Anchor threads can double the insect flight energy absorbed by spider orb webs.

To successfully capture flying insect prey, a spider's orb web must withstand the energy of impact without the silk breaking. In this study, we examined the anchor threads: the silk lines that anchor the main capture area of the web to the surrounding environment. These anchor threads can account for a large portion of the web, yet are usually excluded from experiments and simulations. We compared projectile capture and kinetic energy absorption between webs with and without access to anchor threads. Webs with anchor threads captured significantly more projectiles and absorbed significantly more energy than those with constrained anchors. This is likely because the anchor threads increase web compliance, resulting in webs with the ability to catch high-energy flying insects without breaking. Anchor threads are one example of how different types of web architecture expand the range of possible prey capture strategies by enabling the web to withstand greater impacts.

Slingshot spiders build tensed, underdamped webs for ultrafast launches and speedy halts.

We develop a mathematical model to capture the web dynamics of slingshot spiders (Araneae: Theridiosomatidae), which utilize a tension line to deform their orb webs into conical springs to hunt flying insects. Slingshot spiders are characterized by their ultrafast launch speeds and accelerations (exceeding 1300 [Formula: see text]), however a theoretical approach to characterize the underlying spatiotemporal web dynamics remains missing. To address this knowledge gap, we develop a 2D-coupled damped oscillator model of the web. Our model reveals three key insights into the dynamics of slingshot motion. First, the tension line plays a dual role: enabling the spider to load elastic energy into the web for a quick launch (in milliseconds) to displacements of 10-15 body lengths, but also enabling the spider to halt quickly, attenuating inertial oscillations. Second, the dominant energy dissipation mechanism is viscous drag by the silk lines - acting as a low Reynolds number parachute. Third, the web exhibits underdamped oscillatory dynamics through a finely-tuned balance between the radial line forces, the tension line force and viscous drag dissipation. Together, our work suggests that the conical geometry and tension-line enables the slingshot web to act as both an elastic spring and a shock absorber, for the multi-functional roles of risky predation and self-preservation.

Permanent deformation of triangle weaver silk enables ultrafast tangle-free release of spider webs.

Entanglements are common in both natural and artificial systems and can result in both beneficial and harmful effects. Most spider webs are static structures held under constant tension and do not tangle. However, many spiders actively load tension into their webs by coiling silk threads that are released to "fire" webs at prey. Here we test whether or not tangling occurs during the rapid release of webs built by the triangle spider Hyptiotes cavatus. We use high-speed videography to examine the release of the spider's web, looking for signs of tangling both visually on the videos and on acceleration graphs. The spider tenses the web by pulling on a silken anchor line using a leg-over-leg movement, deforming the silk into permanent coils and storing excess slack in a loose bundle between the spider's legs. This 1-3cm long bundle of coils straightens during the web's release in as few as 4ms. Though the messy silk coils are pressed closely together, the web's release is never impeded by catastrophic tangling. This lack of serious tangling is perhaps due to the permanent coils preventing random movement of the silk. The coils also compact the loose silk, preventing interference with the spider's movement. The ability to coil its anchor line allows H. cavatus to permanently restructure its silk, facilitating its active web-hunting behavior. Our findings broaden our knowledge of silk manipulation by spiders and may give insights into creating tangle-free systems through structural changes.

Size-related increase in inducible mechanical variability of major ampullate silk in a huntsman spider (Araneae: Sparassidae).

Most spiders use major ampullate silk (MAS) to perform many functions across their lifetimes, including prey capture, vibratory signal detection, and safety/dragline. To accommodate their various needs, adult spiders can use inducible variability to tailor MAS with specific mechanical properties. However, it is currently unknown whether this inducible mechanical variability develops gradually or remains consistent across spider size. Supercontraction -a process by which "native-state" silk fibers axially shrink when exposed to water or high humidity-can be used to reveal the extent of inducible variability in MAS. Supercontraction removes some processing effects that occur during the spinning of the solid fiber from its liquid precursor by allowing a native-state MAS fiber to return to a low energy "ground-state". Here, we examined the relative extent of inducible variability of MAS across spider size by assessing supercontraction properties and the difference between ground- and native-state MAS tensile properties using silk from the huntsman spider Heteropoda venatoria (Sparassidae). Stiffness of forcibly pulled native-state silk increased by 200% with spider size. After exposure to 90% RH and subsequent supercontraction, axial shrinkage of native-state silk fibers increased by 15% in larger spiders. Supercontracted, ground-state fibers demonstrated a 200% increase in extensibility across spider size. Our results indicate a gradual increase in inducible variability of MAS mechanical properties across spider size potentially caused by shifts in internal processing or chemical composition. These findings imply both development of inducible variability and changes in use of MAS as a safety line or aiding jumps across a spider's lifetime.

