Capture Spirals Research Articles

Dynamic environments do not appear to constrain spider web building behaviour

Many laboratory experiments demonstrate how orb-web spiders change the architecture of their webs in response to prey, surroundings and wind loading. The overall shape of the web and a range of other web parameters are determined by frame and anchor threads. In the wild, unlike the lab, the anchor threads are attached to branches and leaves that are not stationary but move, which affects the thread tension field. Here we experimentally test the effect of a moving support structure on the construction behaviour and web-parameters of the garden cross spider Araneus diadematus. We found no significant differences in building behaviour between rigid and moving anchors in total time spent and total distance covered nor in the percentage of the total time spent and distance covered to build the three major web components: radials, auxiliary and capture spirals. Moreover, measured key parameters of web-geometry were equally unaffected. These results call for re-evaluation of common understanding of spider webs as thread tensions are often considered to be a major factor guiding the spider during construction and web-operation.

Wind induces variations in spider web geometry and sticky spiral droplet volume

Trap building by animals is rare because it comes at a substantial cost. Using materials with properties that vary across environments maintains trap functionality. The sticky spiral silks of spider orb webs are used to catch flying prey. Web geometry, accompanied by compensatory changes in silk properties, may change across environments to sustain web functionality. We exposed the spider Cyclosa mulmeinensis to wind to test whether wind-induced changes in web geometry are accompanied by changes in aggregate silk droplet morphology, axial thread width or spiral stickiness. We compared: (i) web catching area, (ii) length of total silks, (iii) mesh height, (iv) number of radii, (v) aggregate droplet morphology and (vi) spiral thread stickiness, between webs made by spiders exposed to wind and those made by spiders not exposed to wind. We interpreted co-variation in droplet morphology or spiral stickiness with web capture area, mesh height or spiral length as the silk properties functionally compensating for changes in web geometry to reduce wind drag. Wind-exposed C. mulmeinensis built webs with smaller capture areas, shorter capture spiral lengths and more widely spaced capture spirals, resulting in the expenditure of less silk. Individuals that were exposed to wind also deposited larger droplets of sticky silk but the stickiness of the spiral threads remained unchanged. The larger droplets may be a product of a greater investment in water, or low molecular weight compounds facilitating atmospheric water uptake. Either way, droplet dehydration in wind is likely to be minimized.

Ageing alters spider orb-web construction

Ageing is known to induce profound effects on physiological functions but only a few studies have focused on its behavioural alterations. Orb-webs of spiders, however, provide an easily analysable structure, the result of complex sequences of stereotypical behaviours that are particularly relevant to the study of ageing processes. We chose the orb spider Zygiella x-notata as an invertebrate organism to investigate alterations in web geometry caused by ageing. Parameters taken into account to compare webs built by spiders at different ages were: the length of the capture spiral (CTL), the number of anomalies per cm, and four parameters of web regularity (the angle between radii, the number of spiral thread units connecting two successive radii, the parallelism and the coefficient of variation of the distances between silk threads of two adjacent spiral turns). All web parameters were related to ageing. Two groups of spiders emerged: short- and long-lived spiders (with a higher body mass), with an average life span of 150 and 236 days, respectively. In both short- and long-lived spiders' webs, the CTL and the silk thread parallelism decreased, while the variation of the distances between silk threads increased. However, the number of anomalies per cm and the angle between radii increased in short-lived spiders only. These modifications can be explained by ageing alterations in silk investment and cognitive and/or locomotor functions. Orb-web spiders would therefore provide a unique invertebrate model to study ageing and its processes in the alterations of behavioural and cognitive functions.

