Adaptation and Survival of Marine-Associated Spiders (Araneae)

Aquatic environments are an unusual habitat for most arthropods. Nevertheless, many arthropod species that were once terrestrial dwelling have transitioned back to marine and freshwater environments, either as semiaquatic or, more rarely, as fully aquatic inhabitants. Transition to water from land is exceptional, and without respiratory modifications to allow for extended submergence and the associated hypoxic conditions, survival is limited. In this article, we review marine-associated species that have made this rare transition in a generally terrestrial group, spiders. We include several freshwater spider species for comparative purposes. Marine-associated spiders comprise less than 0.3% of spider species worldwide but are found in over 14% of all spider families. As we discuss, these spiders live in environments that, with tidal action, hydraulic forces, and saltwater, are more extreme than freshwater habitats, often requiring physiological and behavioral adaptations to survive. Spiders employ many methods to survive inundation from encroaching tides, such as air bubble respiration, airtight nests, hypoxic comas, and fleeing incoming tides. While airway protection is the primary survival strategy, further survival adaptations include saltwater-induced osmotic regulation, dietary composition, predator avoidance, reproduction, locomotory responses, and adaptation to extreme temperatures and hydrostatic pressures that challenge existence in marine environments.

Joyful by nature: approaches to investigate the evolution and function of joy in non-human animals

The nature and evolution of positive emotion is a major question remaining unanswered in science and philosophy. The study of feelings and emotions in humans and animals is dominated by discussion of affective states that have negative valence. Given the clinical and social significance of negative affect, such as depression, it is unsurprising that these emotions have received more attention from scientists. Compared to negative emotions, such as fear that leads to fleeing or avoidance, positive emotions are less likely to result in specific, identifiable, behaviours being expressed by an animal. This makes it particularly challenging to quantify and study positive affect. However, bursts of intense positive emotion (joy) are more likely to be accompanied by externally visible markers, like vocalisations or movement patterns, which make it more amenable to scientific study and more resilient to concerns about anthropomorphism. We define joy as intense, brief, and event-driven (i.e. a response to something), which permits investigation into how animals react to a variety of situations that would provoke joy in humans. This means that behavioural correlates of joy are measurable, either through newly discovered 'laughter' vocalisations, increases in play behaviour, or reactions to cognitive bias tests that can be used across species. There are a range of potential situations that cause joy in humans that have not been studied in other animals, such as whether animals feel joy on sunny days, when they accomplish a difficult feat, or when they are reunited with a familiar companion after a prolonged absence. Observations of species-specific calls and play behaviour can be combined with biometric markers and reactions to ambiguous stimuli in order to enable comparisons of affect between phylogenetically distant taxonomic groups. Identifying positive affect is also important for animal welfare because knowledge of positive emotional states would allow us to monitor animal well-being better. Additionally, measuring if phylogenetically and ecologically distant animals play more, laugh more, or act more optimistically after certain kinds of experiences will also provide insight into the mechanisms underlying the evolution of joy and other positive emotions, and potentially even into the evolution of consciousness.

Experimental characterization and automatic identification of stridulatory sounds inside wood

The propagation of animal vocalizations in water and in air is a well-studied phenomenon, but sound produced by bark and wood-boring insects, which feed and reproduce inside trees, is poorly understood. Often being confined to the dark and chemically saturated habitat of wood, many bark- and woodborers have developed stridulatory mechanisms to communicate acoustically. Despite their ecological and economic importance and the unusual medium used for acoustic communication, very little is known about sound production in these insects, or their acoustic interactions inside trees. Here, we use bark beetles (Scolytinae) as a model system to study the effects of wooden tissue on the propagation of insect stridulations and propose algorithms for their automatic identification. We characterize distance dependence of the spectral parameters of stridulatory sounds, propose data-based models for the power decay of the stridulations in both outer and inner bark, provide optimal spectral ranges for stridulation detectability and develop automatic methods for their detection and identification. We also discuss the acoustic discernibility of species cohabitating the same log. The species tested can be acoustically identified with 99% of accuracy at distances up to 20 cm and detected to the greatest extent in the 2-6 kHz frequency band. Phloem was a better medium for sound transmission than bark.

