules and larger proteins. Peptides have important physiological functions as intrinsic signalling molecules, such as neuropeptides and peptide hormones, for cellular or interspecies communication, as toxins to catch prey or as defence molecules to fend off enemies and microorganisms. Clinically, they are gaining popularity as biomarkers or innovative therapeutics; to date there are more than 60 peptide drugs approved and more than 150 in clinical development. The emerging field of peptidomics comprises the comprehensive qualitative and quantitative analysis of the suite of peptides in a biological sample (endogenously produced, or exogenously administered as drugs). Peptidomics employs techniques of genomics, modern proteomics, state-of-the-art analytical chemistry and innovative computational biology, with a specialized set of tools. The complex biological matrices and often low abundance of analytes typically examined in peptidomics experiments require optimized sample preparation and isolation, including in silico analysis. This Primer covers the combination of techniques and workflows needed for peptide discovery and characterization and provides an overview of various biological and clinical applications of peptidomics.

Worldwide Web: High Venom Potency and Ability to Optimize Venom Usage Make the Globally Invasive Noble False Widow Spider Steatoda nobilis (Thorell, 1875) (Theridiidae) Highly Competitive against Native European Spiders Sharing the Same Habitats

Venom compositions include complex mixtures of toxic proteins that evolved to immobilize/dissuade organisms by disrupting biological functions. Venom production is metabolically expensive, and parsimonious use is expected, as suggested by the venom optimisation hypothesis. The decision-making capacity to regulate venom usage has never been demonstrated for the globally invasive Noble false widow Steatoda nobilis (Thorell, 1875) (Theridiidae). Here, we investigated variations of venom quantities available in a wild population of S. nobilis and prey choice depending on venom availability. To partially determine their competitiveness, we compared their attack rate success, median effective dose (ED₅₀) and lethal dose (LD₅₀), with four sympatric synanthropic species: the lace webbed spider Amaurobius similis, the giant house spider Eratigena atrica, the missing sector orb-weaver Zygiella x-notata, and the cellar spider Pholcus phalangioides. We show that S. nobilis regulates its venom usage based on availability, and its venom is up to 230-fold (0.56 mg/kg) more potent than native spiders. The high potency of S. nobilis venom and its ability to optimize its usage make this species highly competitive against native European spiders sharing the same habitats.

Biochemical characterization and insecticidal activity of isolated peptides from the venom of the scorpion Centruroides tecomanus

The venom of scorpions is a mixture of components that constitute a source of bioactive molecules. The venom of the scorpion Centruroides tecomanus contains peptides toxic to insects, however, to date no toxin responsible for this activity has yet been isolated and fully characterized. This communication describes two new peptides Ct-IT1 and Ct-IT2 purified from this scorpion. Both peptides contain 63 amino acids with molecular weight 6857.85 for Ct-IT1 and 6987.77 Da for Ct-IT2. The soluble venom was separated using chromatographic techniques of molecular size exclusion, cationic exchange, and reverse phase chromatography, allowing the identification of at least 99 components of which in 53 the insecticidal activity was evaluated. The LD₅₀ determined for Ct-IT1 is 3.81 μg/100 mg of cricket weight, but low amounts of peptides (0.8 μg of peptide) already cause paralysis in crickets. The relative abundance of these two peptides in the venom is 2.1% for Ct-IT1 and 1% for Ct-IT2. The molecular masses and N-terminal sequences of both insecticidal toxins were determined by mass spectrometry and Edman degradation. The primary structure of both toxins was compared with other known peptides isolated from other scorpion venoms. The analysis of the sequence alignments revealed the position of a highly conserved amino acid residue, Gly39, exclusively present in anti-insect selective depressant β-toxins (DBTXs), which in Ct-IT1 and Ct-IT2 is at position Gly40. Similarly, a three-dimensional structure of this toxins was obtained by homology modeling and compared to the structure of known insect toxins of scorpions. An important similarity of the cavity formed by the trapping apparatus region of the depressant toxin LqhIT2, isolated from the scorpion Leiurus quinquestriatus hebraeus, was found in the toxins described here. These results indicate that Ct-IT1 and Ct-IT2 toxins have a high potential to be evaluated on pests that affect economically important crops to eventually consider them as a potential biological control method.