Spider orb webs rely on radial threads to absorb prey kinetic energy

The kinetic energy of flying insect prey is a formidable challenge for orb-weaving spiders. These spiders construct two-dimensional, round webs from a combination of stiff, strong radial silk and highly elastic, glue-coated capture spirals. Orb webs must first stop the flight of insect prey and then retain those insects long enough to be subdued by the spiders. Consequently, spider silks rank among the toughest known biomaterials. The large number of silk threads composing a web suggests that aerodynamic dissipation may also play an important role in stopping prey. Here, we quantify energy dissipation in orb webs spun by diverse species of spiders using data derived from high-speed videos of web deformation under prey impact. By integrating video data with material testing of silks, we compare the relative contributions of radial silk, the capture spiral and aerodynamic dissipation. Radial silk dominated energy absorption in all webs, with the potential to account for approximately 100 per cent of the work of stopping prey in larger webs. The most generous estimates for the roles of capture spirals and aerodynamic dissipation show that they rarely contribute more than 30 per cent and 10 per cent of the total work of stopping prey, respectively, and then only for smaller orb webs. The reliance of spider orb webs upon internal energy absorption by radial threads for prey capture suggests that the material properties of the capture spirals are largely unconstrained by the selective pressures of stopping prey and can instead evolve freely in response to alternative functional constraints such as adhering to prey.

The influence of prey on size, capture area and mesh height of the orb-web of the garden spider, Argiope aemula (Walckenaer, 1841) (Araneaea: Araneidae)

The orb-web garden spider Argiope aemula (Walckenaer, 1841) is a sit-and-wait predator. It is argued that it can anticipate its future prey environment by detecting the presence of prey and adjusting their web building behavior accordingly. Therefore this study therefore investigates the influence of the different prey sizes and density of the capture area and mesh height of the webs constructed by the spider. In the laboratory, the spiders were given prey with different size and densities to determine their influence on the web architecture. Results show that spider individuals can increase or decrease the sizes of webs, capture area, and mesh height in response to prey size and density. Starved spiders constructed significantly larger webs than well-fed spiders. In the absence of potential prey, the spiders constructed larger capture area. In the presence of small prey, spiders significantly constructed very narrow- meshed webs or tightly spaced capture spirals than the presence of larger prey but larger than in no prey regime. Similarly, the food deprived spiders spun small-spaced mesh height than well-fed spiders. The results of the present study demonstrate that spiders can manipulate their web architecture in response to different prey sizes and food availability (densities).

Indirect Optimization of Three-Dimensional Finite-Burning Interplanetary Transfers Including Spiral Dynamics

The indirect optimization problem for a three-dimensional transfer from low Earth orbit to low Mars orbit is solved. A step-by-step process developed for a two-dimensional model and techniques for accurately estimating the unknown costates for three-dimensional escape and capture spirals are used. Minimum-propellant trajectories for finite-burning engines are calculated. Solutions are considered with and without control limits on specific impulse and compared with previous research. Unlike other research, the entire trajectory, including the Martian capture sequence, is integrated in an Earth-referenced frame. Additionally, the capture sequence is not found by iteratively lowering the final targeted low Mars orbit, but the desired final orbit is directly targeted with no successive iterations of increasingly smaller low Mars orbits. As in the two-dimensional case, more fuel-efficient trajectories are found for the same mission objectives and constraints published in other research, emphasizing the importance of this technique. Whereas previous research only achieved final Martian orbits of 6 Mars radii DU M (20,382 km), the new approach finds solutions for final Martian circular orbits of 1.47―2.00 DU M (5000―6794 km).

