A virtual robot to model the use of regenerated legs in a web-building spider

The garden cross orb-spider, Araneus diadematus, shows behavioural responses to leg loss and regeneration that are reflected in the geometry of the web's capture spiral. We created a virtual spider robot that mimicked the web construction behaviour of thus handicapped real spiders. We used this approach to test the correctness and consistency of hypotheses about orb web construction. The behaviour of our virtual robot was implemented in a rule-based system supervising behaviour patterns that communicated with the robot's sensors and motors. By building the typical web of a nonhandicapped spider our first model failed and led to new observations on real spiders. We realized that in addition to leg position, leg posture could also be of importance. The implementation of this new hypothesis greatly improved the results of our simulation of a handicapped spider. Now simulated webs, like the real webs of handicapped spiders, had significantly more gaps in successive spiral turns compared with webs of nonhandicapped spiders. Moreover, webs built by the improved virtual spiders intercepted prey as well as the digitized real webs. However, the main factors that affected web interception frequency were prey size, size of capture area and individual variance; having a regenerated leg, surprisingly, was relatively unimportant for this trait. Copyright 1999 The Association for the Study of Animal Behaviour.     

Cues that Spiders (Araneae: Araneidae,Tetragnathidae) Use to Build Orbs: Lapses in Attention to One Set of Cues because of Dissonance with Others?

Even for small animals such as spiders, behavioral decisions are sometimes influenced by multiple cues. Orb webs constitute exquisitely precise records of the stimuli the spider experienced and the decisions that it made while building its web. In addition, because spiders appear to sense their webs largely by touch, direct behavioral observations can determine which stimuli they probably sense. Previous studies have shown that when an orb‐weaving spider decides how far apart to space successive sticky lines during orb construction, it responds to at least five different kinds of stimuli, all of which apparently use a cue from the web, the location of the previous, inner loop of sticky spiral (IL location), as a point of reference. Here we show that two additional cues from the web, which are related to the position of the temporary spiral (TS), also influence sticky spiral spacing. A combination of direct observations of spider movements, analyses of complete and partially complete webs, and responses to experimental modifications of the web of two species in different families, Micrathena duodecimspinosa (Araneidae) and Leucauge mariana (Tetragnathidae), indicate that both the TS‐IL distance itself and the short‐term memory of the change in TS‐IL distance compared with that on other recently encountered radii correlate with sticky spiral spacing. When the TS‐IL distance was large, the spiders apparently ceased to attend to other cues. Thus, even the relatively stereotyped behavior of orb construction includes variation that stems from attention‐like mental processes.     

An organic coating keeps orb‐weaving spiders (Araneae,Araneoidea, Araneidae) from sticking to their own capture threads

More than 95% of orb‐weaving spider species ensure prey capture success by producing viscous threads equipped with gluey droplets. However, this trap may bear serious risks for the web‐inhabiting spider as well. The obvious question, how a spider avoids getting stuck in its own capture spiral, has gained little scientific attention up till now. In 1905, the French naturalist Jean‐Henry Fabre concluded from anecdotal observation that orb‐weaving spiders protect themselves by a fatty surface coating. Here, we test this hypothesis by indirectly measuring the force necessary to detach an autotomized spider’s leg from the capture spiral of its own web (here called ‘index of adhesion’, IOA). Three groups of legs, each of the species Araneus diadematus Clerck, 1757 and Larinioides sclopetarius (Clerck, 1757), were tested. One was left untreated, one was washed with distilled water (H₂O), and one was washed with the organic solvent carbon disulphide (CS₂). In both species, we found a weak IOA between the spider leg and the gluey capture spiral in untreated and water‐washed legs without significant differences between the two. The IOA approximately doubled, when spider legs had been washed with carbon disulphide prior to measurement, that is, the CS₂‐washed legs stuck significantly more strongly than the untreated and water‐washed legs. These results provide indirect evidence for a protective anti‐adhesive organic coating on the spider’s body surface and so support Fabre’s hypothesis.     