Spatial acuity-sensitivity trade-off in the principal eyes of a jumping spider: possible adaptations to a 'blended' lifestyle

Jumping spiders (Salticidae) are diurnal visual predators known for elaborate, vision-mediated behaviour achieved through the coordinated work of four pairs of camera-type eyes. One pair ('principal' eyes) is responsible for colour and high spatial acuity vision, while three pairs ('secondary' eyes) are mostly responsible for motion detection. Based on its unusual capacity to visually discriminate specific prey in very low, but also under bright light settings, we investigated the structure of the principal and one pair of secondary eyes (antero-lateral eyes) of Cyrba algerina to determine how these eyes achieve the sensitivity, while maintaining spatial acuity, needed to sustain behaviour in low light. Compared to salticids that live in bright light, the principal eyes of C. algerina have a short focal length, and wide contiguous twin rhabdomeres that support optical pooling, overall favouring sensitivity (0.39 μm²), but without fully compromising acuity (12.4 arc min). The antero-lateral eye retinae have large receptors surrounded by pigment granules, providing effective shielding from scattered light. These adaptations may be beneficial for a xeric salticid species with a 'blended' lifestyle: generally living and hunting under stones in the dark, but sometimes venturing above them, in dramatically different light conditions.

Brown marmorated stink bug overwintering aggregations are not regulated through vibrational signals during autumn dispersal

The brown marmorated stink bug, Halyomorpha halys (Heteroptera: Pentatomidae), is regarded as one of the world's most pernicious invasive pest species, as it feeds on a wide range of economically important crops. During the autumn dispersal period, H. halys ultimately moves to potential overwintering sites, such as human-made structures or trees where it will alight and seek out a final overwintering location, often aggregating with other adults. The cues used during this process are unknown, but may involve vibrational signals. We evaluated whether vibrational signals regulate cluster aggregation in H. halys in overwintering site selection. We collected acoustic data for six weeks during the autumn dispersal period and used it to quantify movement and detect vibrational communication of individuals colonizing overwintering shelters. Both movement and vibrational signal production increased after the second week, reaching their maxima in week four, before decaying again. We found that only males produced vibrations in this context, yet there was no correlation between movement and vibrational signals, which was confirmed through playback experiments. The cues regulating the formation of aggregations remain largely unknown, but vibrations may indicate group size.

Widespread army ant aversion among East African jumping spiders (Salticidae)

Jumping spiders (Salticidae) typically prey on a variety of arthropods of similar size to themselves, but rarely on ants. Using 28 salticid species from East Africa, we first investigated vision-based aversion to ants by recording latency to enter a transparent sealed chamber flanked by chambers containing living army ants (Dorylus sp.) or tsetse flies (Glossina pallidipes) of comparable size. For all species, entry latency was significantly longer when the stimuli were ants. In another experiment, we used dead ants and tsetse flies mounted in a life-like posture as stimuli; except for Goleba puella, a species with unusual retinal ultrastructure, we again found significantly longer entry latency when the stimuli were ants. Our findings imply that these salticids express an aversion specifically to ants even when restricted to using vision alone and, except for G. puella, even when relying on solely the static appearance of the insects. Having used salticids from laboratory cultures with no prior experience with ants, our findings are consistent with vision-based aversion to army ants being innate.

Caffeine affects the vigilance decrement of Trite planiceps jumping spiders (Salticidae)

In jumping spiders (Salticidae), the vigilance decrement, or decrease in response to a repeated visual stimulus over time, directly parallels that found in humans. Explanations for the vigilance decrement in the human literature are heavily mentalistic and central nervous system (CNS) based, whereas response decrements in invertebrates are typically thought of as habituation at the sensory periphery. Here we explored whether the salticid vigilance decrement could be CNS modulated by using caffeine, which is a well-known CNS stimulant for both vertebrates and invertebrates. We used paired tests in which Trite planiceps Simon spiders were randomly given a drop of either caffeinated water or distilled water and were then shown dot stimuli presented on monitors. We measured both general "walking," or activity, as a control for physical fatigue, and optomotor responses to the stimuli. We found that the vigilance decrement was significantly shallower (i.e., spiders were more responsive for longer) when spiders were administered caffeine compared with water; furthermore, these spiders were also generally more active throughout the testing period and thus were not physically impaired. Our results suggest that, in at least some invertebrates, CNS modulation of the vigilance decrement is likely. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Distance assessment of detours by jumping spiders

