Biomechanical Variation of Silk Links Spinning Plasticity to Spider Web Function Cecilia Boutryã, Todd A

ARTICLE IN PRESS ZOOLOGY Zoology ] (]]]]) ]]]–]]] www.elsevier.de/zool

Biomechanical variation of silk links spinning plasticity to spider web function Cecilia BoutryÃ, Todd A. Blackledge

Department of Biology and Integrated Bioscience Program, University of Akron, Akron, OH 44325-3908, USA

Received 15 December 2008; received in revised form 24 February 2009; accepted 2 March 2009

Abstract

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. r 2009 Elsevier GmbH. All rights reserved.

Keywords: Achaearanea tepidariorum; Spider silk; Spider webs; Sticky gumfooted threads; Supporting threads

Introduction ‘‘model’’ biomaterial whose properties scientists seek to replicate in synthetic ﬁbers. Spider silk is an outstanding material. It can be Spiders can spin silks from up to seven different types almost as strong as steel but also as much as 30 times of glands and each type of silk possesses unique material more extensible (Gosline et al., 1986). The combination properties (Blackledge and Hayashi, 2006). However, of strength and extensibility allows spider silk to absorb silk spun from the same gland can also exhibit extreme more kinetic energy without breaking than most known variability, even at the intra-individual level (Madsen materials (Gosline et al., 1999). While spiders depend et al., 1999). Both the potential adaptive value and the upon these properties for survival, they also make silk a mechanisms causing this variation are unclear (but see Tso et al., 2007; Boutry and Blackledge, 2008). ÃCorresponding author. Physiological mechanisms by which spiders might E-mail address: [email protected] (C. Boutry). control silk variation include modulation of the forces

Please cite this article as: Boutry, C., Blackledge, T.A., Biomechanical variation of silk links spinning plasticity to spider web function. Zoology (2009), doi:10.1016/j.zool.2009.03.003 ARTICLE IN PRESS 2 C. Boutry, T.A. Blackledge / Zoology ] (]]]]) ]]]–]]] applied on the silk (Knight and Vollrath, 1999)and previous studies showing similarities in morphology changes in pH (Dicko et al., 2004), water (Tillinghast (Benjamin and Zschokke, 2002) and mechanical perfor- et al., 1984) and ion content (Knight and Vollrath, 2001) mance (Blackledge et al., 2005a, b). Thus, any differ- within the spinning duct as well as variation of the ratio ences in performance related to the structures’ divergent of the different proteins composing silk (Rising et al., functions likely result from spiders actively modifying 2005; Boutry and Blackledge, 2008). Understanding the silk while it is spun. how spiders control silk properties through spinning is We tested whether the mechanical performance, in critical for industrial production of silk. Moreover, terms of both structural and material properties, of understanding how spiders control silk properties for Achaearanea silk differed between supporting threads different uses also provides insight into how evolu- and sticky gumfooted threads. We expected supporting tionary forces have shaped silk performance. threads to break at higher loads than sticky gumfooted Spiders modify their silk when spinning under a threads, allowing them to resist the weights of both variety of different conditions. For instance, producing spiders and prey. In contrast, we predicted that sticky silk that can bear higher loads (improved mechanical gumfooted threads would absorb more energy before performance) may allow webs to sustain the spiders’ breaking, allowing them to better stop moving prey. We own weight better as they capture larger prey (Boutry also expected supporting threads to resist yielding (i.e. and Blackledge, 2008). Spiders can improve the load permanently deforming) better than sticky gumfooted bearing capacity of silk by increasing either ultimate threads because supporting threads are longer-lasting strength (an intrinsic material property) or thread and function repeatedly in webs. diameter (a structural property). For example, silk spun by spiders housed in taller cages exhibits greater tensile strength than silk spun by spiders housed in shorter cages (Pan et al., 2004). Silk spun by spiders climbing Material and methods vertical walls is thicker than silk spun by spiders walking on horizontal surfaces, again allowing the threads to Spiders sustain higher loads (Garrido et al., 2002). However, most of these studies focused on silk spun as a safety line Adult A. tepidariorum (n ¼ 26) were collected at the or as a way to mark paths, rather than on silk used to University of Akron’s Bath Field Station (Bath, OH) construct webs. Even though webs are critical for and surrounding homes (Akron, OH). Spiders were survival of many spiders, the potential for spiders to housed in 38 cm 20 cm 23 cm clear plastic cages alter the mechanical properties of silk while spinning (Kritter Keepers; Lee’s Aquarium & Pet Products, San webs remains poorly investigated. Marcos, CA, USA), with cardboard frames to support Cobwebs are three-dimensional networks of silk webs (Fig. 1). The frames consisted of two 18 cm 20 composed of several architecturally and functionally cm cardboard sheets, on top and bottom, joined by distinct regions (Eberhard et al., 2008). The common 34 cm high wooden sticks (two on back and one on house spider Achaearanea tepidariorum (Araneae: Ther- front). This design allowed the spiders to spin webs idiidae) spins a cobweb consisting of a retreat suspended between the top and bottom cardboard sheets, while within a series of tangled supporting threads from which assuring easy access to silk. Spiders were housed at a project near-vertical sticky gumfooted threads (Benja- constant temperature of 24 1C under a 15:9 light/dark min and Zschokke, 2003). The spider rests and cycle. The spiders were fed one cricket (purchased from consumes prey within the supporting threads. Therefore, Fluker’s Cricket Farm, Port Allen, LA, USA) every 2 supporting threads must be able to bear the weight of days. both the spider and prey (Boutry and Blackledge, 2008). In contrast, sticky gumfooted threads are gluey along their lower 1–2 cm and are used solely in prey capture. Silk collection Prey stick to the glue and are held by the threads, which resist the prey’s struggles (Szlep, 1965). Sticky gum- Prior to silk collection, the whole web was destroyed footed threads function only once because they detach and the spiders were given two nights to rebuild their from the substrate when contacted by prey (Argintean web. Three samples of sticky gumfooted threads and et al., 2006). In contrast, supporting threads remain in three samples of the uppermost supporting threads were the web through many days and must maintain their collected from each 2-day-old web. Silk was secured on function nearly continuously. Although some authors both ends of a 10.7 mm hole in cardboard mounts, using disagree (Moore and Tran, 1999), most evidence cyanoacrylate glue, and then cut free from the web with suggests that both supporting threads and sticky a hot soldering iron. Only the dry upper regions of gumfooted threads are spun using the same type of sticky gumfooted threads were collected because the dragline silk. Evidence for their homology comes from glue droplets alter mechanical performance of the silk