Linking toxicity and predation in a venomous arthropod: the case of Tityus fuhrmanni (Scorpiones: Buthidae), a generalist predator scorpion

Background:

Scorpions are arachnids that have a generalist diet, which use venom to subdue their prey. The study of their trophic ecology and capture behavior is still limited compared to other organisms, and aspects such as trophic specialization in this group have been little explored.

Methods:

In order to determine the relationship between feeding behavior and venom toxicity in the scorpion species Tityus fuhrmanni, 33 specimens were offered prey with different morphologies and defense mechanisms: spiders, cockroaches and crickets. In each of the experiments we recorded the following aspects: acceptance rate, immobilization time and the number of capture attempts. The median lethal dose of T. fuhrmanni venom against the three different types of prey was also evaluated.

Results:

We found that this species does not have a marked difference in acceptance for any of the evaluated prey, but the number of capture attempts of spiders is higher when compared to the other types of prey. The immobilization time is shorter in spiders compared to other prey and the LD₅₀ was higher for cockroaches.

Conclusions:

These results indicate that T. fuhrmanni is a scorpion with a generalist diet, has a venom with a different potency among prey and is capable of discriminating between prey types and employing distinct strategies to subdue them.

Armed stem to stinger: a review of the ecological roles of scorpion weapons

Scorpions possess two systems of weapons: the pincers (chelae) and the stinger (telson). These are placed on anatomically and developmentally well separated parts of the body, that is, the oral appendages and at the end of the body axis. The otherwise conserved body plan of scorpions varies most in the shape and relative dimensions of these two weapon systems, both across species and in some cases between the sexes. We review the literature on the ecological function of these two weapon systems in each of three contexts of usage: (i) predation, (ii) defense and (iii) sexual contests. In the latter context, we will also discuss their usage in mating. We first provide a comparative background for each of these contexts of usage by giving examples of other weapon systems from across the animal kingdom. Then, we discuss the pertinent aspects of the anatomy of the weapon systems, particularly those aspects relevant to their functioning in their ecological roles. The literature on the functioning and ecological role of both the chelae and the telson is discussed in detail, again organized by context of usage. Particular emphasis is given on the differences in morphology or usage between species or higher taxonomic groups, or between genders, as such cases are most insightful to understand the roles of each of the two distinct weapon systems of the scorpions and their evolutionary interactions. We aimed to synthesize the literature while minimizing conjecture, but also to point out gaps in the literature and potential future research opportunities.

The biology and evolution of spider venoms

Spiders are diverse, predatory arthropods that have inhabited Earth for around 400 million years. They are well known for their complex venom systems that are used to overpower their prey. Spider venoms contain many proteins and peptides with highly specific and potent activities suitable for biomedical or agrochemical applications, but the key role of venoms as an evolutionary innovation is often overlooked, even though this has enabled spiders to emerge as one of the most successful animal lineages. In this review, we discuss these neglected biological aspects of spider venoms. We focus on the morphology of spider venom systems, their major components, biochemical and chemical plasticity, as well as ecological and evolutionary trends. We argue that the effectiveness of spider venoms is due to their unprecedented complexity, with diverse components working synergistically to increase the overall potency. The analysis of spider venoms is difficult to standardize because they are dynamic systems, fine-tuned and modified by factors such as sex, life-history stage and biological role. Finally, we summarize the mechanisms that drive spider venom evolution and highlight the need for genome-based studies to reconstruct the evolutionary history and physiological networks of spider venom compounds with more certainty.

