Previous foraging success influences web building in the spider Stegodyphus lineatus (Eresidae)

Stegodyphus lineatus (Eresidae) is a desert spider that buildsan aerial capture web on bushes in the Negev desert in southernIsrael. Web building for spiders is costly in energy, time,and risk of predation. Spiders should trade-off these costswith the benefits in terms of prey capture. We tested the hypothesisthat the previous foraging success of the spider influencesthe effort invested in foraging. Specifically, we asked whetheran increase in food intake causes spiders to reduce web renewalactivity and web size. Alternatively, time constraints on foragingand development, resulting from a short growing season, couldinduce spiders to continue foraging even when supplemented withprey. The cost of web building was measured as time and massloss. To build an average size web (about 150 cm²), we calculatedthat a spider requires 6 h and that spiders lose 3%-7% of their weight.In field experiments, spiders responded differently to food supplementationin 2 different years. In 1994, they improved their condition comparedto individuals whose webs were removed to reduce foraging opportunitiesand compared to control spiders. In 1995, spiders tested earlier inthe season than the previous year did not improve their conditionin response to prey supplementation. Nonetheless, in both years, food-supplementedspiders built significantly smaller webs than food-deprived andcontrol spiders. This result was confirmed in a laboratory experiment whereprey intake was controlled. We conclude that for S. lineatus immediateforaging risks outweigh the potential time constraints on foraging.     

Attraction between social crab spiders: silk pheromones in Diaea socialis

The snare web is used as a medium for communication betweenindividuals within colonies of social spiders and has thereforebeen suggested as necessary for the evolution of sociality inthe Araneae. The social spider Diaea socialis (Thomisidae) isan exception because it does not build a snare web. Experimentsdemonstrate that silk attracts all spiders and that a chemicaldeposited onto the silk attracts adult female spiders, suggestingthat the group living of this species is mediated by a pheromone.The pheromone attracts spiders differentially: females are notattracted to juvenile silk, and it repels gravid females. Thepheromone appears to be stable but volatile, is ether-soluble,and retains its viability after dissolution. Molecular-ionicmasses for 7-8 different compounds were found in the range 220–281atomic units; the pheromone may be one or a combination of severalof these.     

Costs and benefits of facultative aggregating behaviour in the orb-spinning spider Gasteracantha minax Thorell (Araneae: Araneidae)

Abstract The potential costs and benefits of foraging in aggregations are examined for the orb-spinning spider Gasteracantha minax. Web-site tenacity is low in this species; individuals frequently move among sites, thereby joining aggregations of different sizes. Female spiders in aggregations suffered lower predation rates and attracted more males than their solitary counterparts. However, aggregated eggsacs, probably produced by females in aggregations, experienced higher rates of parasitism than solitary eggsacs. We found no evidence of higher prey capture success rates among spiders in aggregations. However, we demonstrate a novel way in which spiders can increase their foraging efficiency by decreasing silk investment. A spider spinning a web within an existing aggregation can attach the support threads of its web to those of other webs, thereby exploiting the silk produced by other spiders.     

Biomechanical variation of silk links spinning plasticity to spider web function

Spider silk is renowned for its high tensile strength, extensibility and toughness. However, the variability of these material properties has largely been ignored, especially at the intra-specific level. Yet, this variation could help us understand the function of spider webs. It may also point to the mechanisms used by spiders to control their silk production, which could be exploited to expand the potential range of applications for silk. In this study, we focus on variation of silk properties within different regions of cobwebs spun by the common house spider, Achaearanea tepidariorum. The cobweb is composed of supporting threads that function to maintain the web shape and hold spiders and prey, and of sticky gumfooted threads that adhere to insects during prey capture. Overall, structural properties, especially thread diameter, are more variable than intrinsic material properties, which may reflect past directional selection on certain silk performance. Supporting threads are thicker and able to bear higher loads, both before deforming permanently and before breaking, compared with sticky gumfooted threads. This may facilitate the function of supporting threads through sustained periods of time. In contrast, sticky gumfooted threads are more elastic, which may reduce the forces that prey apply to webs and allow them to contact multiple sticky capture threads. Therefore, our study suggests that spiders actively modify silk material properties during spinning in ways that enhance web function.     