Uncoiling springs promote mechanical functionality of spider cribellate silk.

Composites, both natural and synthetic, achieve novel functionality by combining two or more constituent materials. For example, the earliest adhesive silk in spider webs - cribellate silk - is composed of stiff axial fibers and coiled fibers surrounded by hundreds of sticky cribellate nanofibrils. Yet, little is known of how fiber types interact to enable capture of insect prey with cribellate silk. To understand the roles of each constituent fiber during prey capture, we compared the tensile performance of native-state and manipulated threads produced by the cribellate spider Psechrus clavis, and the adhesion of native threads along a smooth surface and hairy bee thorax. We found that the coiled fiber increases the work to fracture of the entire cribellate thread by up to 20-fold. We also found that the axial fiber breaks multiple times during deformation, an unexpected observation that indicates: (i) the axial fiber continues to contribute work even after breakage, and (ii) the cribellate nanofibrils may perform a previously unidentified role as a binder material that distributes forces throughout the thread. Work of adhesion increased on surfaces with more surface structures (hairy bee thorax) corresponding to increased deformation of the coiled fiber. Together, our observations highlight how the synergistic interactions among the constituents of this natural composite adhesive enhance functionality. These highly extensible threads may serve to expose additional cribellate nanofibrils to form attachment points with prey substrata while also immobilizing prey as they sink into the web due to gravity.

Phylogeny of the orb-weaving spider family Araneidae (Araneae: Araneoidea).

We present a new phylogeny of the spider family Araneidae based on five genes (28S, 18S, COI, H3 and 16S) for 158 taxa, identified and mainly sequenced by us. This includes 25 outgroups and 133 araneid ingroups representing the subfamilies Zygiellinae Simon, 1929, Nephilinae Simon, 1894, and the typical araneids, here informally named the "ARA Clade". The araneid genera analysed here include roughly 90% of all currently named araneid species. The ARA Clade is the primary focus of this analysis. In taxonomic terms, outgroups comprise 22 genera and 11 families, and the ingroup comprises three Zygiellinae and four Nephilinae genera, and 85 ARA Clade genera (ten new). Within the ARA Clade, we recognize ten informal groups that contain at least three genera each and are supported under Bayesian posterior probabilities (≥ 0.95): "Caerostrines" (Caerostris, Gnolus and Testudinaria), "Micrathenines" (Acacesia, Micrathena, Ocrepeira, Scoloderus and Verrucosa), "Eriophorines" (Acanthepeira, Alpaida, Eriophora, Parawixia and Wagneriana), "Backobourkiines" (Acroaspis, Backobourkia, Carepalxis, Novakiella, Parawixia, Plebs, Singa and three new genera), "Argiopines" (Arachnura, Acusilas, Argiope, Cyrtophora, Gea, Lariniaria and Mecynogea), "Cyrtarachnines" (Aranoethra, Cyrtarachne, Paraplectana, Pasilobus and Poecilopachys), "Mastophorines" (Celaenia, Exechocentrus and Mastophora,), "Nuctenines" (Larinia, Larinioides and Nuctenea), "Zealaraneines" (Colaranea, Cryptaranea, Paralarinia, Zealaranea and two new genera) and "Gasteracanthines" (Augusta, Acrosomoides, Austracantha, Gasteracantha, Isoxya, Macracantha, Madacantha, Parmatergus and Thelacantha). Few of these groups are currently corroborated by morphology, behaviour, natural history or biogeography. We also include the large genus Araneus, along with Aculepeira, Agalenatea, Anepsion, Araniella, Cercidia, Chorizopes, Cyclosa, Dolophones, Eriovixia, Eustala, Gibbaranea, Hingstepeira, Hypognatha, Kaira, Larinia, Mangora, Metazygia, Metepeira, Neoscona, Paraplectanoides, Perilla, Poltys, Pycnacantha, Spilasma and Telaprocera, but the placement of these genera was generally ambiguous, except for Paraplectanoides, which is strongly supported as sister to traditional Nephilinae. Araneus, Argiope, Eriophora and Larinia are polyphyletic, Araneus implying nine new taxa of genus rank, and Eriophora and Larinia two each. In Araneus and Eriophora, polyphyly was usually due to north temperate generic concepts being used as dumping grounds for species from southern hemisphere regions, e.g. South-East Asia, Australia or New Zealand. Although Araneidae is one of the better studied spider families, too little natural history and/or morphological data are available across these terminals to draw any strong evolutionary conclusions. However, the classical orb web is reconstructed as plesiomorphic for Araneidae, with a single loss in "cyrtarachnines"-"mastophorines". Web decorations (collectively known as stabilimenta) evolved perhaps five times. Sexual dimorphism generally results from female body size increase with few exceptions; dimorphic taxa are not monophyletic and revert to monomorphism in a few cases.