Do pit vipers assess their venom? Defensive tactics of Deinagkistrodon acutus shift with changed venom reserve

To maximize survival probability, animals must assess predation risks and adopt flexible defensive strategies based on specific conditions. Pit vipers utilize venom for predation and self-defense, and venom status significantly influences its effectiveness. Thus, pit vipers may evaluate their venom reserve and adopt corresponding defensive tactics. Twenty-three sharp-snouted pit vipers (Deinagkistrodon acutus) were grouped by different venom status and were subjected to eight behavior trials. Subjects' defensive behaviors were recorded and analyzed. Results showed that the normal venom group displayed stable responses across the trials. The low venom group showed fewer strikes and more fleeing behaviors at the end of experiments. After given prolonged intervals for replenishing the venom, significant increases of strike behaviors were observed in the replenishing venom group. These results demonstrated the capability of adopting flexible defensive tactics based on varied venom reserve and provided new evidence for venom-status-recognition.

Do predators react differently to dangerous and larger prey? The case of a mygalomorph generalist spider preying upon insects

Prey morphology and size are known to influence a predator's decision to attack and consume particular prey; however, studies that evaluate both traits simultaneously are uncommon. Here, we first described the trophic niche in the mygalomorph spider Paratropis sp. These spiders have a narrow trophic niche and feed mainly on sympatric species such as larvae of lepidopterans and of beetles such as carabids, passalids and scarabeids. Second, we evaluated the effect of prey taxon and size on acceptance and immobilization duration, and built an ethogram of the predator's behaviors. For each prey taxa, we offered large (approximately same size than the spider) and small larvae (approximately half of the size of the spider) of the aforementioned prey. We classified carabid beetle larvae as the most dangerous prey because of their sharp mandibles and predatory habits, followed by scarabeid larvae; lepidopteran and passalids larvae were considered to be non-dangerous prey. We did not find a significant effect of prey taxon or size on spiders' acceptance. Prey size did not affect the time invested on each behavioral category, but prey taxon did. Moreover, although spiders used a similar strategy for capturing prey, they spent more time biting carabid larvae than other prey. Our results suggest that, at least in our study area, prey that are dangerous or the predator's size do not seem to affect Paratropis sp. acceptance per se, but can cause the predator to change the time budget allocated to each behavior.

Venom Collection from Spiders and Snakes: Voluntary and Involuntary Extractions ("Milking") and Venom Gland Extractions

Venom collection (often called "milking") provides the toxic secretions essential for studying animal venoms and/or generating venom products. Methods of venom collection vary widely, falling into three broad categories: voluntary venom extraction (inducing the animal to willingly release its venom), involuntary venom extraction (glandular massage, electrical stimulation, or administration of induction chemicals to promote venom expulsion), and venom gland extraction (surgical aspiration or trituration of homogenized gland tissue). Choice of method requires consideration of animal species, animal welfare, human safety (avoiding envenomation), venom yield and composition desired, and level of toxin purity required. Here, we summarize the materials and methods used to obtain venom by each of these approaches from spiders and snakes.

Context-dependent venom deployment and protein composition in two assassin bugs

The Heteroptera are a diverse suborder of phytophagous, hematophagous, and zoophagous insects. The shift to zoophagy can be traced back to the transformation of salivary glands into venom glands, but the venom is used not only to kill and digest invertebrate prey but also as a defense strategy, mainly against vertebrates. In this study, we used an integrated transcriptomics and proteomics approach to compare the composition of venoms from the anterior main gland (AMG) and posterior main gland (PMG) of the reduviid bugs Platymeris biguttatus L. and Psytalla horrida Stål. In both species, the AMG and PMG secreted distinct protein mixtures with few interspecific differences. PMG venom consisted mostly of S1 proteases, redulysins, Ptu1-like peptides, and uncharacterized proteins, whereas AMG venom contained hemolysins and cystatins. There was a remarkable difference in biological activity between the AMG and PMG venoms, with only PMG venom conferring digestive, neurotoxic, hemolytic, antibacterial, and cytotoxic effects. Proteomic analysis of venom samples revealed the context-dependent use of AMG and PMG venom. Although both species secreted PMG venom alone to overwhelm their prey and facilitate digestion, the deployment of defensive venom was species-dependent. P. biguttatus almost exclusively used PMG venom for defense, whereas P. horrida secreted PMG venom in response to mild harassment but AMG venom in response to more intense harassment. This intriguing context-dependent use of defensive venom indicates that future research should focus on species-dependent differences in venom composition and defense strategies among predatory Heteroptera.