黄绿绿僵菌对两种稻田蜘蛛捕杀褐飞虱作用的影响

在室内研究了喷施黄绿绿僵菌Metarhizium anisopliae var. acridum Driver et Milner孢子悬液对稻田蜘蛛捕杀褐飞虱作用的影响。结果表明,黄绿绿僵菌不感染拟水狼蛛Pirata subpiraticus Bsenberg et Strand和食虫沟瘤蛛Ummeliata inse cticeps Bsenberg et Strand,只对蜘蛛喷施菌液不影响它们的捕杀褐飞虱的能力。而对褐飞虱喷施黄绿绿僵菌液后,褐飞虱活力明显下降,导致蜘蛛对其捕杀效果显著提高。在喷施黄绿绿僵菌106 、107、108/mL孢子浓度后,拟水狼蛛的平均捕杀量分别为10.5头/d、11.1头/d和11.4头/d,食虫沟瘤蛛的平均捕杀量分别为3.8头/d、4.3头/d和4.7头/d,均显著大于对照组。对蜘蛛和褐飞虱同时喷施黄绿绿僵菌不影响前者的捕杀力。这些结果提示,在稻田施用黄绿绿僵菌防治稻飞虱对蜘蛛天敌没有不良影响。     

“Veils” Against Predators: Modified Web Structure of a Host Spider Induced by an Ichneumonid Parasitoid, Brachyzapus nikkoensis (Uchida) (Hymenoptera)

When the larva of Brachyzapus nikkoensis, a koinobiont ectoparasitoid of Agelena limbata forming a funnel web, is a penultimate instar, “veils” of very fine and dense threads covering the spider and parasitoid larva were observed in the tunnel of funnel web. As the veils are not formed until the parasitoid became penultimate instar and the parasitoid larvae are on active hosts at this stage, the veils are considered to be formed by the spider under the manipulation of the parasitoid larva. Removal of the veil indicated that penultimate larvae and pupae were more likely to fail to emerge in manipulated webs compared to the controls. The modified web seems resistant against predators and scavengers such as ants. This is the first record of web modification demonstrating its resistance to predators.     

Size-dependent survivorship in the web-building spiderAgelena limbata

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

How Did the Spider Cross the River? Behavioral Adaptations for River-Bridging Webs in Caerostris darwini (Araneae: Araneidae)

Background

Interspecific coevolution is well described, but we know significantly less about how multiple traits coevolve within a species, particularly between behavioral traits and biomechanical properties of animals'' “extended phenotypes”. In orb weaving spiders, coevolution of spider behavior with ecological and physical traits of their webs is expected. Darwin''s bark spider (Caerostris darwini) bridges large water bodies, building the largest known orb webs utilizing the toughest known silk. Here, we examine C. darwini web building behaviors to establish how bridge lines are formed over water. We also test the prediction that this spider''s unique web ecology and architecture coevolved with new web building behaviors.

Methodology

We observed C. darwini in its natural habitat and filmed web building. We observed 90 web building events, and compared web building behaviors to other species of orb web spiders.

Conclusions

Caerostris darwini uses a unique set of behaviors, some unknown in other spiders, to construct its enormous webs. First, the spiders release unusually large amounts of bridging silk into the air, which is then carried downwind, across the water body, establishing bridge lines. Second, the spiders perform almost no web site exploration. Third, they construct the orb capture area below the initial bridge line. In contrast to all known orb-weavers, the web hub is therefore not part of the initial bridge line but is instead built de novo. Fourth, the orb contains two types of radial threads, with those in the upper half of the web doubled. These unique behaviors result in a giant, yet rather simplified web. Our results continue to build evidence for the coevolution of behavioral (web building), ecological (web microhabitat) and biomaterial (silk biomechanics) traits that combined allow C. darwini to occupy a unique niche among spiders.     

Emergent properties in the behaviour of a virtual spider robot

Using a virtual spider robot, we studied hypotheses about the weaving behaviour of orb spiders. Our model spiders built virtual webs that mimicked perfectly the visual architecture of real webs of the garden cross spider Araneus diadematus. The matching of capture spiral and auxiliary spiral pitch was an apparently emergent property in both types of web. This validated our interpretation of the garden spider''s web-building decision rules, which use strictly local interactions with previously placed threads to generate global architecture.     