Low-Thrust Interplanetary Transfers, Including Escape and Capture Trajectories

O PTIMAL low-thrust interplanetary transfers in the heliocentric frame have been studied in many documented articles in which the planetary escape and capture maneuvers were not addressed. For example, [1–4] demonstrated different methods to find optimum steering programs for heliocentric orbital transfers. Moreover, a number of state-of-the-art low-thrust trajectory optimization tools are also available for mission designers. The Jet Propulsion Laboratory (JPL) developed the well-known SEPTOP and VARITOP [5] (SEPTOP’s predecessor) programs that have been used for decades. More recently, JPL has demonstrated devotion to developing new low-thrust programs such as MYSTIC [6] and MALTO [7], which are more sophisticated and capable of designing complicated low-thrust trajectories. On the other hand, optimizing low-thrust planetocentric escape and capture trajectories has also been studied (see [8–11]). These prior efforts and studies reveal that mission designers are interested in low-thrust transfers from a lowEarth orbit (LEO) to a low-planet orbit. However, the literature dealing with optimizing the entire mission, including both heliocentric and planetocentric transfer trajectories, still appears to be somewhat limited. The most commonly used method is the patched-conic approximation, which was used by Melbourne and Sauer [12] in the 1960s. The interplanetary leg and planetocentric spirals are optimized separately and then patched together. Currently, this approximation method is still in wide use by mission designers. In this Note, an approach is proposed to estimate the mission performances (e.g., flight time and propellant mass) of low-thrust interplanetary transfers, including escape and capture trajectories. During the escape or capture phases, the continuous tangential or antitangential thrust (null in shadow) is used and J2 perturbations are considered. The low-thrust escape or capture spirals are computed by a fast algorithm (an analytic orbital averaging technique plus a shortduration integration process) that significantly reduces the trajectory propagation time. With the patched-conic approximation, the proposed approach makes it possible to formulate the entire lowthrust interplanetary transfer trajectory in a single nonlinear programming parameter optimization problem. It should be noted that the shadow conditions may significantly affect propellant consumption, flight time, and the ephemeris time that the capture or escape begins. The inclusion of shadowing distinguishes this work from most previous research. The purpose of the proposed approach is not to generate highfidelity trajectory solutions but to provide a preliminary analysis tool for mission designers to quickly estimate the mission performances of low-thrust interplanetary transfers that include escape and capture trajectories. Themany-revolution planetocentric spiral trajectories in the presence of shadowing and J2 perturbations are propagated using orbital averaging, which results in much fewer steps than precise numerical integration. The proposed approach is particularly useful for determining a particular mission’s trendwith respect to variations in system parameters. An optimal Earth–Mars transfer is obtained in this Note to provide an example. Furthermore, the preliminary solutions obtained with the proposed method could be used as good initial guesses for further trajectory optimization to generate highfidelity solutions.

Mesh Width Influences Prey Retention in Spider Orb Webs

AbstractOrb‐weaving spiders depend upon the sticky capture spirals of webs to retain insects long enough to be captured. However, insects often escape from orb webs before the spiders can attack them. Therefore, the architectures of orb webs likely reflect strong selective pressure to increase retention times of insects. We experimentally increased the mesh width of one side of an orb web while maintaining the original mesh width on the other side as a control, and then tested the effect of this manipulation on the retention times of four different taxa of insects. We found evidence that increased mesh width of Argiope aurantia orb webs resulted in a general reduction in the retention times of insects. However, retention times for different taxa of insects were not predicted by any one specific morphological or flight characteristic. The influence of mesh width on the retention times of insects is very complex, but our results suggest that mesh width can act to selectively favor the capture of certain taxa of insect prey over others. This effect may help to explain both species level differences in web‐building behaviors and variation in the architectures of webs spun by individual spiders on different days.

The effects of capture spiral composition and orb-web orientation on prey interception

Cribellar prey capture threads found in primitive, horizontal orb-webs reflect more light, including ultraviolet wavelengths, than viscous threads found in more derived, vertical orb-webs. Low web visibility and vertical orientation are each thought to increase prey interception and may represent key innovations that contributed to the greater diversity of modern, araneoid orb-weaving spiders. This study compares prey interception rates of cribellate orb-webs constructed by Uloborus glomosus (Uloboridae) with viscous orb-webs constructed by Leucauge venusta (Tetragnathidae) and Micrathena gracilis (Araneidae). We placed sectors of cribellar and viscous threads side by side in frames that were oriented either horizontally or vertically. The webs of both U. glomosus and L. venusta intercepted more prey when vertically oriented. In each orientation L. venusta webs intercepted more insects than did U. glomosus. Although this is consistent with the greater visibility of cribellar threads, the more closely spaced capture spirals of L. venusta may have contributed to this difference. Micrathena gracilis webs intercepted more prey than did U. glomosus webs, although web orientation did not affect the performance of this araneoid species. The stickier and more closely spaced capture spirals of M. gracilis may have enhanced the interception rates of this species and accounted for the greater number of smaller dipterans retained in its webs. The tendency for these slow, weak flight insects to be blown into both horizontal and vertical webs may account for similar interception rates of horizontal and vertical M. gracilis webs. These observations support the enhanced prey interception of vertically oriented orb-webs, but offer only qualified support for the contributions of lower visibility viscous capture threads.