To take an indirect route (detour) in order to reach a specific target requires complex cognitive processes. Yet more demanding, from the cognitive point of view, is when the goal is only visible at the beginning of the detour. In spiders from the family Salticidae, vision is a key sensory modality mediating navigation and prey search. Their acute vision allows them to perform complicated detours, possibly as a consequence of the multitude of potential routes in their typically complex 3-dimensional habitats. We used a 4-route choice test, in which routes differed in being either short or long and in the presence or absence of a lure of a prey item, to investigate route assessment in 2 salticid species, Trite planiceps and Marpissa marina. Although both species showed evidence of motivation to follow lured-routes, judging by the number of times they re-oriented toward them while detouring, we found that Trite chose short routes in preference to long routes, but did not prefer the lured-routes. In contrast, Marpissa exhibited random route choice, although it oriented toward lured-routes more often than control routes (lure absent). Our results suggest that decision-making processes about which route to take occurs before embarking on a route, but this is cognitively challenging. Spiders exhibited cognitive limitations in which the lack of visibility of the goal affected success. However, the severity of cognitive limitations depended on species. We suggest that variability in spatial ability across the Salticidae may be related to the habitat complexity inhabited by each species.

Risk assessment and the use of novel shortcuts in spatial detouring tasks in jumping spiders

Selection on individuals that incorporate risk to quickly and accurately make a priori navigational assessments may lead to increased spatial ability. Jumping spiders (Araneae: Salticidae) are characterized by their highly acute vision, which mediates many behaviors, including prey capture and navigation. When moving to a specific goal (prey, nest, a potential mate, etc.), salticids rely on visual cues and spatial memory to orient in 3-dimensional space. Salticid spatial ability has been studied in homing and detour tasks, with Portia being considered one of the most skillful genera in terms of spatial ability in the family. Commonly living in complex environments, salticids are likely to encounter a wide variety of routes that could lead to a goal, and, as selection favors individuals that can accurately make assessments, they may be able to assess alternative route distances to select the most efficient route. Here, we tested whether 2 salticid species (Portia fimbriata and Trite planiceps) can discriminate and assess between different available routes by their length, and riskiness to escape from a stressful scenario. Results suggest that while Portia is more likely to choose the easiest and shortest escape routes, Trite is faster in both decision making about which route to take, and to escape. However, some individuals were able to use novel shortcuts instead of the routes expected, with Portia containing a higher proportion of shortcut-takers than Trite. These differences in spatial ability seem to correspond with the environmental complexity inhabited by each species.

Dim-light vision in jumping spiders (Araneae, Salticidae): identification of prey and rivals

Jumping spiders (family Salticidae) are known for their intricate vision-based behavior during encounters with prey and conspecific individuals. This is achieved using eyes specialized for discerning fine detail, but there has been minimal research on the capacity that salticids might have for visual performance under low ambient light levels. Here, we investigated the capacity of two salticid species, Cyrba algerina from Portugal and Cyrba ocellata from Kenya, to perform two specific visual tasks under low ambient light levels. We used lures made from spiders and midges in prey-identification experiments and mirror images (virtual conspecifics) in rival-identification experiments. These experiments were implemented under a range of ambient light levels (234, 1.35, 0.54, 0.24 cd m^-2). In most instances, C algerina and C ocellata were proficient at performing both of these visual tasks when ambient light was 234 and 1.35 cd m^-2, and a minority performed these tasks at 0.54 cd m^-2, but none succeeded when the light level was 0.24 cd m^-2 Cyrba algerina and C. ocellata showed vision-based discrimination under low ambient light levels previously associated with nocturnal species.

First report of luminous stimuli eliciting sound production in weevils

Light-based stimuli elicited acoustic responses in male Hylesinus aculeatus Say (Curculionidae: Scolytinae: Hylesinina) instantaneously, with 100% reliability. Stridulations were elicited with a white light beam in a dark environment and recorded with an ultrasonic microphone. Acoustic responses were consistent, and, when compared with sounds produced under stressful conditions (i.e. physical stimulation), no significant differences were found. Hylesinus aculeatus possess an elytro-tergal stridulatory organ and acoustic communication is only present in males. This is also the first report of acoustic communication for this species. Instantaneous light-elicited acoustic communication has potential applications in the development of electronic traps and real-time acoustic detection and identification of beetles, border biosecurity, and noise-reduction in acoustic data collection.