Hunting Behavior in the Ctenizidae

The most primitive way of hunting in Mygalomorph spiders seemsto be the free roaming and catching of encountered prey Theraphosidae.The trap-door spiders Ctenizidae, Actinopodidae and Barychelidawhich are entirely sedentary, lie in wait behind the trap-doorand leap at prey that happens to pass close to the door. Somespecies spin radial silk threads outside the door which functionas stumbleor signal-lines, and some Australian species use grassblades and other litter in the same way. A further evolutionarystep leads to species which do not build a trap-door but crowntheir burrow by a funnel-shaped web. The Dipluridae finallyare real web-builders, which depend on a sheet-web to catchtheir prey. Generally they do not dig a burrow but hide in asmall retreat from which a funnel-web leads to the net. With a few exceptions Ctenizidae are entirely nocturnal. Theirrhythm of activity has been analyzed. The "Zeitgeber" is thedaylight during the last half hour before sunset. Most trapdoorspiders never leave their burrow during their whole life. Theyneed three to four years from hatching to become adults. Adultmales die during or at the end of the restricted mating-season;they take no food when adult. Females, which undergo post-adultmolts, can probably live for 15–20 years. Nemesia caementaria, like most other species, hunts during thewhole night. The mean time of activity is about Si/o hours,consisting of periods of lying in wait and periods of intermediaterests. The spiders make an average of three leaps per nightto catch a prey, but only about 10% of all bounds are successful.A hungry animal, lurking in vain, shows unmotivated leaps. The effects of light, moisture, and temperature on hunting activityare analyzed. Ctenizidae hunt during autumn, winter, and springbut interrupt their activity in summer for an estivation whichlasts usually two months. The females of some species capture the male after mating andeat him; others never attack him. This difference in behaviorlias repercussions on reproduction. Among certain species the young nymphs of the third instar refuseall food until they have made their own burrow and can huntby themselves. The young of other species stay with the motherfor one year and leave her at the fifth or sixth instar to maketheir own burrow. The young of Nemesia caementaria can remainin the burrow of the mother until they are almost adult, i.e.,for two or three years, and feed on the prey the mother hascaught. The behavior of Ctenizidae can be grouped. Some species lurkbehind the closed or almost closed trap-door. Another groupopens the trap-door and puts the pedipalpi and the two anteriorpairs of legs radially out onto the rim of the burrow, whilethe cephalothorax is hidden behind the trap-door. A third groupspins threads of silk radially about the entrance and uses themas signal-lines. A fourth group can come out and pursue theprey, then drags it to the trap-door and into the burrow. The nature of the prey depends on the bio tope. The Ctenizidaefeed almost exclusively on insects, mainly ants and beetles;in the laboratory crickets are accepted as prey. Ctenizidae have no tarsal organ. They have different types oftrichobothria, transversal and longitudinal slit-organs, andlyriform organs. With these three kinds of sense-organs, thefunctions of which are not clearly understood, the trap-doorspiders are able to perceive the approach of prey, to judgeits distance from the trap-door, and to locate it in direction.They seem to have an organ of smell, since certain groups ofinsects are repulsive to them. Sight is not used for hunting.     

Causes for mass occurrences of the jellyfish Periphylla periphylla: a hypothesis that involves optically conditioned retention

The mesopelagic coronate medusa Periphylla periphylla has beenvery abundant (20–320 individuals m^–2), with asmany as 2.5 individuals m^–3, for more than a decade incertain Norwegian fjords. These abundances are two to threeorders of magnitude higher than reported from open ocean environments.Comparisons of the size, density and behavior of this jellyfishin three fjords (Lurefjorden, Sognefjorden and Halsafjorden)support the hypothesis that retention of P. periphylla is relatedto basin topography, light attenuation and photosensitivity.Furthermore, we hypothesize that the abundances of P. periphyllain Lurefjorden and Halsafjorden, but not in Sognefjorden, havebeen stimulated by increased light absorbance of water massesformed in the North Sea.     

Food consumption and diet composition of the web-building spider Agelena limbata in two habitats

Summary Prey capture rate, food consumption, and diet composition of all developmental stages of the funnelweb spider Agelena limbata were estimated in woody and open habitats by a sight-count method. Prey availability was evaluated on the basis of two indices, i.e. the ratios of daily food consumption to dry weight of predator and to daily standard metabolic rate. These indices varied seasonally and between instars in this spider. Comparison of these indices between arthropod predators suggests that A. limbata live under conditions of relatively limited food supply. In the open habitat, the spiders reduced foraging activities to avoid heat stress at midday in summer because the sheet web was exposed to the direct rays of the sun and its temperature exceeded 40°C. The daily food consumption of adult spiders in the open habitat was about half of that in the woody habitat. The lower rate of energy intake of spiders in the open habitat may cause the observed smaller size of adults and lower fecundity. A. limbata captured a great range of prey comprising ten orders of arthropods and ate chemically defended insects, e.g. stink bugs, lady beetles, and ants which were rejected by many spiders. This generalistic foraging may be associated with limited and heterogeneous food supply in this spider.     