Spider Genes and Fossils Spin Tales of the Original Worldwide Web

EVOLUTION Building an orb web is no simple affair. Spiders suspend the silky equivalent of guy wires, attach radial spokes, and then weave in a sticky spiral to snare prey. Two groups of spiders—deinopoids and araneoids—make such webs. Their use of different kinds of adhesives for the “capture spiral” once made biologists think that the two spider lineages had evolved orb weaving independently. But the discovery of similar construction techniques made a single origin of orb webs seem more likely, and a new study of silk genetics on page [1762][1] strengthens the case. ![Figure][2] Better flytrap. After orb webs evolved, araneoid spiders improved them by adding gluey silk. CREDIT: MARK CHAPPELL “It's really cool to see this matched by the genetic evidence,” says Gustavo Hormiga, an arachnologist at George Washington University in Washington, D.C., about a study led by Jessica Garb, a postdoc at the University of California, Riverside (UCR). Two other new papers describe fossils of spiders and their webs that further emphasize the antiquity of orb webs. Deinopoids follow the more ancient silk recipe. They swathe their capture spirals in dry silk. First, a spider oozes fibrils just tens of nanometers in diameter from thousands of spigots on its abdomen. Then the spider combs the threads like cotton candy onto a support line that makes up the spiral. When a fly or other prey brushes up against the fibrils, electrostatic forces pin it to the web. Araneoids simplified the process. Using a pair of glands that deinopoids lack, they simply dab a viscous glue onto the support line. That approach takes about one-tenth the effort of making dry silk, and the adhesive is 13 times stickier per unit volume. The web is also less visible to insects, because the silk doesn't reflect ultraviolet light. All these advantages may help explain why araneoids are 10 times more diverse than the deinopoids. Scientists have extensively studied the genes and proteins that make spider silk stretchy and strong ( Science , 25 February 2000, [p. 1378][3]), but most of the work has focused on araneoids. Garb decided to take the web less traveled by. Working with Cheryl Hayashi of UCR and others, she studied complementary DNA from silk glands of two kinds of deinopoids, Deinopis spinosa and Uloborus diversus . The team found that the deinopoids had genes (known as Flag , MaSp1 , and MaSp2 ) quite similar to those that araneoids use to make silk for the capture spiral and radial spokes—additional evidence for a single origin of orb webs. That's not a surprise to spider biologists, but it's pleasant confirmation that their previous observations “are as valid as we thought they were,” says Brent Opell of Virginia Polytechnic Institute and State University in Blacksburg. Two new fossils described this week underscore the long-lived success of orb webs. On page [1761][4], a team led by David Grimaldi of the American Museum of Natural History in New York City reports the oldest example of spider silk entrapping prey. In a chunk of 110-million-year-old amber from Spain, they found a fly and a mite ensnared in strands of gluey spider silk, possibly from an orb web. Meanwhile, in the 14 June online issue of Biology Letters , David Penney of the University of Manchester, U.K., and Vicente Ortuno of the Universidad de Alcala, Madrid, describe the oldest true orb-weaving spider: an araneoid found in 115-million-year-old Spanish amber from a different site. The 2-millimeter-long spider, which they name Mesozygiella dunlopi , is remarkably similar to a living spider—showing that the basic, and successful, body plan appeared long ago. [1]: /lookup/doi/10.1126/science.1127946 [2]: pending:yes [3]: /lookup/doi/10.1126/science.287.5457.1378a [4]: /lookup/doi/10.1126/science.1126628

EVALUATION OF FORMULAE TO ESTIMATE THE CAPTURE AREA AND MESH HEIGHT OF ORB WEBS (ARANEOIDEA, ARANEAE)

Abstract We evaluated several formulae to estimate the capture area (the area of the web covered by capture spirals) and the mesh height (the distance between capture spirals) of orb webs constructed by Argiope keyserlingi Karsch. The accuracy of the various formulae was estimated through regression analyses. Accordingly, we propose two new formulae specifically suited for asymmetric orb webs, which provide accurate estimates of capture area and mesh height.