to support forces, which directly determines how threads interact with prey and spiders. We also measured the material properties of threads. The material properties describe the intrinsic quality of Supporting the silk spun by spiders, and interact with the structural Threads aspects of threads (such as diameter and number of strands) to determine how threads interact with prey. We calculated six material properties for each sample (Fig. 2): Young’s modulus, yield stress, yield strain, extensibility (breaking strain), ultimate strength (break- Sticky ing stress) and toughness (breaking energy). Young’s Gumfooted modulus measures the initial stiffness of the material. Threads Silk with a higher Young’s modulus better resists deformation under a given load. The yield point measures the transition of the material from elastic to plastic behavior. After yield, the material is irreversibly deformed and its future mechanical performance is permanently altered. Yield stress and yield strain are the Glue true stress and true strain at yield, respectively. Similarly, ultimate strength and extensibility are the drops true stress and true strain at failure, respectively. We used true stress and true strain, rather than Fig. 1. Achaearanea tepidariorum cobweb. The web is com- engineering stress and engineering strain, because posed of a network of supporting threads holding near-vertical these are more reliable for very elastic materials such sticky gumfooted threads that are coated with glue droplets on as spider silk (Blackledge et al., 2005b). True stress their lower 1–2 cm. Note: the glue drops are slightly exaggerated in the ﬁgure. (s) measures the force supported per area of thread and is calculated as

(Blackledge et al., 2005a). Supporting threads were F collected from the top 1–5 cm of the web. s ¼ , A