MECHANICAL PROPERTIES OF MUSSEL BYSSUS THREADS

The byssus threads of the common mussel, Mytilus edulis L.,have been tested mechanically and the results from the testsrelated to the ecology of the animal. The threads are mechanicallysimilar to other crystalline polymers such as polyethylene havinga modulus of about 10⁸N m^–2 and a long relaxation time.Resilience of 60% is similar to tendon; ultimate strain is aboutfive times that of tendon at 0.44. The thread is laid down witha prestrain of 10% and so guys the mussel in position. Calculationshows that a mussel with 50 byssus threads would be able toresist all but severe winter storms (Received 1 December 1978;     

Does individual internal state affect the presence of a barrier web in Argiope bruennichii (Araneae: Araneidae)?

A barrier web created by some orb-web spiders has a three-dimensional web structure made of nonsticky threads, which is believed to provide a defensive function against predators. To test for a possible antipredatory function, we investigated relationships between the presence of a barrier web and an individuals internal state in the orb-web spider Argiope bruennichii, which builds orb-webs with and without a barrier web. Hunger level and molting as measures of internal state were represented by relative abdominal width and relative web size, respectively. Field survey showed that individuals with a wider abdomen and a smaller web tended to construct a barrier web. The positive relationship between body width and the presence of a barrier web supports the hypothesis of an antipredatory function as well-fed individuals are likely to expend more effort for defense. The small web size, however, may not be representative of molting because a correlation existed between web size and potential web substrates; therefore, the effect of molting on the presence of barrier web remains unclear.     

Vacuolar Na/K Exchange, its Occurrence in Root Cells of Hordeum, Atriplex and Zea and its Significance for K/Na Discrimination in Roots

Using excised low-salt roots of barley and Atriplex hortenslsthe transport of endogenous potassium through the xylem vesselswas studied It was enhanced by nitrate and additionally by sodiumions which apparently replaced vacuolar potassium which wasthen available in the symplasm of root cells for transport tothe shoot Vacuolar Na/K exchange also has been investigatedby measurements of longitudinal ion profiles in single rootsof both species. In Atriplex roots a change in the externalsolution from K⁺ to Na⁺ induced an exchange of vacuolar K⁺ forNa⁺, in particular in the subapical root tissues and led toincreased K⁺ transport and loss of K⁺ from the cortex. In inverseexperiments a change from Na⁺ to K⁺ did not induce an exchangeof vacuolar Na⁺; merely in meristematic tissues Na⁺—apparentlyfrom the cytoplasm—was extruded in exchange for K⁺. Inroots of barley seedlings without caryopsis, as in excised roots,a massive exchange of K⁺ for Na⁺ was observed in the continuouspresence of external 1.0 mM Na and 0.2 mM K. This exchange alsowas attributed to the vacuole and was most pronounced in theyoung subapical tissues. It did not occur, however, in the correspondingtissues in roots of fully intact barley seedlings. In these,the young tissues retained a relatively high K/Na ratio alsoin their vacuoles. Similarly, contrasting results were obtainedwith intact and excised roots of Zea mays L. Based on theseresults a scheme of the events that lead to selective cationuptake in intact barley roots is proposed. In this scheme acrucial factor of selectivity is sufficient phloem recirculationof K⁺ by the aid of which K⁺ rich cortical cells are formednear the root tip. When matured these cells are suggested tomaintain a high cytoplasmic K/Na ratio due to K⁺ dependent sodiumextrusion at the plasmalemma and due to recovery of vacuolarK⁺ by Na/K exchange across the tonoplast. Key words: Potassium/Sodium selectivity, Vacuolar exchange, Xylem transport, Hordeum, Zea, Atriplex     

Relative impacts of copepods, cladocerans and nutrients on the microbial food web of a mesotrophic lake