Design Variability in Web Geometry of an Orb-Weaving Spider

We studied the effect of several variables (environmental and physiological) on web geometry in the garden cross spider Araneus diadematus. Variables were: web support, wind, temperature, humidity, and silk supply. All had an effect. The spiders generally attempted to fit their webs to the shape of the supporting frame (standard, small, vertical, or horizontal). Windy conditions (0.5 m s −1) during web construction caused spiders to build smaller and rounder webs, laying down fewer capture spirals while increasing the distances between capture-spiral meshes. Decreasing temperature from 24° to 12°C caused the capture spiral to have fewer and wider spaced meshes, which did not change overall capture area but reduced the length of capture-spiral threads laid down. Subsequent increase of temperature to 24°C restored the number of meshes laid down, but the wider mesh was retained, causing the capture area to be increased over initial control values. Decreased humidity (from 70 to 20% rH) had the effect of reducing web and capture-spiral size, the latter by reducing mesh number while keeping mesh spacing constant. Subsequent increase of humidity to control level (70%) restored web and capture area. However, this was achieved by laying down capture meshes at larger distances, rather than returning to initial mesh numbers. Silk supply also had a strong effect. Webs built in unnaturally rapid succession by the same spider (4 in 24 h when 1 is the norm) became sequentially smaller, had fewer radii, shorter capture spirals, and were wider meshed.

Thread biomechanics in the two orb‐weaving spiders Araneus diadematus (Araneae, Araneidae) and Uloborus walckenaerius (Araneae, Uloboridae)

AbstractThe absorption of high kinetic energy by a small amount of material depends not only on the quality of the material but also on the structural design of the elements involved. Using a rapid response microbalance, we measured the tensions of radial threads in webs of the garden cross spider, Araneus diadematus. We also measured the stress‐strain characteristics of dry radius and wet spiral threads laid down by A. diadematus, as well as of the very different and dry spiral threads laid down by the hackled‐band weaver, Uloborus walckenaerius.The radius threads of A. diadematus showed good extensibility (e = 39. 4%), high tensile strength (s = 1153. 8 MPa) and large hysteresis (56%) which indicates that they can function as shock absorbers and structural elements. Although fewer radii were built in the upper than in the lower half of the Araneus web, our method found no systematic difference between the average pretensions of individual radius threads in these two halves. However, pretension in the upper half of the web showed greater variation.Orb weavers employ two different mechanisms to increase the energy‐absorbing capacity of their respective capture spirals. The sticky spiral of Araneus diadematus absorbed energy by large extensibility (about 475%) of the wetted thread which developed substantial force only after 100–200% extension, and the entire thread failed suddenly. The hackled band of Uloborus walckenaerius had shorter extensibility (about 125%) and it absorbed energy by friction of the fine hackled fibres, many of which needed to break in succession before a thread failed. © 1995 Wiley‐Liss, Inc.

Functional organization of the spinning apparatus of Cyrtophora citricola with regard to the evolution of the web (Araneae, Araneidae)

The spinning apparatus of Cyrtophora citricola closely corresponds to that of orb-weaving Araneidae, two peculiarities excepted. Firstly the spigots of the piriform glands differ extremely in size, the smallest of them being numerous and having a unique location on the anterior spinnerets. Secondly, the triad complex (on the posterior spinnerets) used by other Araneidae for producing gluey capture threads is lacking. Both these characteristics are correlated with the construction of a fine meshed sheet of dry silk by Cyrtophora instead of orbwebs with capture spirals. The sheet can be understood as being a very much enlarged central area of orb-webs. Since vestiges of triads could be found in early developmental stages of C. citricola, the origin of the meshed sheet from orb-webs with gluey capture threads is clearly demonstrated. The paper includes a study on how the spider produces thread attachments by means of the secretions of the piriform glands.