Diameter measurements where F is the force exerted on the material, and A the instantaneous cross-sectional area of the silk ﬁber at Polarized light microscopy was used to take two digital photographs of each sample at 1000 . The 1400 Failure point diameter of each strand in a sample was measured three 1200 times from each photograph using ImageJ (Rasband, Supporting Thread 1997–2007). The multiple measurements accounted for 1000 the slight shape anisotropy of silk so that cross-sectional area could be approximated by a single average 800 diameter (Blackledge et al., 2005c). From these mea- 600 Gumfooted Thread

surements, both the total cross-sectional area of the silk Stress (MPa) sample and the average diameter of single strands within 400 Yield point the sample were calculated. 200 Toughness Mechanical and material properties of the silk 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 The mechanical performance of silk threads was then Strain (mm/mm) tested on a Nano Bionix UTM (MTS Systems Corp., Fig. 2. Stress–strain curves for sticky gumfooted thread silk Eden Prairie, MN, USA) using established protocols and supporting thread silk. The yield point is the first inflexion (Blackledge et al., 2005b). All fibers were extended at a 1 in the slope of the curve. The failure point appears as a sudden strain rate of 0.01 s and the resulting force values drop to zero stress. The area under the curve (hatched for the measured. We recorded raw failure load, which repre- sticky gumfooted thread curve) measures toughness, i.e. the sents the force resisted by the thread upon breaking energy the silk can absorb before failure. The samples without correction for thread diameter. Failure load displayed present extreme behavior (i.e. very strong supporting thus gave us information about the capacity of a thread thread silk and very elastic sticky gumfooted thread silk).

Statistics 1

0 Mean single-strand diameter was compared between Supporting Gumfooted supporting threads and sticky gumfooted threads Threads Threads collected from the same web using a paired t-test (the 7 data were normally distributed, Shapiro-Wilk test, Fig. 3. Mean ( SE) failure load for silk forming supporting threads and sticky gumfooted threads in Achaearanea cob- P ¼ 0.3102 for supporting threads, P ¼ 0.4727 for webs. Asterisk indicates a significant difference at a ¼ 5%. sticky gumfooted threads). Failure load was not normally distributed for sticky gumfooted threads than supporting threads (Fig. 5). In contrast, ultimate because of a single outlier. After this outlier was strength, toughness and yield strain did not differ removed, the data were normal (Shapiro-Wilk test, significantly between supporting threads and sticky P ¼ 0.1529 for supporting threads and P ¼ 0.8867 for gumfooted threads (Fig. 5). We observed non-major sticky gumfooted threads). Therefore, failure load was ampullate silk, likely minor ampullate, in a few samples compared between the two regions of the web using a (7 out of 140), but the inclusion/exclusion of these paired t-test, which controlled for the strong effect that samples in the statistical analysis did not affect our spider size has on thread diameter (Osaki, 2003). results. Material properties were compared among supporting threads and sticky gumfooted threads using a MANO- VA and post-hoc Tukey’s HSD tests were used to identify specific differences. 2.5

∗ Results 2.0 μ m) Supporting threads broke at higher loads than sticky gumfooted threads (failure load, mean7SE: 8.2570.89 1.5 and 3.8170.21 m N, respectively, t ¼5.66, df ¼ 24, Po0.0001) (Fig. 3). Supporting threads were also thicker than sticky gumfooted threads (single-strand 1.0 diameter: 1.88570.091 and 1.57170.059 mm, respectively, t ¼ 4.26, df ¼ 25, P ¼ 0.0003) (Fig. 4). Material properties differed between supporting Single-strand Diameter ( 0.5 threads and sticky gumfooted threads (MANOVA, Wilk’s l ¼ 0.0053, n ¼ 24 and 22, respectively, P ¼ 0.0025). Post-hoc Tukey’s HSD tests showed that 0.0 sticky gumfooted threads were more elastic (extensi- Supporting Gumfooted bility: 0.435970.0152 and 0.349870.0106 mm/mm, Threads Threads respectively, Po0.0001), less stiff (Young’s modulus: Fig. 4. Mean (7SE) single-strand diameter of silk forming 9.8670.37 and 11.4170.57 GPa, respectively, supporting threads and sticky gumfooted threads in Achaear- P ¼ 0.0239) and yielded more easily (yield stress: anea cobwebs. Asterisk indicates a signiﬁcant difference at 22679 and 280713 MPa, respectively, P ¼ 0.0013) a ¼ 5%.