Risk Assessment and the Effects of Refuge Availability on the Defensive Behaviors of the Southern Unstriped Scorpion (Vaejovis carolinianus)

Selection should favor individuals that acquire, process, and act on relevant environmental signals to avoid predation. Studies have found that scorpions control their use of venom: both when it is released and the total volume expelled. However, this research has not included how a scorpion’s awareness of environmental features influences these decisions. The current study tested 18 Vaejovis carolinianus scorpions (nine females and nine males) by placing them in circular arenas supplied with varying numbers (zero, two, or four) of square refuges and by tracking their movements overnight. The following morning, defensive behaviors were elicited by prodding scorpions on the chelae, prosoma, and metasoma once per second over 90 s. We recorded stings, venom use, chelae pinches, and flee duration. We found strong evidence that, across all behaviors measured, V. carolinianus perceived prods to the prosoma as more threatening than prods to the other locations. We found that stinging was a common behavior and became more dominant as the threat persisted. Though tenuous, we found evidence that scorpions’ defensive behaviors changed based on the number of refuges and that these differences may be sex specific. Our findings suggest that V. carolinianus can assess risk and features of the local environment and, therefore, alter their defensive strategies accordingly.

Biology, venom composition, and scorpionism induced by brazilian scorpion Tityus stigmurus (Thorell, 1876) (Scorpiones: Buthidae): A mini-review

Scorpionism is a serious public health problem in various regions of the world. In Brazil, a high number of accidents by scorpions have been reported. From 2014 to 2018, about 547,000 cases were recorded, resulting in 466 deaths. The scorpion Tityus stigmurus is the predominant species in the northeast of Brazil, being responsible for most scorpionism cases in this region. With the aid of the transcriptomic approach of the venom gland of this species, components as neurotoxins, antimicrobials, metal chelating peptides and hypotensins, have been identified and characterized in silico, showing different biologic activity in vitro. In addition, the neuronal, pancreatic, renal, and enzymatic effects have been demonstrated for the crude T. stigmurus venom. Therefore, the T. stigmurus scorpion venom constitutes a rich arsenal of bioactive molecules with high potential for therapeutic and biotechnological application.

Scorpion envenomation: state of the art

Scorpion envenomation is common in the tropical and subtropical regions. It poses a major public health problem with some patients having serious clinical manifestations and severe complications including death. Old World and New World scorpions are usually contrasted because of differences in venom composition, clinical presentation and severity, and, accordingly, different therapeutic approaches. The majority of scorpion stings are either dry or result in low amounts of injected venom, thus explaining why up to 95% of scorpion stings ensue only in local signs. For a clinical envenomation to occur, it has been suggested that the interaction between the quantity of venom introduced in the body of the prey and the distribution volume should ensue in a critical threshold of scorpion toxin plasma concentration. In this case, there is a massive release of neurohormonal mediators (mainly catecholamine), with systemic vasoconstrictor effects eliciting a sharp increase in systemic arterial pressure and LV-filling pressure and decreased cardiac output. This early phase of cardiac dysfunction, also called "vascular phase", is followed by a severe cardiomyopathy, a form of Takotsubo cardiomyopathy, involving both ventricles and reversible in days to weeks. The more comprehensive understanding of the disease pathophysiology has allowed for a well-codified symptomatic treatment, thus contributing to a substantial reduction in the death toll of scorpion envenomation over the past few decades. The standard intensive-care treatment (when available) overcomes envenomation's consequences such as acute pulmonary edema and cardiogenic shock. Even though it continues to inspire many evaluative studies, immunotherapy seems less attractive because of the major role held by mediators in the pathogenesis of envenomation, and unfavorable pharmacokinetic properties to existing sera compared to venom. Meta-analyses of controlled trials of immunotherapy in severe scorpion envenomation reached similar conclusions: there is an acceptable level of evidence in favor of the use of scorpion antivenom (Fab'₂) against Centruroides sp. in USA/Mexico, while there is still a need for a higher level of evidence for immunotherapy in the Old World envenomation.