Vigilance all the way down: Vigilance decrement in jumping spiders resembles that of humans

The inability to maintain signal detection performance with time on task, or vigilance decrement, is widely studied in people. Despite suggestions that limitations in sustained attention may be a fundamental characteristic of animal cognition, there has been limited research on the vigilance decrement in other animals. We conducted two experiments to explore vigilance in jumping spiders. Our first experiment established that the vigilance decrement, decline in signal detections with time on task, occurs in these spiders in laboratory settings. Our second experiment tested whether this phenomenon was simply the result of habituation of sensory receptors by employing two dishabituation manipulations. Neither dishabituation manipulation appeared to have an effect. Thus, the vigilance decrement in spiders appears to be due to something more than simply peripheral sensory habituation. We suggest that limitations in sustained attention may be a widespread phenomenon among animals that needs addressing when theorising about the vigilance decrement.

Positive emotional contagion in a New Zealand parrot

Positive emotional contagions are outwardly emotive actions that spread from one individual to another, such as glee in preschool children [1] or laughter in humans of all ages [2]. The play vocalizations of some animals may also act as emotional contagions. For example, artificially deafened rats are less likely to play than their non-hearing-impaired conspecifics, while no such effect is found for blinded rats [3]. As rat play vocalizations are also produced in anticipation of play, they, rather than the play itself, may act as a contagion, leading to a hypothesis of evolutionary parallels between rat play vocalizations and human laughter [4]. The kea parrot (Nestor notabilis) has complex play behaviour and a distinct play vocalization [5]. We used acoustic playback to investigate the effect of play calls on wild kea, finding that play vocalizations increase the amount of play among both juveniles and adults, likely by acting as a positive emotional contagion.

Audiogram of the kea parrot, Nestor notabilis

Vocal communication requires the sender to produce a sound, which transmits through the environment and is perceived by the receiver. Perception is dependent on the quality of the received signal and the receiver's frequency and amplitude sensitivity; hearing sensitivity of animals can be tested using behavioural detection tasks, showing the physical limitations of sound perception. Kea parrots (Nestor notabilis) were tested for their ability to hear sounds that varied in terms of both frequency and amplitude by means of a simple auditory detection task. Audiograms for three kea were similar, with the region of highest sensitivity (1-5 kHz) corresponding to the frequency of the highest amplitude in kea calls. Compared with other parrots and other bird taxa, the overall shape of the kea audiogram follows a similar pattern. However, two potentially interesting differences to the audiograms of other birds were found: an increase in sensitivity at approximately 12 kHz and a decreased sensitivity to frequencies below 1 kHz.

Innate pattern recognition and categorization in a jumping spider

The East African jumping spider Evarcha culicivora feeds indirectly on vertebrate blood by preferentially preying upon blood-fed Anopheles mosquitoes, the vectors of human malaria¹, using the distinct resting posture and engorged abdomen characteristic of these specific prey as key elements for their recognition. To understand perceptual categorization of objects by these spiders, we investigated their predatory behavior toward different digital stimuli - abstract ‘stick figure’ representations of Anopheles constructed solely by known key identification elements, disarranged versions of these, as well as non-prey items and detailed images of alternative prey. We hypothesized that the abstract images representing Anopheles would be perceived as potential prey, and would be preferred to those of non-preferred prey. Spiders perceived the abstract stick figures of Anopheles specifically as their preferred prey, attacking them significantly more often than non-preferred prey, even when the comprising elements of the Anopheles stick figures were disarranged and disconnected from each other. However, if the relative angles between the elements of the disconnected stick figures of Anopheles were altered, the otherwise identical set of elements was no longer perceived as prey. These data show that E. culicivora is capable of making discriminations based on abstract concepts, such as the hypothetical angle formed by discontinuous elements. It is this inter-element angle rather than resting posture that is important for correct identification of Anopheles. Our results provide a glimpse of the underlying processes of object recognition in animals with minute brains, and suggest that these spiders use a local processing approach for object recognition, rather than a holistic or global approach. This study provides an excellent basis for a comparative analysis on feature extraction and detection by animals as diverse as bees and mammals.