Calanoid copepods, rather than cladocerans, frequently dominatethe zooplankton of lakes in New Zealand. The potential consequencesof this domination for the microbial community of mesotrophicLake Mahinerangi, New Zealand, were determined by field experimentsin which Boeckella and Daphnia were added to in situ enclosuresin the presence and absence of added nutrients. Boeckella hamataat ambient densities (2 and 81^–1) rapidly and severelysuppressed ciliate population growth over 4 days, even whenmicrobial growth was enhanced by added nutrients, but effectsof copepods on other components of the microbial community (bacteria,photosynthetic picoplankton, heterotrophic nano-flagellates,algae) were slight. In contrast, Daphnia carinata at the samedensities (but 3-fold higher biomasses per litre) had a relativelyweak effect on ciliates, suppressing ciliate abundance onlyafter 4 days at 8 Daphnia 1^–1 (330 µg 1^–1);this daphniid density also depressed abundances of large bacterialrods, some photosynthetic picoplankton and the dominant alga,Cyclotella. These results highlight the relative importanceof specific trophic linkages in a microbial food web; they alsosuggest that the dominance of Boeckella in many southern hemispherelakes may account for relatively low ciliate abundances in theselakes.     

Mesh selection of copepodite and nauplius stages of four calanoid copepod species

High-speed plankton nets, using mesh sizes of 61, 90, 124,190and 270 µn, were deployed in the North Sea to catch Calanusspp., Pseudocalanus minutus, Paracalanus parvus and Temora longicomis.These species were identified to their nauplii, copepodite andadult stages, and the numbers caught in each stage were convertedto numbers m^–3 seawater filtered. The width and lengthof a representative sample of each stage was measured, and theproportion caught by the different mesh sizes is presented.The selection pattern was described by a mathematical model,which showed that a mesh size of 75% of the copepod carapacewidth catches 95% of the individuals of that size present inthe seawater. Some recommendations for mesh sizes required tosample copepods are given.     

Web tuning of an orb-web spider, Octonoba sybotides, regulates prey-catching behaviour

An uloborid spider (Oclonoba sybotides constructs two types of web which are distinguished by linear or spiral stabilimenta. Food-deprived spiders tend to construct webs with spiral stabilimenta and food-satiated spiders tend to construct webs with linear stabilimenta. I experimentally examined the influence of web type on the speed of a spider's response to small and large flies. The results indicated that web type rather than the spiders' energetic condition influences the response speed to small or large Drosophila flies. I also examined whether thread tension affects the response speed of spiders by increasing the tension of the radial threads. The results showed that spiders on an expanded web responded to small prey as quickly as spiders on webs with spiral stabilimenta. The tension of the radial threads may be regulated by the degree of distortion of the radial threads at the hub. O. sybotides seems to construct orb webs which induce different responses for smaller, less-profitable prey according to its energetic state. The spider appears to increase the tension of the radial threads so that it can sense smaller prey better when hungry.     

The colonial cyanobacterium Trichodesmium as a physical and nutritional substrate for the harpacticoid copepod Macrosetella gracilis

The pelagic harpacticoid copepod Macrosetella gracilis usesthe colonial cyanobacterium Trichodesmium not only as a physicalsubstrate for juvenile development, but also as a food source.By associating itself with a buoyant colonial cyanobacterium,M.gracilis has developed a successful mode of life for existencein the plankton. Further evidence of M.gracills' dependenceon Trichodesmium as a physical substrate is demonstrated bypreviously undescribed microscopic observations of a gravidM.gracilis female attaching eggs to a Trichodesmium colony.Shipboard experiments investigating the ingestion and assimilationof Trichodesmium carbon (C) were conducted in September 1991and January/February 1992 in waters of the Bahamas and the Caribbean,respectively. Macrosetella gracilis not only ingested, but rapidlyincorporated, cyanobacterial organic matter into its own cellularmaterial. Utilization of ingested Trichodesmium by M.graciliswas investigated by assessing the metabolic partitioning andincorporation of ¹⁴C-labelled Trichodesmium into copepod lipids,proteins, polysaccharides and low-molecular-weight (LMW) compoundsusing sequential biochemical fractionation techniques. Despitevariations in grazing rates between the two sites and times(September 1991,0.017 µg C^* µg^–1 C h^–1;January 1992, 0.134 µg C ^* µg^–1 C h^–1,the partitioning of incorporated C into the different biochemicalfractions was relatively consistent. There was rapid assimilationof ingested C into the LMW ({small tilde}60%) and polysaccharidefractions ({small tilde}30%) in the first few hours, with asubsequent increase in the percent C incorporated into protein.On average, {small tilde}21% of the Trichodesmium C ingestedby M.gracilis was assimilated. Therefore, M.gracilis is an importantsecondary link in the food web of oligotrophic waters whereTrichodesmium is abundant.