The Diversity of Venom: The Importance of Behavior and Venom System Morphology in Understanding Its Ecology and Evolution

Venoms are one of the most convergent of animal traits known, and encompass a much greater taxonomic and functional diversity than is commonly appreciated. This knowledge gap limits the potential of venom as a model trait in evolutionary biology. Here, we summarize the taxonomic and functional diversity of animal venoms and relate this to what is known about venom system morphology, venom modulation, and venom pharmacology, with the aim of drawing attention to the importance of these largely neglected aspects of venom research. We find that animals have evolved venoms at least 101 independent times and that venoms play at least 11 distinct ecological roles in addition to predation, defense, and feeding. Comparisons of different venom systems suggest that morphology strongly influences how venoms achieve these functions, and hence is an important consideration for understanding the molecular evolution of venoms and their toxins. Our findings also highlight the need for more holistic studies of venom systems and the toxins they contain. Greater knowledge of behavior, morphology, and ecologically relevant toxin pharmacology will improve our understanding of the evolution of venoms and their toxins, and likely facilitate exploration of their potential as sources of molecular tools and therapeutic and agrochemical lead compounds.

Evolutionary Ecology of Fish Venom: Adaptations and Consequences of Evolving a Venom System

Research on venomous animals has mainly focused on the molecular, biochemical, and pharmacological aspects of venom toxins. However, it is the relatively neglected broader study of evolutionary ecology that is crucial for understanding the biological relevance of venom systems. As fish have convergently evolved venom systems multiple times, it makes them ideal organisms to investigate the evolutionary ecology of venom on a broader scale. This review outlines what is known about how fish venom systems evolved as a result of natural enemy interactions and about the ecological consequences of evolving a venom system. This review will show how research on the evolutionary ecology of venom in fish can aid in understanding the evolutionary ecology of animal venoms more generally. Further, understanding these broad ecological questions can shed more light on the other areas of toxinology, with applications across multiple disciplinary fields.

Target-Specificity in Scorpions; Comparing Lethality of Scorpion Venoms across Arthropods and Vertebrates

Scorpions use their venom in defensive situations as well as for subduing prey. Since some species of scorpion use their venom more in defensive situations than others, this may have led to selection for differences in effectiveness in defensive situations. Here, we compared the LD₅₀ of the venom of 10 species of scorpions on five different species of target organisms; two insects and three vertebrates. We found little correlation between the target species in the efficacy of the different scorpion venoms. Only the two insects showed a positive correlation, indicating that they responded similarly to the panel of scorpion venoms. We discuss the lack of positive correlation between the vertebrate target species in the light of their evolution and development. When comparing the responses of the target systems to individual scorpion venoms pairwise, we found that closely related scorpion species tend to elicit a similar response pattern across the target species. This was further reflected in a significant phylogenetic signal across the scorpion phylogeny for the LD₅₀ in mice and in zebrafish. We also provide the first mouse LD₅₀ value for Grosphus grandidieri.