Evolutionary implications of deception in mimicry and masquerade

Aggressive mimicry occurs when an organism resembles some aspect of another organism (the model) in order to obtain prey through its deceptive resemblance. This may function either through the overt response of the receiver or through the lack of response of the receiver. Reviewing selected examples, I discuss some of the difficulties in ascribing a model for the mimic. I also discuss how a single animal can have multiple ploys in its armoury of deceptive signals, thus belonging within two or more categories of deceptive signalling. In addition to aggressive mimicry, these may include crypsis or camouflage, masquerade (mimicry of inanimate objects), and Batesian or protective mimicry. Each of these examples of deception has multiple evolutionary pathways, and some deceptive signals may be more costly to receivers than others, but no single organism is subject to a single selection pressure, leading to the reality that many evolutionary pathways contribute to the diversity we see around us. New technologies are opening new channels of investigation into deceptive signaling in many different sensory modalities, and this is reflected in the recent increase in studies investigating the structure and function of deceptive signals. In turn, these studies are beginning to expose the fascinating complexity of deceptive signaling systems, allowing us to discover the myriad, non-mutually exclusive, solutions that can be selected for to obtain prey.

Vocal repertoire of the New Zealand kea parrot Nestor notabilis

The unique alpine-living kea parrot Nestor notabilis has been the focus of numerous cognitive studies, but its communication system has so far been largely neglected. We examined 2,884 calls recorded in New Zealand’s Southern Alps. Based on audio and visual spectrographic differences, these calls were categorised into seven distinct call types: the non-oscillating ‘screech’ contact call and ‘mew’; and the oscillating ‘trill’, ‘chatter’, ‘warble’ and ‘whistle’; and a hybrid ‘screech-trill’. Most of these calls contained aspects that were individually unique, in addition to potentially encoding for an individual’s sex and age. Additionally, for each recording, the sender’s previous and next calls were noted, as well as any response given by conspecifics. We found that the previous and next calls made by the sender were most often of the same type, and that the next most likely preceding and/or following call type was the screech call, a contact call which sounds like the ‘kee-ah’ from which the bird’s name derives. As a social bird capable of covering large distances over visually obstructive terrain, long distance contact calls may be of considerable importance for social cohesion. Contact calls allow kea to locate conspecifics and congregate in temporary groups for social activities. The most likely response to any given call was a screech, usually followed by the same type of call as the initial call made by the sender, although responses differed depending on the age of the caller. The exception was the warble, the kea’s play call, to which the most likely response was another warble. Being the most common call type, as well as the default response to another call, it appears that the ‘contagious’ screech contact call plays a central role in kea vocal communication and social cohesion.

How spiders practice aggressive and Batesian mimicry

To understand communication, the interests of the sender and the receiver/s of signals should be considered separately. When our goal is to understand the adaptive significance of specific responses to specific signals by the receiver, questions about signal information are useful. However, when our goal is to understand the adaptive significance to the sender of generating a signal, it may be better to envisage the receiver’s response to signals as part of the sender’s extended phenotype. By making signals, a sender interfaces with the receiver’s model of the world and indirectly manipulates its behaviour. This is especially clear in cases of mimicry, where animals use deceptive signals that indirectly manipulate the behaviour of receivers. Many animals adopt Batesian mimicry to deceive their predators, or aggressive mimicry to deceive their prey. We review examples from the literature on spiders to illustrate how these phenomena, traditionally thought of as distinct, can become entangled in a web of lies.

The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider

Evarcha culicivora is an East African jumping spider that feeds indirectly on vertebrate blood by choosing blood-fed female Anopheles mosquitoes as prey. Previous studies have shown that this predator can identify its preferred prey even when restricted to using only visual cues. Here, we used lures and virtual mosquitoes to investigate the optical cues underlying this predator's prey-choice behaviour. We made lures by dissecting and then reconstructing dead mosquitoes, combining the head plus thorax with different abdomens. Depending on the experiment, lures were either moving or motionless. Findings from the lure experiments suggested that, for E. culicivora, seeing a blood-fed female mosquito's abdomen on a lure was a necessary, but not sufficient, cue by which preferred prey was identified, as cues from the abdomen needed to be paired with cues from the head and thorax of a mosquito. Conversely, when abdomens were not visible or were identical, spiders based their decisions on the appearance of the head plus thorax of mosquitoes, choosing prey with female characteristics. Findings from a subsequent experiment using animated 3D virtual mosquitoes suggest that it is specifically the mosquito's antennae that influence E. culicivora's prey-choice decisions. Our results show that E. culicivora uses a complex process for prey classification.

