Prey classification by an araneophagic ant-like jumping spider (Araneae: Salticidae)

What to attack is one of the most basic decisions predators must make, and these decisions are reliant upon the predator's sensory and cognitive capacity. Active choice of spiders as preferred prey, or araneophagy, has evolved in several distantly related spider families, including jumping spiders (Salticidae), but has never been demonstrated in ant-like jumping spiders. We used prey-choice tests with motionless lures to investigate prey-choice behaviour in Myrmarachne melanotarsa , an East African ant-like salticid that normally lives in aggregations and often associates with other spider species. We show that M . melanotarsa chooses spiders as prey in preference to insects and, furthermore, discriminates between different types of spiders. Myrmarachne melanotarsa 's preferred prey were juvenile hersiliids and its second most preferred were other salticids. To date, all documented examples of araneophagic salticids have been from the basal subfamily Spartaeinae. Myrmarachne melanotarsa is the first non-spartaeine and also the first ant-like salticid for which araneophagy has been demonstrated.     

Prey size,prey perishability and group foraging in a social spider

Summary Selection might favor group foraging and social feeding when prey are distributed in patches that do not last long enough for a solitary individual to consume more than a small fraction of them (Pulliam and Millikan 1982; Pulliam and Caraco 1984). Here we considered the foraging behavior of a social spider, Anelosimus eximius, in light of this ephemeral resource hypothesis. This species builds large webs in which members cooperate to capture a wide variety of different sizes and types of prey, many of which are very large. The capture success of this species was very high across all prey sizes, presumably due to the fact that they foraged in groups. Group consumption times in natural colonies for all prey larger than five mm were less than the time that dead insects remained on the plastic sheets that we used as artificial webs. Solitary consumption estimates, calculated from the rate at which laboratory individuals extracted insect biomass while feeding, were the same as the residence times of insects on artificial webs in the field for insects between 6 and 15 mm in length and were significantly longer than the persistence of insects on plastic sheets for all larger insects. Large prey, that contribute substantially to colony energy supplies, appeared to be ephemeral resources for these spiders that could not be consumed by a single spider in the time they were available. These factors made the food intake of one spider in a group less sensitive to scavenging by others and could act to reinforce the social system of this species.     

Prey abundance and the strength of interference in a foraging shorebird

Interference is an important component of food competition but is often difficult to detect and measure in natural animal populations. Although interference has been shown to occur between oystercatchers Haematopus ostralegus L. feeding on mussels Mytilus edulis L., four previous studies have not detected interference between oystercatchers feeding on cockles Cerastoderma edule L. In contrast, this study detected interference between cockle-feeding oystercatchers in the Baie de Somme, France. Prey stealing (kleptoparasitism), one of the main causes of interference between mussel-feeders, also occurred between oystercatchers in the Baie de Somme. The kleptoparasitism rate was related to the natural variation in the food supply, tending to be higher when cockles were rare. Feeding rate was negatively related to competitor density, so providing evidence for interference, but, as in mussel-feeders, only above a threshold density of about 50–100 birds ha⁻¹. The strength of interference at a fixed competitor density was related to the cockle food supply, usually being greater when cockles were rare. Previous studies probably failed to detect interference between cockle-feeders because competitor densities were too low, or cockles were too abundant, or because they were not conducted during late winter when interference is most intense. The study shows that natural variation in the food supply can influence the strength of interference within an animal population and provides support for those behaviour-based interference models which predict that the strength of interference will be greatest when competitor densities are high and prey scarce.     

The role of prey size and abundance in the geographical distribution of spider sociality

1. Social species in the spider genus Anelosimus predominate in lowland tropical rainforests, while congeneric subsocial species occur at higher elevations or higher latitudes. 2. We conducted a comparative study to determine whether differences in total biomass, insect size or both have been responsible for this pattern. 3. We found that larger average insect size, rather than greater overall biomass per se, is a key characteristic of lowland tropical habitats correlating with greater sociality. 4. Social species occupied environments with insects several times larger than the spiders, while subsocial species nearing dispersal occupied environments with smaller insects in either high or low overall biomass. 5. Similarly, in subsocial spider colonies, individuals lived communally at a time when they were younger and therefore smaller than the average insect landing on their webs. 6. We thus suggest that the availability of large insects may be a critical factor restricting social species to their lowland tropical habitats.     