A biomechanical view on stinger diversity in scorpions

Scorpions have elongated metasomas that bear a telson, which is used as a stinger for venom injection. There is a remarkable diversity in the use of the stinger among scorpions, comprising defensive behavior, prey subjugation and mating. This diversity could be reflected by the shape of the telson, as different stinging behaviors will result in very different functional demands. Here we explored the diversity of telson shapes in scorpions by providing morphological measurements, such as curvature and tip angle, as well as by testing stingers under load using finite element analysis (FEA). FEA models were loaded with forces scaled to the surface area of the models, to allow comparison of the relative strain energy based on shape alone. Load force angle was rotated to identify the optimal stinging angle based on the lowest strain energy. Aculeus length and mean aculeus height correlated with minimal strain energy. Optimal stinging angle correlated with tip angle, and differed from the tip angle by about 28.4 ± 6.22 °. We found that species that are more venomous have long aculei (stinger barbs) with a larger radius of curvature. FEA models of these longer aculei showed basal stress concentrations, indicating a potential greater risk of basal breakage due to shape alone. Telsons with shorter and thicker aculeus shapes showed stress concentrations at the tip only. Despite these marked differences in shape, we found no difference in the scaled strain energy between groups of species that are more venomous and less venomous groups of species. These results show that scorpion stingers may be biomechanically optimized, and this may indicate different usage of the stinger in different species.

Sex Differences in Defensive Behavior and Venom of The Striped Bark Scorpion Centruroides vittatus (Scorpiones: Buthidae)

Studies of venom variability have advanced from describing the mechanisms of action and relative potency of medically important toxins to understanding the ecological and evolutionary causes of the variability itself. While most studies have focused on differences in venoms among taxa, populations, or age-classes, there may be intersexual effects as well. Striped bark scorpions (Centruroides vittatus) provide a good model for examining sex differences in venom composition and efficacy, as this species exhibits dramatic sexual dimorphism in both size and defensive behavior; when threatened by an enemy, larger, slower females stand and fight while smaller, fleeter males prefer to run. We here add evidence suggesting that male and female C. vittatus indeed have different defensive propensities; when threatened via an electrical stimulus, females were more likely to sting than were males. We reasoned that intersexual differences in defensive phenotypes would select for venoms with different functions in the two sexes; female venoms should be effective at predator deterrence, whereas male venoms, less utilized defensively, might be better suited to capturing prey or courting females. This rationale led to our predictions that females would inject more venom and/or possess more painful venom than males. We were wrong. While females do inject more venom than males in a defensive sting, females are also larger; when adjusted for body size, male and female C. vittatus commit equal masses of venom in a sting to a potential enemy. Additionally, house mice (Mus musculus) find an injection of male venom more irritating than an equal amount of female venom, likely because male venom contains more of the toxins that induce pain. Taken together, our results suggest that identifying the ultimate causes of venom variability will, as we move beyond adaptive storytelling, be hard-won.

The dilemma of choosing a reference character for measuring sexual size dimorphism, sexual body component dimorphism, and character scaling: cryptic dimorphism and allometry in the scorpion Hadrurus arizonensis

Sexual differences in morphology, ranging from subtle to extravagant, occur commonly in many animal species. These differences can encompass overall body size (sexual size dimorphism, SSD) or the size and/or shape of specific body parts (sexual body component dimorphism, SBCD). Interacting forces of natural and sexual selection shape much of the expression of dimorphism we see, though non-adaptive processes may be involved. Differential scaling of individual features can result when selection favors either exaggerated (positive allometry) or reduced (negative allometry) size during growth. Studies of sexual dimorphism and character scaling rely on multivariate models that ideally use an unbiased reference character as an overall measure of body size. We explored several candidate reference characters in a cryptically dimorphic taxon, Hadrurus arizonensis. In this scorpion, essentially every body component among the 16 we examined could be interpreted as dimorphic, but identification of SSD and SBCD depended on which character was used as the reference (prosoma length, prosoma area, total length, principal component 1, or metasoma segment 1 width). Of these characters, discriminant function analysis suggested that metasoma segment 1 width was the most appropriate. The pattern of dimorphism in H. arizonensis mirrored that seen in other more obviously dimorphic scorpions, with static allometry trending towards isometry in most characters. Our findings are consistent with the conclusions of others that fecundity selection likely favors a larger prosoma in female scorpions, whereas sexual selection may favor other body parts being larger in males, especially the metasoma, pectines, and possibly the chela. For this scorpion and probably most other organisms, the choice of reference character profoundly affects interpretations of SSD, SBCD, and allometry. Thus, researchers need to broaden their consideration of an appropriate reference and exercise caution in interpreting findings. We highly recommend use of discriminant function analysis to identify the least-biased reference character.