Hyperacute motion detection by the lateral eyes of jumping spiders

Jumping spiders (Salticidae) are renowned for their high performing visual system. In addition to their prominent forward-facing telescope-like principal eyes, salticids possess two or three pairs of secondary eyes used for wide-angle motion detection. Salticids orient towards relevant sources of motion detected by the secondary eyes, enabling them to inspect the stimulus with their spatially acute principal eyes. The anteriormost pair of secondary eyes, the anterior lateral (AL) eyes, also faces forward and has higher spatial acuity than the other, laterally-facing, secondary eyes. We used small computer-generated targets to elicit orienting saccades from tethered jumping spiders in order to examine the perceptual limits of the AL eyes. We describe the contrast thresholds of male and female spiders, investigate the reaction time between stimulus appearance and initiation of orientation, as well as the minimum distance a stimulus must travel before eliciting a saccade. Our results show that female spiders react to lower contrast stimuli than males and demonstrate that the secondary eyes can detect stimulus displacements considerably smaller than the inter-receptor angle.

Translocation of threatened New Zealand falcons to vineyards increases nest attendance, brooding and feeding rates

Anthropogenic landscapes can be rich in resources, and may in some cases provide potential habitat for species whose natural habitat has declined. We used remote videography to assess whether reintroducing individuals of the threatened New Zealand falcon Falco novaeseelandiae into a highly modified agricultural habitat affected the feeding rates of breeding falcons or related breeding behavior such as nest attendance and brooding rates. Over 2,800 recording hours of footage were used to compare the behavior of falcons living in six natural nests (in unmanaged, hilly terrain between 4 km and 20 km from the nearest vineyard), with that of four breeding falcon pairs that had been transported into vineyards and nested within 500 m of the nearest vineyard. Falcons in vineyard nests had higher feeding rates, higher nest attendance, and higher brooding rates. As chick age increased, parents in vineyard nests fed chicks a greater amount of total prey and larger prey items on average than did parents in hill nests. Parents with larger broods brought in larger prey items and a greater total sum of prey biomass. Nevertheless, chicks in nests containing siblings received less daily biomass per individual than single chicks. Some of these results can be attributed to the supplementary feeding of falcons in vineyards. However, even after removing supplementary food from our analysis, falcons in vineyards still fed larger prey items to chicks than did parents in hill nests, suggesting that the anthropogenic habitat may be a viable source of quality food. Although agricultural regions globally are rarely associated with raptor conservation, these results suggest that translocating New Zealand falcons into vineyards has potential for the conservation of this species.

Evarcha culicivora chooses blood-fed Anopheles mosquitoes but other East African jumping spiders do not

Previous research using computer animation and lures made from dead prey has demonstrated that the East African salticid Evarcha culicivora Wesolowska & Jackson (Araneae: Salticidae) feeds indirectly on vertebrate blood by actively choosing blood-carrying female mosquitoes as prey, and also that it singles out mosquitoes of the genus Anopheles (Diptera: Culicidae) by preference. Here, we demonstrate that E. culicivora's preference is expressed when the species is tested with living prey and that it is unique to E. culicivora. As an alternative hypothesis, we considered the possibility that the preference for blood-fed female anopheline mosquitoes might be widespread in East African salticids. When live-prey choice tests were carried out in 19 additional species, there were no instances in which blood-carrying mosquitoes were chosen significantly more often than other prey. Combined with the findings of previous work, these results suggest that it is possible that specialized predators play a role in the biological control of disease vectors.

The role of numerical competence in a specialized predatory strategy of an araneophagic spider

Although a wide range of vertebrates have been considered in research on numerical competence, little is known about the role of number-related decisions in the predatory strategies of invertebrates. Here, we investigate how numerical competence is expressed in a highly specialized predatory strategy adopted by the small juveniles of Portia africana when practicing communal predation, with the prey being another spider, Oecobius amboseli. Two or more P. africana juveniles sometimes settle by the same oecobiid nest and then share the meal after one individual captures the oecobiid. Experiments were designed to clarify how these predators use number-related cues in conjunction with non-numerical cues when deciding whether to settle at a nest. We used lures (dead spiders positioned in lifelike posture) arranged in a series of 24 different scenes defined by the type, configuration and especially number of lures. On the whole, our findings suggest that P. africana juveniles base settling decisions on the specific number of already settled conspecific juveniles at the nest and express a preference for settling when the number is one instead of zero, two or three. By varying the size of the already settled juveniles and their positions around the nest, we show that factors related to continuous variables and stimulus configuration are unlikely explanations for our findings.