Aging and foraging efficiency in an orb-web spider

Aging is often associated with reduced behavioral performance such as decreased locomotion or food consumption, related to a deterioration in physiological functions. In orb-web spiders, webs are used to capture prey and aging can affect web-building behavior and web structure. Here, we investigated the effect of aging on prey capture in the orb-web spider Zygiella x-notata. The ability of adult females to capture flies was examined at different ages. The rate of prey capture did not change with age, but older spiders took more time to subdue and capture the prey. Alterations which appeared in web structure with age (increase in the number of anomalies affecting radii and capture spiral) affected prey capture behavior. Furthermore, the analysis of individual performance (carried out on 17 spiders at two different ages) showed that older females spent more time handling the prey and finding it in the web. Our results suggest that, in the laboratory, age does not affect prey capture rates but it influences prey capture behavior by affecting web structure or/and spider motor functions.     

Gravity as an orientation guide during web-construction in the orb spiderAraneus diadematus (Araneae,Araneidae)

summary The effect of gravity on the web building behaviour of the common garden spiderAraneus diadematus was studied in three ways: (i) frames with partially completed vertical webs were swivelled into a horizontal position, (ii) by rotating frames with spiders in a vertical klinostat (1 rpm), and (iii) by vertically rotating a partially completed web treadmill fashion keeping the building animal in a certain position in space.(i) In the horizontal, radial wheels are not constructed, however, a more or less irregular spiral is added to a completed wheel; (ii) in the klinostat the radial wheel lacks the up/down distinction of normal webs, and the spiral is irregular; (iii) in the treadmill the spiral course is abnormal, and the degree of deviation depends on the position of the animal. If the body axis is parallel to gravity the spiral path deviates to both sides of the norm. In ag perpendicular body position the path deviates predominantly to one side, spiralling sharply inwards towards the hub. The observations suggest that the cyclic changes in the body position of spiral-buildingAraneus are an important component of the animal's orientation during this phase of web-construction.     

The influence of clinostat rotation on the fertilized amphibian egg

Unrestrained, fertilized eggs ofRana pipiens andXenopus laevis were rotated in a plane parallel to the normal gravity vector. InR. pipiens rotation at 1/4 rpm for 5 days at 18°C produced a significantly increased number of commonly occurring abnormalities. Rotation at 1/15, 1/8, 1, 2, 5 and 10 rpm did not significantly affect normal development.X. laevis eggs reacted similarly.R. pipiens eggs were most sensitive to rotation at 1/4 rpm when exposure was initiated before first cleavage. Mixing of intracellular constituents apparently occurred only at 1/4 rpm inR. pipiens (of the clinostat speeds studied), and may have been the cause of the increased abnormality observed at this rate.     

Forces in the spider orb web

Summary Pretensile forces were measured in individual threads of intact spider webs. In the orb web of Araneus diadematus forces decrease from mooring threads to frame threads and radii, a typical ratio being 1071. The smaller number of radii in the upper than in the lower half of the orb is paralleled by force ratios of 21 to 31. A similar difference between radii built first during web construction and radii added after completion of the frame underlines the importance of the former as part of the scaffolding. High tensions in the auxiliary spiral stabilize the radii in addition to providing a pathway for the spider when inserting the sticky spiral. Radial pretension (F) changes with spider mass (m). F/m is similar for different animals indicating an adaptation of radial forces to those resulting from spider mass. Several observations suggest tension control by the spider. When forced to anchor its web to thin flexible rods tension in the threads remains in the normal range. Tension values are similar in the webs of A. diadematus, Zygiella x-notata, Nuctenea umbratica, and Nephila clavipes indicating independence from details of web geometry. Only the mooring threads of Nephila show unusually large forces suggesting a narrower working range of tensions for the catching area than for the scaffolding.     

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.     

Energetic cost of web construction and its effect on web relocation in the web-building spider Agelena limbata

Summary Although spider webs may be effective in trapping prey, they require energy for construction. The design of webs varies in complexity from species to species. I assume that the energetic cost of web construction is significantly different among web types or species. This cost may constrain foraging tactics, particularly web relocation, because web relocation also requires energy to make a new web. To clarify the effect of the cost of web construction on web relocation, the energy cost of web construction and the rate of web relocation were estimated for the spider Agelena limbata. This spider constructs a sheet-funnel web consisting of a tight mesh of silk threads. This web was costly in terms of the energy needed for construction, which ranged from 9 to 19 times the daily maintenance energy. The daily rate of web relocation was below 1%, indicating high web-site tenacity. Relocation rates of species which built different types of web were compared in relation to cost of web construction. Orbweavers, which produce less costly webs than sheet-funnel weavers, relocate webs more frequently. Sheetweavers, which make webs of intermediate cost, appear to relocate webs more frequently than sheetfunnel weavers but less frequently than orbweavers. These results suggest that the energy cost of web construction is important in determining the frequency of web relocation.     