Venom-spraying behavior of the scorpion Parabuthus transvaalicus (Arachnida: Buthidae)

Many animals use chemical squirting or spraying behavior as a defensive response. Some members of the scorpion genus Parabuthus (family Buthidae) can spray their venom. We examined the stimulus control and characteristics of venom spraying by Parabuthus transvaalicus to better understand the behavioral context for its use. Venom spraying occurred mostly, but not always, when the metasoma (tail) was contacted (usually grasped by forceps), and was absent during stinging-like thrusts of the metasoma apart from contact. Scorpions were significantly more likely to spray when contact was also accompanied by airborne stimuli. Sprays happened almost instantaneously following grasping by forceps (median=0.23s) as a brief (0.07-0.30s, mean=0.18s), fine stream (<5° arc) that was not directed toward the stimulus source; however, rapid independent movements of the metasoma and/or telson (stinger) often created a more diffuse spray, increasing the possibility of venom contact with the sensitive eyes of potential scorpion predators. Successive venom sprays varied considerably in duration and velocity. Collectively, these results suggest that venom spraying might be useful as an antipredator function and can be modulated based on threat.

Variation in venom yield and protein concentration of the centipedes Scolopendra polymorpha and Scolopendra subspinipes

Venom generally comprises a complex mixture of compounds representing a non-trivial metabolic expense. Accordingly, natural selection should fine-tune the amount of venom carried within an animal's venom gland(s). The venom supply of scolopendromorph centipedes likely influences their venom use and has implications for the severity of human envenomations, yet we understand very little about their venom yields and the factors influencing them. We investigated how size, specifically body length, influenced volume yield and protein concentration of electrically extracted venom in Scolopendra polymorpha and Scolopendra subspinipes. We also examined additional potential influences on yield in S. polymorpha, including relative forcipule size, relative mass, geographic origin (Arizona vs. California), sex, time in captivity, and milking history. Volume yield was linearly related to body length, and S. subspinipes yielded a larger length-specific volume than S. polymorpha. Body length and protein concentration were uncorrelated. When considering multiple influences on volume yield in S. polymorpha, the most important factor was body length, but yield was also positively associated with relative forcipule length and relative body mass. S. polymorpha from California yielded a greater volume of venom with a higher protein concentration than conspecifics from Arizona, all else being equal. Previously milked animals yielded less venom with a lower protein concentration. For both species, approximately two-thirds of extractable venom was expressed in the first two pulses, with remaining pulses yielding declining amounts, but venom protein concentration did not vary across pulses. Further study is necessary to ascertain the ecological significance of the factors influencing venom yield and how availability may influence venom use.

Role of the animal model on the pharmacokinetics of equine-derived antivenoms

Antivenom pharmacokinetics has been studied in heterologous models in which the animal species used as immunoglobulin source is different from that used as recipient. In these models, after intravenous administration of antivenom, the plasma concentration of immunoglobulins shows a rapid initial declining-phase followed by a slower terminal-phase, which has been associated with antivenom distribution and elimination, respectively. We have compared pharmacokinetic parameters for equine-derived antivenom in homologous (horse) and heterologous (cow) models. It was found that the maximum concentration is lower in cows than in horses. Additionally, the steady-state distribution volume is higher in cows as compared to horses. On the other hand, models were not different in the time required to reach the maximum concentration, the area under the concentration/time curve, the half-life of decay during the slowest phase, the systemic clearance and the mean residence time. Similar results were obtained in a rabbit model, in which the pharmacokinetics was also affected by passive immunization of rabbits with anti-equine IgG. We conclude that, in addition to other physiological differences (e.g. cardiac frequency, plasmatic volume, glomerular filtration rate, etc.) between animal models, the ability to remove foreign immunoglobulins might influence the way in which the plasma concentration of antivenom decreases over time, thereby distorting the pharmacokinetic predictions based on non-compartmental models.