Prey attraction as a possible function of the silk decoration of the uloborid spider Octonoba sybotides

Both laboratory experiments and field observations were usedto examine the prey-attraction hypothesis for the function ofthe silk decoration on the orb web of Octonoba sybotides. Thereflectance spectrum of the decorative silk showed that thedecorations reflect relatively more ultraviolet (UV) light.Choice experiments were conducted using Drosophila melanogaster,a common prey species of the spider, to determine whether webswith silk decoration attract more flies than undecorated webs.The choice experiment showed that webs with silk decorationattract more flies in light that includes UV rays. However,flies choose their flight direction randomly in light withoutUV rays. This suggests that the silk decoration might attract preyinsects that tend to fly toward UV-reflecting objects. Fieldobservations comparing the prey capture rate between webs withand without a silk decoration showed that more prey are caughtin decorated webs. In this study, no difference between thetwo forms of silk decoration, linear and spiral, was detectedeither in prey attraction in the choice experiment or in theprey capture rate in the field observations.     

The functions of societies and the evolution of group living: spider societies as a test case

Many models have been advanced to suggest how different expressions of sociality have evolved and are maintained. However these models ignore the function of groups for the particular species in question. Here we present a new perspective on sociality where the function of the group takes a central role. We argue that sociality may have primarily a reproductive, protective, or foraging function, depending on whether it enhances the reproductive, protective or foraging aspect of the animal's life (sociality may serve a mixture of these functions). Different functions can potentially cause the development of the same social behaviour. By identifying which function influences a particular social behaviour we can determine how that social behaviour will change with changing conditions, and which models are most pertinent. To test our approach we examined spider sociality, which has often been seen as the poor cousin to insect sociality. By using our approach we found that the group characteristics of eusocial insects is largely governed by the reproductive function of their groups, while the group characteristics of social spiders is largely governed by the foraging function of the group. This means that models relevant to insects may not be relevant to spiders. It also explains why eusocial insects have developed a strict caste system while spider societies are more egalitarian. We also used our approach to explain the differences between different types of spider groups. For example, differences in the characteristics of colonial and kleptoparasitic groups can be explained by differences in foraging methods, while differences between colonial and cooperative spiders can be explained by the role of the reproductive function in the formation of cooperative spider groups. Although the interactions within cooperative spider colonies are largely those of a foraging society, demographic traits and colony dynamics are strongly influenced by the reproductive function. We argue that functional explanations help to understand the social structure of spider groups and therefore the evolutionary potential for speciation in social spiders.     

Prey stage preference and feeding behaviour of Oligota kashmirica benefica (Coleoptera: Staphylinidae), an insect predator of the spider mite Tetranychus urticae (Acari: Tetranychidae)

We studied the prey stage preference and feeding behaviour of the first to third instar larvae and adult females of Oligota kashmirica benefica Naomi (Coleoptera: Staphylinidae), a predator of the spider mite Tetranychus urticae Koch (red form) (Acari: Tetranychidae), on leaves of the kudzu vine (Pueraria lobata (Wild.) Ohwi (Leguminosae)) under laboratory conditions. The number of mites eaten increased with the growth of predator larvae. Third instar larvae preyed on all stages of spider mite, whereas first instar larvae preyed mainly on immobile stages (eggs and quiescent stages). The predator larvae showed two types of foraging behaviour (active searching and ambush behaviour) when targeting the mobile stages (larval, nymph and adult stages of prey). Although no difference was found in the number of prey consumed by adult females and third instar larvae of the predator, the adult females mainly attacked and consumed the immobile stages.     

Coordination of Behavioral Sequences Between Individuals During Prey Capture in a Social Spider, Anelosimus eximius

The analysis of collaborative predation sequences performed by groups of 10 individuals (females) in a nonterritorial permanent-social spider, A. eximius, shows that prey-captures are organized in successive steps. Spiders begin by throwing sticky silk, which hinders the prey in the web; they then throw dry silk, which completes the immobilization of the prey. The third step is characterized by bites that paralyze the prey that will be then carried. A concordance test reveals a coordination of the individual's acts that explains the collaborative prey-capture efficiency. No individual specialization in one type of act has been shown. On the contrary, by using living preys or artificially dead vibrated preys, we show that all individuals have equipotential behaviors. Furthermore, each spider is able to adjust its behavior to the state of the prey. Individuals already involved in prey transportation can again display bites or sticky silk throwing if the prey is artificially vibrated. This mechanism, which corresponds to stimergic processes responsible for self-organized phenomena, already described in social insects, permits a coordination of individual acts without the recourse of direct communication. These results permit us to understand better how individuals coordinate their acts and lead us to support the hypothesis that the transition between solitary species and social species in spiders could have been sudden.     