350 0.04 ∗ 300

250 0.03

200 0.02 150

Yield Stress (MPa) 100 0.01 Yield Strain (mm/mm) 50

0 0 Supporting Gumfooted Supporting Gumfooted Threads Threads Threads Threads

0.5 1800 ∗

1500 0.4

1200 0.3

900 0.2 600

Extensibility (mm/mm) 0.1 Ultimate Strength (MPa) 300

0 0 Supporting Gumfooted Supporting Gumfooted Threads Threads Threads Threads

14 350 ∗ 12 300

10 250

8 200

6 150

4 Toughness (MPa) 100 Young’s Modulus (GPa) 2 50

0 0 Supporting Gumfooted Supporting Gumfooted Threads Threads Threads Threads

Fig. 5. Mean (7SE) material properties of silk forming the supporting threads and sticky gumfooted threads in Achaearanea cobwebs. Asterisks indicate signiﬁcant differences at a ¼ 5%.

Discussion We predicted that variation in silk performance should relate to the different functions of each region We found that the mechanical performance of silk of the cobweb. For instance, supporting threads have to varied in different regions of cobwebs spun by the spider resist the static weight of both spiders and subdued prey, A. tepidariorum. Evidence suggested that the whole web and should therefore be able to bear higher loads while is made of dragline silk, in which case the spiders could sticky gumfooted threads function primarily to absorb actively modulate the properties of a single type of silk kinetic energy of prey during capture, suggesting that while spinning different regions of webs. they should be tougher. We found that supporting

Please cite this article as: Boutry, C., Blackledge, T.A., Biomechanical variation of silk links spinning plasticity to spider web function. Zoology (2009), doi:10.1016/j.zool.2009.03.003 ARTICLE IN PRESS 6 C. Boutry, T.A. Blackledge / Zoology ] (]]]]) ]]]–]]] threads from Achaearanea cobwebs were stiffer and changes, such as spinning thicker threads, are likely broke under higher loads than sticky gumfooted more costly to spiders because they require more silk threads, but we did not find a difference in silk proteins to spin equivalent lengths of threads. On the toughness between the two regions of cobwebs. Thus, other hand, altering material properties could poten- spiders can control at least some of the properties of tially adjust the silk’s performance without increasing their silk in ways that fit web function. production costs. Thus, it is surprising that spiders favor Table 1 compares the mechanical performance of adjusting structural, rather than material, properties of Achaearanea silk to cobweb silk of another theridiid, the silk. However, this pattern appears to be common, both western black widow Latrodectus hesperus, showing at the intra-specific level (Osaki, 1996, 1999, 2003; remarkable similarities. Moore and Tran (1999) report Boutry and Blackledge, 2008) and at the inter-specific values for Young’s modulus, extensibility and toughness level (Craig, 1987a, b). Denny (1976) suggested that orb about half that of our own study or Blackledge et al.’s webs use the minimum possible amount of silk necessary (2005a, b). This may in part be attributed to differences to function. This predicts that, in active web-spinners, in methods. Moore and Tran assumed constant there has been strong selection historically to maximize diameter instead of constant volume (as we did), which the material properties of silk. Quite simply, there may results in underestimated stress values for stretchy be few physiological mechanisms left by which spiders materials. can further increase silk tensile strength, thereby Sticky gumfooted threads and supporting threads explaining why structural properties are commonly so exhibit different material properties, but we argue that much more variable. both still consist of major ampullate silk because these Sticky gumfooted threads are only used once for prey differences are subtle compared with the divergence in capture while supporting threads must remain func- material properties of silks spun from different glands. tional through several capture events. Therefore, in For instance, flagelliform silk extends to 1.50 times its contrast to sticky gumfooted threads, supporting original length (Swanson et al., 2007) while tubuliform threads must support the weight of both spider and silk’s strength is only 0.5 GPa (Blackledge and Hayashi, prey without deforming permanently. This may explain 2006), compared with cobweb silk’s extensibility of 0.4 why the stiffness and yield stress are higher for and strength of 1.5 GPa. Minor ampullate silk is closest supporting threads than for sticky gumfooted threads. in performance to major ampullate silk (Blackledge and Measurements on western black widow cobwebs show Hayashi, 2006). However, minor ampullate threads are that supporting threads do not pass their yield point 50% thinner, such that silk threads from these two during prey capture, contrary to sticky gumfooted different glands are easily distinguished (pers. obs. on threads (Argintean et al., 2006). L. hesperus). Thus, the differences we found in mechan- As predicted, sticky gumfooted threads are less stiff ical performance of sticky gumfooted threads and and more elastic than supporting threads, but not supporting threads likely resulted from effects of how tougher. This pattern is also found in another theridiid, spiders assembled the silk threads rather than variation the western black widow (Blackledge et al., 2005b), in the chemical composition of the silk. suggesting that it may result from selective forces or The higher failure load of supporting threads was not constraints. In particular, toughness directly depends on achieved through increased ultimate strength, an in- extensibility. Yet, more extensible sticky gumfooted trinsic material property of the silk, but rather through threads are not tougher, a quality that we predicted to structural change in thread diameter. Structural be important for stopping moving prey. Is there any