Asymmetry in spider orb webs: a result of physical constraints?

A typical feature of most vertical orb webs is that the upper web region is smaller and contains less silk than the lower web region, creating an asymmetrical web. The degree of web asymmetry changes during the spider's development: small juveniles construct more symmetrical webs, but older and larger individuals decrease the upper web region. This implies that weight may control the extent of web asymmetry. Using two species, Argiope keyserlingi and Larinioides sclopetarius, we tested the effect of weight increase on web asymmetry by naturally increasing weight through feeding and by artificially adding lead weights to the abdomen of the spiders. Weight increase (natural or artificial) resulted in more asymmetric webs through a reduction of the upper web region. Added weight may interfere with spiral placement in the upper region, because the spider has to lift its abdomen above the carapace during the process. In the lower region, however, the position of the spider is mostly head up during spiral placement. Therefore, amongst other factors, weight and gravitational forces may be physical constraints during web construction. Copyright 1999 The Association for the Study of Animal Behaviour.     

Orientation in a vertical plane: The use of light cues by an orb-weaving spider, Araneus diadematus clerk

The use of non-vibrational cues for orientation on a vertical orb-web was examined in Araneus diadematus. By rotating webs during prey capture, it was shown that this orb-weaving spider uses cues that are independent of the web for finding its way back to the web's hub after capturing prey. Further, it was demonstrated that light is an important orientation cue. Light is used to discriminate between the sides of the web, and the significance of this is discussed in relation to compass orientation within the web's vertical plane.     

Design features of the orb web of the spider, Araneus diadematus

Analysis of orb webs of the garden cross spider (Araneus diadematus)showed that these vertical webs have a significant up/down asymmetry.Experiments demonstrated that the spider runs down faster thanup, and thus confers a relatively higher foraging value to sectionsbelow the hub. Simulations suggested that the density of capturespiral spacing, prey size, and the density of prey should allaffect the capture efficiency of a web. Webs lose effectivecapture area because of overlap of the capture zone around eachthread; the smaller the prey, the finer the mesh can be withoutlosing effective area. Lower sectors of the web have a particularmesh size (height and length of capture spiral segments) throughout,whereas in the upper sectors the mesh size changes, wideningfrom the hub towards the periphery.     

Habitat selection and web plasticity by the orb spider Argiope keyserlingi (Argiopidae): Do they compromise foraging success for predator avoidance?

Abstract Orb web spiders face a dilemma: forage in open habitats and risk predation or forage in closed habitats to minimize risk but at reduced foraging profitability. We tested whether Argiope keyserlingi opts for safer habitats at the expense of foraging success by (i) determining habitat selection indices in open and closed habitats; (ii) marking and releasing individual juvenile, subadult and adults over two 4‐week periods to determine if life‐history stage influences habitat selection; and (iii) determining the biotic and abiotic environmental parameters that relate to A. keyserlingi abundance. We found that A. keyserlingi selected closed habitats. Sedge and anthropogenic structures were selected and trees were avoided. Juveniles were never found in open habitats, most likely because of high postdispersal mortality. Subadults and adults may shift from closed to open habitats while juveniles never shifted habitat. Foliage density, plant height, potential prey abundance, and mantid and bird abundance were correlated with A. keyserlingi abundance, with only bird abundance explaining habitat selection. We measured web capture area, spiral distance (distance between spiral threads) and the number of decoration arms (0, 1, 2, 3 or 4) in the field and did laboratory experiments to test the influence of (i) space and vegetation; (ii) prey abundance; and (iii) web damage, on web architecture. Argiope keyserlingi webs exhibited geometric plasticity by having larger prey capture areas and spiral distances in open habitats. Decoration design did not differ between habitats however. Variation in space availability, air temperature, prey abundance and web damage explained the variations in web architecture. Potential prey size and diversity differed between habitats but prey abundance did not. As large prey may be important for spider survivorship, foraging success appears to be compromised by occupying closed habitats.     