Pouncing spider, flying mosquito: blood acquisition increases predation risk in mosquitoes

Female mosquitoes dramatically increase their mass when bloodfeeding on their hosts. Such an increase could impact mosquitomortality risk by reducing escape speed and/or agility. We usedtwo laboratory-based experiments to test this notion. In thefirst, we allowed mature female Anopheles gambiae mosquitoesto feed from 0 to 4 min and then attacked those females withan artificial predator. We videotaped subsequent escape responseof each mosquito. Analysis of those responses clearly demonstratedan inverse relationship between increased mass and escape speed.In the second experiment, we exposed both blood-engorged andunfed A.gambiae females to single zebra spiders (Salticus scenicus)in small plexiglass cages. Here, we focused on mosquito escapesfrom searching and pouncing spiders. We found that engorgedmosquitoes were three times less likely to escape searchingspiders compared to unfed conspecifics. Thus we conclude thatblood feeding has substantial state-dependent risk both at thehost (experiment 1) and after feeding (experiment 2). Such riskcan be extended to a broad range of taxa.     

Prey capture in Dysdera crocata (Araneae: Dysderidae), a long fanged spider

Abstract

The manner in which Dysdera crocata used its large chelicerae was described. Previous claims that the chelicerae are adaptations enabling the spider to penetrate the dorsal carapace of its prey (woodlice) were not supported.     

Social behaviour in a web-building lynx spider, Tapinillus sp. (Araneae: Oxyopidae)

Social behaviour involving cooperative prey capture and communal feeding is reported for the first time in the spider family Oxyopidae (lynx spiders), in a web-building species of the genus Tapinillus. This social spider inhabits communal webs that may contain several dozen individuals, including adults of both sexes and juveniles of different cohorts. Its colonies occur in clusters and appear to be long-lived, much like those of non-territorial permanently social species such as Anelosimus eximius (Theridiidae) or Agelena consociata (Agelenidae). However, unlike colonies of these other cooperative spiders, the colonies of the social Tapinillus do not have highly female-biased sex ratios. The possible explanations for this difference are discussed.     

Seasonal abundance and depth distribution of Blennius fluviatilis and introduced Lepomis gibbosus, in Lake Banyoles (Catalonia, Spain)

Temporal and spatial occurrence of limnetic larval fish in Lake Banyoles in the spring and summer of 1993 and 1994 indicated that pumpkinseed spawned over 15 weeks (from mid-May to mid-August) and freshwater blenny over 14 weeks (from mid-May until early August). We noted that the spawning period of pumpkinseed commenced earlier in Lake Banyoles than in more northern lakes. Pumpkinseed and freshwater blenny larvae were collected by tow net in both the limnetic (at 4–4.5 m) and littoral zones (at 0–0.5 m), during daylight (no samples were taken during night). Freshwater blenny larvae were more abundant than pumpkinseed larvae. Both of them were more abundant in 1993 than in 1994 (pumpkinseed: 1.61 ind. 100 m−3 vs. 0.70 ind. 100 m−3; freshwater blenny: 3.95 ind. 100 m−3 vs. 1.90 ind. 100 m−3) and in both years they were uniformly distributed in the lake, without differences between the areas considered. The pumpkinseed larvae migrated at 3.5 mm TL into the limnetic zone, and returned to the vegetation of the littoral zone as juveniles. The freshwater blenny larvae were also planktonic (3.5–14 mm TL) but were distributed in both the littoral and limnetic zone before benthic settlement. The larval distribution is discussed on the basis of a trade-off between foraging and predation risk. This revised version was published online in July 2006 with corrections to the Cover Date.     

Toxin constraint explains diet choice,survival and population dynamics in a molluscivore shorebird

Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d''Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.