Effects of introducing threatened falcons into vineyards on abundance of passeriformes and bird damage to grapes

Agricultural landscapes are becoming an important focus of animal conservation, although initiatives to conserve predators to date have rarely provided economic benefits to agricultural producers. We examined whether introduction to vineyards of the New Zealand Falcon (Falco novaeseelandiae), a species listed as threatened by the New Zealand Department of Conservation, affected the abundance of 4 species of Passeriformes that are considered vineyard pests or affected the amount of economic loss due to grape (Vitis vinifera) damage. Three of the species were introduced and remove whole grapes from bunches (Blackbird [Turdus merula], Song Thrush [Turdus philomelos], and Starling [Sturnus vulgaris]), whereas the one native species (Silvereye [Zosterops lateralis]) pecks holes in grapes. The introduction of falcons to vineyards was associated with a significant decrease in the abundance of introduced passerines and with a 95% reduction in the number of grapes removed relative to vineyards without falcons. Falcon presence was not associated with a change in the number of Silvereyes, but there was a 55% reduction in the number of grapes pecked in vineyards with falcons. Our results indicate that, relative to damage in vineyards without falcons, the presence of a falcon could potentially result in savings of US$234/ha for the Sauvignon Blanc variety of grapes and $326/ha for Pinot Noir variety of grapes.

The role of the anterior lateral eyes in the vision-based behaviour of jumping spiders

Jumping spiders, or salticids, sample their environment using a combination of two types of eyes. The forward-facing pair of 'principal' eyes have narrow fields of view, but exceptional spatial resolution, while the two or three pairs of 'secondary' eyes have wide fields of view and function especially well as motion analysers. Motion detected by the secondary eyes may elicit an orienting response, whereupon the object of interest is examined further using the high-acuity principal eyes. The anterior lateral (AL) eyes are particularly interesting, as they are the only forward-facing pair of secondary eyes. In this study, we aimed to determine characteristics of stimuli that elicit orienting responses mediated by the AL eyes. After covering all eyes except the AL eyes, we measured orienting responses to dot stimuli that varied in size and contrast, and moved at different speeds. We found that all stimulus parameters had significant effects on orientation propensity. When tethered flies were used as prey, we found that visual information from the AL eyes alone was sufficient to elicit stalking behaviour. These results suggest that, in terms of overall visual processing, the relevance of spatial vision in the AL eyes has been underestimated in the literature. Our results also show that female spiders are significantly more responsive than males. We found that hunger caused similar increases in orientation propensity in the two sexes, but females responded more often than males both when sated and when hungry. A higher propensity by females to orient toward moving objects may be related to females tending to experience higher nutritional demands than males.

Specialized prey selection behavior of two East African assassin bugs, Scipinnia repax and Nagusta sp. that prey on social jumping spiders

The prey choice behavior and predatory strategies of two East African assassin bugs, Scipinnia repax (Stäl 1961) and Nagusta sp. (Hemiptera: Reduviidae), were investigated in the field and the laboratory. Both of these species are from the subfamily Harpactorinae and specialize in eating spiders. They prey especially often on social jumping spiders (Salticidae) that build nest complexes (nests connected by silk) in vegetation near the shoreline of Lake Victoria. Both reduviid species associate with these nest complexes and prey on the resident salticids. Nagusta sp., but not S. repax, form groups on nest complexes with 2-3 individuals of Nagusta sometimes feeding together on a single salticid. In addition to social salticids, Nagusta sp. preys on Portia africana, an araneophagic salticid that often invades the same nest complexes. S. repax preys on salticid eggs and also on Nagusta. Although they avoid ants, Nagusta and especially S. repax prey on ant-mimicking salticids, suggesting that sensory modalities other than vision play a dominant role in prey detection.