Gumfooted lines in black widow cobwebs and the mechanical properties of spider capture silk

Orb-weaving spiders produce webs using two types of silk that have radically different mechanical properties. The dragline silk used to construct the supporting frame and radii of the web is stiff and as strong as steel, while the capture spiral is much weaker but more than ten times as extensible. This remarkable divergence in mechanical properties has been attributed to the aqueous glue that coats the capture spiral, which is thought to decrease capture spiral stiffness and increase its extensibility. However, discerning the effect of the aqueous glue on fiber performance is complicated because dragline silk and the capture spiral are assembled from different proteins, which may also affect mechanical performance. Here, we use the sticky gumfooted lines of black widow cobwebs to test the effect of the addition of aqueous glue on the mechanical properties of dragline silk. We also surveyed orb-webs spun by a broad range of species for bundles of looped silk. Such bundles, termed windlasses, have been thought to increase capture spiral extensibility by "paying out" additional lengths of silk. Our results suggest that neither plasticization of silk by aqueous glue nor excess silk in windlasses can by themselves account for the remarkable extensibility of orb-weaver capture silk compared to other spider silks. This argues that the unique amino acid motifs of the flagelliform fibroins that constitute the core of the capture spiral play an essential role in capture silk's extreme extensibility.     

Energy costs of the predation strategy of the web-spinning spider Lepthyphantes zimmermanni bertkau (Linyphiidae)

Summary The energy costs of the predation strategy of the web-spinning spider Lypthyphantes zimmermanni were investigated in the laboratory. The standard respiratory costs associated with the stationary aspect of the strategy were estimated by means of a Gilson respirometer run at the different temperatures prevailing month by month in the beech woodland litter layer which comprises the spider's natural habitat. Respiration rate is related to weight by an exponent with a mean value of 0.7398. The Q ₁₀ of respiration rate is 2.41 between 5°C and 10°C and 1.97 between 10°C and 15°C. The energy costs of producing a web comprise the active respiratory costs associated with the locomotory activity involved in spinning a web together with the energy value of the silk used in the web manufacture. The former were evaluated by allowing a spider to spin a web in a respirometer, subtracting the calculated standard respiratory energy costs for a spider of equivalent weight and multiplying by a correction factor for web size. The relationship between spider weight and area of web produced was established in the laboratory. The respiratory cost of spinning a web is effectively constant with temperature at 724.46·10^-3 J for an adult (4 mg) spider. The energy value of spider silk was estimated by means of a bomb calorimeter and found to be 17,435 J g^-1. The energy content of the silk of a single adult's web is 1.16 J, giving energy cost of web production of 1.88 J at all temperatures.     

Effect of abiotic factors on the foraging strategy of the orb‐web spider Argiope keyserlingi (Araneae: Araneidae)

Abstract Environmental conditions such as light level, background contrast and temperature might influence a spider's prey capture success and risk of predation. Thus it may often be advantageous for spiders to adjust web‐building behaviour in response to variation in these environmental conditions. This hypothesis was examined in a study of the construction of webs and web decorations (conspicuous strands of silk at the hub of the web) of the orb‐web spider Argiope keyserlingi. Web decorations are thought to have one or more separate functions. They may attract prey, deter predators or advertise the web to oncoming birds, thus preventing web damage. In this series of experiments, relationships between weather parameters and the construction of webs and web decorations were considered. In complementary laboratory experiments, A. keyserlingi spiders were exposed to two different light levels (700 and 90 lx), background contrasts (black and white) and temperature conditions (20 and 26°C). Of the available weather parameters, only temperature was significantly related to web decorating behaviour but not to web size. In the laboratory, temperature also influenced web‐decorating behaviour, and spiders in dim light (700 lx) constructed larger webs and longer decorations. Background contrast did not significantly alter web size or web decorations. These data suggest that when prey availability is reduced at low temperatures, spiders may use web decorations to attract prey to the web. Similarly, in dim light, spiders may build more and larger decorations to increase the visual signal to approaching prey or to advertise the web to oncoming birds.     