Table 1. Comparison of silk material properties from different regions of the cobwebs of Achaearanea tepidariorum and the western black widow Latrodectus hesperus.

Silk Young’s modulus Ultimate strength Extensibility Toughness Source (GPa) (MPa) (mm/mm) (MPa)

Sticky gumfooted threads A. tepidariorum 9.9 1569 0.44 271 Current study L. hesperus 8.9 1316 0.39 231 Blackledge et al. (2005a) L. hesperus 9.2 957 0.47 236 Blackledge et al. (2005b) Supporting threads A. tepidariorum 11.4 1638 0.35 285 Current study L. hesperus 6 1100 0.22 136 Moore and Tran (1999) L. hesperus 10.7 1069 0.42 238 Blackledge et al. (2005b)

All of these threads are produced from the same type of silk gland.

Rg sinð180 as2Þ Rs1 ¼ Rg= sinðas1 þ as2 180Þ 10 mN and αg ground Rg sinð180 as1Þ F= 5 mN Rs2 ¼ sinðas1 þ as2 180Þ Supporting threads

Fig. 6. (A) Model of force transmission in the cobweb. F (in Rs1=37 mN red) is the force exerted by the prey pulling on the sticky Rs2=34 mN gumfooted thread. This force creates a reaction force which is projected on the sticky gumfooted thread (Rg, in blue). The resulting tension generates additional reaction forces that are Sticky gumfooted thread projected on the supporting threads (Rs1 and Rs2, in green). The values of these reaction forces depend on the angles between the threads, ag, as1 and as2. More compliant sticky gumfooted threads increase ag and ultimately result in less force acting on the supporting threads. (B, C) Example of Rg= forces transmitted in the web for different compliances of silk. In both cases, the prey applies the same force F of 5 m N, 30 mN which displaces the sticky gumfooted thread from its original position (dashed grey line). In (B), the silk is very compliant αg and the sticky gumfooted thread displaces more, so that the ground angle ag ¼ 1201, while in (C), the silk is less compliant and F= 5 mN ag ¼ 1001.

force from prey then the total force supported by the sticky gumfooted thread decreases both because b is reduced and because as1 tends to 1801. More compliant silk allows as1 to approach 1801 and b to decrease under lower forces, thereby reducing the total reaction force in the supporting threads. To conclude, the more the sticky gumfooted thread silk extends under a given force generated by prey, the lower the tensions in the sticky gumfooted thread and supporting threads. Thus, F compliant sticky gumfooted thread silk may allow the cobweb to resist higher forces exerted by stronger, Force in gumfooted threads (Rg) 0 heavier prey. αg (°) 180 Our model can be applied to compare cobwebs with diverse architecture, spun by different species. For Rs1 Rs2 example, longer sticky gumfooted threads allow the total Rs angle ag of our model to increase when prey stuck to the thread crawls the same distance, thereby reducing the forces supported by the web (Rg and Rs) and Rg allowing the web to support heavier prey. Sticky gumfooted threads’ length interacts with silk compliance and angles between supporting threads (b in our model) to determine the ability of webs to bear large, heavy prey. This combination of factors may help explain the