Complex cocktails: the evolutionary novelty of venoms

Venoms have evolved on numerous occasions throughout the animal kingdom. These 'biochemical weapon systems' typically function to facilitate, or protect the producing animal from, predation. Most venomous animals remain unstudied despite venoms providing model systems for investigating predator-prey interactions, molecular evolution, functional convergence, and novel targets for pharmaceutical discovery. Through advances in 'omic' technologies, venom composition data have recently become available for several venomous lineages, revealing considerable complexity in the processes responsible for generating the genetic and functional diversity observed in many venoms. Here, we review these recent advances and highlight the ecological and evolutionary novelty of venom systems.

Prey orientation and the role of venom availability in the predatory behaviour of the centipede Scolopendra subspinipes mutilans (Arthropoda: Chilopoda)

Many animal phyla contain clades in which most or all species are venom-injecting predators. An example, in the arthropods, is the class Chilopoda, containing the approximately 3500 species of centipedes. Very little ecological or behavioural work yielding quantitative data has been conducted on centipede predation. Here, we describe a study of this kind. Our experiments employed one centipede species - a large tropical one, Scolopendra subspinipes mutilans - and two species of prey - a cricket, Gryllus assimilis, and a locust, Schistocerca gregaria. We conducted two experiments. The first was aimed at investigating the extent to which the centipedes attacked prey in particular tagmata as opposed to at random over the whole body surface. The results showed that the centipedes were highly selective, preferring to attack the head or thorax rather than the abdomen; indeed, they often reoriented the prey in order to achieve this. A possible explanation of this behaviour is to maximize the speed with which the neurotoxins in the venom reach either the brain or the thoracic ganglia that control limb movement. The second experiment was aimed at investigating the effect of venom-extraction on the attack rate, and specifically at testing if the magnitude of any such effect differed between the two types of prey, which differ considerably in size. The results showed a major effect of venom extraction in relation to both types of prey, but with the time taken to return to a 'normal' attack rate being longer in the case of the larger prey-type, namely the locust. We discuss these results in relation to the 'venom optimization hypothesis' and, more generally, to the principle of minimizing the production/use of venom, which is an energetically expensive resource.

Investigating the chemical profile of regenerated scorpion (Parabuthus transvaalicus) venom in relation to metabolic cost and toxicity

We investigated the biochemical profile of regenerated venom of the scorpion Parabuthus transvaalicus in relation to its metabolic cost and toxicity. Using a closed-system respirometer, we compared oxygen consumption between milked and unmilked scorpions to determine the metabolic costs associated with the first 192 h of subsequent venom synthesis. Milked scorpions had a substantially (21%) higher mean metabolic rate than unmilked scorpions, with the largest increases in oxygen consumption occurring at approximately 120 h, 162 h, and 186 h post-milking. Lethality tests in crickets indicated that toxicity of the regenerated venom returned to normal levels within 4 d after milking. However, the chemical profile of the regenerated venom, as evaluated by FPLC and MALDI-TOF mass spectrometry, suggested that regeneration of different venom components was asynchronous. Some peptides regenerated quickly, particularly those associated with the scorpion's "prevenom," whereas others required much or all of this time period for regeneration. This asynchrony could explain the different spikes detected in oxygen consumption of milked scorpions as various peptides and other venom components were resynthesized. These observations confirm the relatively high metabolic cost of venom regeneration and suggest that greater venom complexity can be associated with higher costs of venom production.