Homogeneous batch cultures of<Emphasis Type="Italic"> Aspergillus oryzae</Emphasis> by elimination of wall growth in the Variomixing bioreactor

A novel principle for mixing and aeration in stirred bioreactors, named Variomixing, was developed. Four baffles are rotated intermittently at a rotational speed slower or similar to the speed of a centrally placed axial flow impeller. Rotational speeds of the baffles and impeller of 5–10 and 500–600 rpm, respectively, results in the highly turbulent flow regime characteristic of conventional bioreactors with high mixing and mass transfer capacities. Stagnant zones around crevices and crannies in which wall growth may commence are avoided since the baffles are never completely at rest. Increasing the rotational speed of the baffles (5 s every 5 min), so that it follows the speed of the impeller (500–600 rpm), cancels the effect of the baffles and a deep vortex and high peripheral liquid flow rates at the reactor wall develop. The vortex ensures that also the head-space of the reactor wall is flushed and any deposits removed. The filamentous fungus Aspergillus oryzae has been grown in batch cultures in the Variomixing bioreactor. Compared to conventional laboratory-scale bioreactors, in which more than 30% of all biomass was found attached to walls, less than 2% of the total A. oryzae biomass was found on the walls in the Variomixing bioreactor.     

Using Past Experience in Web Relocation Decisions Enhances the Foraging Efficiency of the Spider Cyclosa argenteoalba

The purpose of this study is to test whether the integration of past and present foraging experience in web relocation decision enhances foraging efficiency of the spider Cyclosa argenteoalba in its natural environment. We measured daily changes in the prey availability at several fixed sites in a natural environment and constructed a model environment based on these observational data. In the model environment, we simulated the behavior of spiders that foraged and relocated their webs according to several decision rules, which differed in terms of how a spider used its past experience. Results of the simulation revealed that the less past experience is discounted in making web relocation decisions, the more prey the spider is expected to capture. The expected number of web relocations decreased as spiders kept past foraging experience longer. These results suggest that C. argenteoalba enhances foraging efficiency by using past foraging experience for long times in the decision of web relocation in its natural environment.     

Web-building behaviour in the orb-weaving spider Zygiella x-notata: influence of experience

Zygiella x-notata is an orb-weaving spider that often renews its trap daily. Web building has associated costs and benefits, and building successive webs may have consequences for lifetime reproductive success. In the laboratory, we tested the ability of Z. x-notata to modify its building behaviour in response to various stages in predation (prey detection, capture and ingestion) experienced with a previous web. We determined which stages provided information for the spiders. Spiders that detected, captured and ingested prey and then rebuilt their web used less silk and made a smaller capture area than in the previous web. There was no effect of prey detection alone on the next web. Capture without feeding gave the same results as capture followed by feeding. The spiders that ate prey without detection and capture (feeding by hand) had the same energetic gains as spiders that caught prey but delayed building a new web. The spiders thus showed plasticity in web-building behaviour and in the amount of silk used (energetic investment) in the short term (from one web to the next). Changes in body condition may therefore influence web construction. Moreover, information gained during prey capture appeared to influence the size and structure of the next web. This ability should enable spiders to adapt their web building to maximize their fitness. Copyright 2000 The Association for the Study of Animal Behaviour.     

Influence of aging on brain and web characteristics of an orb web spider

In animals, it is known that age affects the abilities of the brain. In spiders, we showed that aging affects web characteristics due to behavioral alterations during web building. In this study, we investigated the effects of age on the associations between morphological changes to the spider brain and changes in web characteristics. The orb web spider Zygiella x-notata (Araneae, Araneidae) was used to test these relationships. Experiments were conducted on young (19 ± 2 days after adult molt, N = 13) and old (146 ± 32 days, N = 20) virgin females. The brain volume decreased with age (by 10%). Age also had an impact on the number of anomalies in the capture area generated during web building. The statistical relationships between the volume of the brain and web characteristics showed that there was an effect of age on both. Our results showed that in spiders, aging affects the brain volume and correlates with characteristics (anomalies) of the web. As web building is the result of complex behavioral processes, we suggest that aging affects spider behavior by causing some brain alterations.