Force in supporting threads large differences in cobweb architecture among species 0 (Eberhard et al., 2008). αs1 (°) 180 Insects typically escape quickly from webs, before they are attacked by spiders (Rypstra, 1982). In orb Fig. 7. Effect of angles on the forces experienced by the sticky webs, even a small increase in the number of sticky gumfooted threads and supporting threads: (A) Change in the threads contacted by prey can make escape signiﬁcantly reaction force in the sticky gumfooted thread, R , as a function g less likely (Blackledge and Zevenbergen, 2006). The high of the angle ag. The force F exerted by the prey is indicated by a thin horizontal line. (B) Change in the reaction forces in the extensibility of sticky gumfooted threads may facilitate prey movement within webs, allowing them to contact supporting threads, Rs1, Rs2 and total Rs (Rs1+Rs2), as a multiple sticky gumfooted threads, causing the prey to function of the angle as1. The tension in the sticky gumfooted thread, Rg, is indicated by a thin horizontal line. The reaction become even more entrapped. However, other factors, force in the sticky gumfooted thread (Rg) and the total reaction such as increased sticky gumfooted thread density or force in the supporting threads (total Rs) decrease as the angles length, may also facilitate prey retention. Thus, spiders ag and as1 increase, i.e. as the sticky gumfooted thread aligns may be able to manipulate the retention time of prey in with the direction of the prey, and as one of the supporting their webs by altering these architectural properties, threads aligns with the sticky gumfooted thread. much as orb web spiders can by altering the spacing between sticky spirals. (Detailed calculations of these equations are given in We argue that spiders may have limited mechanisms Appendix A which is available as an electronic supple- for improving silk tensile strength, due in part to ment at doi:10.1016/j.zool.2009.03.003.) potentially strong selective pressures on this property Fig. 7B shows how Rs1 and Rs2 vary as a function of in the past. However, other material properties (Young’s as1. We assume that the angle b between the two modulus, extensibility and yield stress) are variable at supporting threads remains constant, and therefore, as2 the intra-individual level. While it seems likely that high is negatively correlated with as1.Asas1 increases, Rs1 tensile strength is always desirable for silk performance, increases while Rs2 decreases. The sum of Rs1 and Rs2 these other properties may in fact experience selection (total Rs) reaches its minimum when as1 ¼ 1801, Rs1 ¼ F for both ‘‘increased’’ and ‘‘decreased’’ performance, and Rs2 ¼ 0. This occurs when the sticky gumfooted depending on the function of the silk. For instance, we thread is aligned with one of the supporting threads, previously showed how compliant sticky gumfooted which is facilitated by reduced silk stiffness. In reality threads may allow the web to bear higher forces from though, b does not remain constant. Instead, the angle prey. Yet, high compliance in supporting threads may decreases as the prey pulls on the sticky gumfooted result in a web that deforms easily. thread. Total Rs decreases as b decreases. Thus, if a We found strong support for intra-individual varia- sticky gumfooted thread stretches further under a given tion in silk properties of 2-day-old webs, while other

Please cite this article as: Boutry, C., Blackledge, T.A., Biomechanical variation of silk links spinning plasticity to spider web function. Zoology (2009), doi:10.1016/j.zool.2009.03.003 ARTICLE IN PRESS C. Boutry, T.A. Blackledge / Zoology ] (]]]]) ]]]–]]] 9 studies documented intra-individual variation in the thank Chiara Benvenuto, Ingi Agnarsson and an dragline silk of spiders spun across several days anonymous reviewer for comments on the manuscript. (Madsen et al., 1999). The fact that spiders could vary the material properties of their silk in less than 48 h, and probably within single bouts of web spinning, argues persuasively that the intra-individual variation we found Appendix A. Supplementary data is not due to changes in internal body condition or amino acid intake, as suggested in previous studies (see Supplementary data associated with this article can be Tso et al. (2007) for a discussion of how prey intake found in the online version at doi:10.1016/j.zool. relates to changes in silk amino acid composition and 2009.03.003. material properties). Instead, Achaearanea seems to control silk material properties during web spinning using nearly instantaneous physiological mechanisms (detailed in Vollrath and Knight, 2001; Ortlepp and Gosline, 2004; Perez-Rigueiro et al., 2005). Learning how spiders control silk properties could allow material References scientists and polymer scientists to replicate those Argintean, S., Chen, J., Kim, M., Moore, A.M.F., 2006. mechanisms to fine-tune silk production, broadening Resilient silk captures prey in black widow cobwebs. Appl. the range of potential applications for synthetic silk. Phys. A 82, 235–241. Moreover, it is clearly essential for understanding the Benjamin, S.P., Zschokke, S., 2002. 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Please cite this article as: Boutry, C., Blackledge, T.A., Biomechanical variation of silk links spinning plasticity to spider web function. Zoology (2009), doi:10.1016/j.zool.2009.03.003