Aggressive use of Batesian mimicry by an ant-like jumping spider

Batesian and aggressive mimicry are united by deceit: Batesian mimics deceive predators and aggressive mimics deceive prey. This distinction is blurred by Myrmarachne melanotarsa, an ant-like jumping spider (Salticidae). Besides often preying on salticids, ants are well defended against most salticids that might target them as potential prey. Earlier studies have shown that salticids identify ants by their distinctive appearance and avoid them. They also avoid ant-like salticids from the genus Myrmarachne. Myrmarachne melanotarsa is an unusual species from this genus because it typically preys on the eggs and juveniles of ant-averse salticid species. The hypothesis considered here is that, for M. melanotarsa, the distinction between Batesian and aggressive mimicry is blurred. We tested this by placing female Menemerus sp. and their associated hatchling within visual range of M. melanotarsa, its model, and various non-ant-like arthropods. Menemerus is an ant-averse salticid species. When seeing ants or ant mimics, Menemerus females abandoned their broods more frequently than when seeing non-ant-like arthropods or in control tests (no arthropods visible), as predicted by our hypothesis that resembling ants functions as a predatory ploy.

A predator from East Africa that chooses malaria vectors as preferred prey

Background

All vectors of human malaria, a disease responsible for more than one million deaths per year, are female mosquitoes from the genus Anopheles. Evarcha culicivora is an East African jumping spider (Salticidae) that feeds indirectly on vertebrate blood by selecting blood-carrying female mosquitoes as preferred prey.

Methodology/Principal Findings

By testing with motionless lures made from mounting dead insects in lifelike posture on cork discs, we show that E. culicivora selects Anopheles mosquitoes in preference to other mosquitoes and that this predator can identify Anopheles by static appearance alone. Tests using active (grooming) virtual mosquitoes rendered in 3-D animation show that Anopheles' characteristic resting posture is an important prey-choice cue for E. culicivora. Expression of the spider's preference for Anopheles varies with the spider's size, varies with its prior feeding condition and is independent of the spider gaining a blood meal.

Conclusions/Significance

This is the first experimental study to show that a predator of any type actively chooses Anopheles as preferred prey, suggesting that specialized predators having a role in the biological control of disease vectors is a realistic possibility.

Compound mimicry and trading predators by the males of sexually dimorphic Batesian mimics

Sexual dimorphism is pronounced in Myrmarachne, a large genus of ant-like jumping spiders (Salticidae) and one of the major animal groups in which Batesian mimicry of ants has evolved. Although adult females and juveniles of both sexes are distinctly ant-like in appearance, Myrmarachne males have elongated chelicerae that might appear to detract from their resemblance to ants. Experimental findings suggest that the Myrmarachne male's solution is to adopt compound mimicry (i.e. the male's model seems to be not simply an ant worker but a combination of an ant and something carried in the ant's mandibles: an "encumbered ant"). By becoming a mimic of a particular subset of worker ants, Myrmarachne males may have retained their Batesian-mimicry defence against ant-averse predators, but at the price of receiving the unwanted attention of predators for which encumbered ants are preferred prey. Two salticid species were used as predators in the experiments. Portia fimbriata is known to choose other salticids as preferred prey and to avoid unencumbered ants and their mimics (Myrmarachne females). In experiments reported here, P. fimbriata avoided encumbered ants and Myrmarachne males. Ants are the preferred prey of Chalcotropis gulosus. In our experiments, C. gulosus chose safer encumbered ants in preference to more dangerous unencumbered ants, chose Myrmarachne males more often than Myrmarachne females and showed no evidence of distinguishing between Myrmarachne males and encumbered ants. The cost of reconciling sexual dimorphism with Batesian mimicry appears to be that Myrmarachne males attract the unwanted attention of specialist predators of their compound model.

A spider that feeds indirectly on vertebrate blood by choosing female mosquitoes as prey

Spiders do not feed directly on vertebrate blood, but a small East African jumping spider (Salticidae), Evarcha culicivora, feeds indirectly on vertebrate blood by choosing as preferred prey female mosquitoes that have had recent blood meals. Experiments show that this spider can identify its preferred prey by sight alone and by odor alone. When presented with two types of size-matched motionless lures, E. culicivora consistently chose blood-fed female mosquitoes in preference to nonmosquito prey, male mosquitoes, and sugar-fed female mosquitoes (i.e., females that had not been feeding on blood). When the choice was between mosquitoes of different sizes (both blood- or both sugar-fed), small juveniles chose the smaller prey, whereas adults and larger juveniles chose the larger prey. However, preference for blood took precedence over preference for size (i.e., to get a blood meal, small individuals took prey that were larger than the preferred size, and larger individuals took prey that were smaller than the preferred size). When presented with odor from two prey types, E. culicivora approached the odor from blood-fed female mosquitoes significantly more often the odor of the prey that were not carrying blood.