Tuning orb spider glycoprotein glue performance to habitat humidity.

Orb-weaving spiders use adhesive threads to delay the escape of insects from their webs until the spiders can locate and subdue the insects. These viscous threads are spun as paired flagelliform axial fibers coated by a cylinder of solution derived from the aggregate glands. As low molecular mass compounds (LMMCs) in the aggregate solution attract atmospheric moisture, the enlarging cylinder becomes unstable and divides into droplets. Within each droplet an adhesive glycoprotein core condenses. The plasticity and axial line extensibility of the glycoproteins are maintained by hygroscopic LMMCs. These compounds cause droplet volume to track changes in humidity and glycoprotein viscosity to vary approximately 1000-fold over the course of a day. Natural selection has tuned the performance of glycoprotein cores to the humidity of a species' foraging environment by altering the composition of its LMMCs. Thus, species from low-humidity habits have more hygroscopic threads than those from humid forests. However, at their respective foraging humidities, these species' glycoproteins have remarkably similar viscosities, ensuring optimal droplet adhesion by balancing glycoprotein adhesion and cohesion. Optimal viscosity is also essential for integrating the adhesion force of multiple droplets. As force is transferred to a thread's support line, extending droplets draw it into a parabolic configuration, implementing a suspension bridge mechanism that sums the adhesive force generated over the thread span. Thus, viscous capture threads extend an orb spider's phenotype as a highly integrated complex of large proteins and small molecules that function as a self-assembling, highly tuned, environmentally responsive, adhesive biomaterial. Understanding the synergistic role of chemistry and design in spider adhesives, particularly the ability to stick in wet conditions, provides insight in designing synthetic adhesives for biomedical applications.

Role of Hygroscopic Low Molecular Mass Compounds in Humidity Responsive Adhesion of Spider's Capture Silk.

The aggregate glue in spider webs is composed of hygroscopic low molecular mass compounds (LMMCs), glycoproteins and water. The LMMCs absorb atmospheric water and solvate the glycoproteins to spread and adhere to flying insects upon contact. The glue viscosity varies with humidity and there is an optimum range of viscosity where the adhesion is maximum. LMMCs composition and the humidity at which glue viscosity is optimized vary greatly among spider species. These findings suggest that spiders adapt to forage in diverse habitats by "tuning" LMMCs composition or how LMMCs interact with glycoproteins to control water uptake and adhesion. To test these hypotheses, we analyzed the LMMCs for spiders from diverse habitats and performed water uptake studies on intact glue droplets, isolated glue constituents, and synthetic LMMCs. Even though glue droplets showed differences in water uptake among spider species, we found no differences among species in hygroscopicity of natural or synthetic LMMCs mixtures. This demonstrates that LMMCs composition alone is insufficient to explain interspecific differences in water uptake of spider glues and instead support the hypothesis that an interaction between LMMCs and glycoproteins mediate differences in water uptake and adhesion.

Physicochemical Property Variation in Spider Silk: Ecology, Evolution, and Synthetic Production.

The unique combination of great stiffness, strength, and extensibility makes spider major ampullate (MA) silk desirable for various biomimetic and synthetic applications. Intensive research on the genetics, biochemistry, and biomechanics of this material has facilitated a thorough understanding of its properties at various levels. Nevertheless, methods such as cloning, recombination, and electrospinning have not successfully produced materials with properties as impressive as those of spider silk. It is nevertheless becoming clear that silk properties are a consequence of whole-organism interactions with the environment in addition to genetic expression, gland biochemistry, and spinning processes. Here we assimilate the research done and assess the techniques used to determine distinct forms of spider silk chemical and physical property variability. We suggest that more research should focus on testing hypotheses that explain spider silk property variations in ecological and evolutionary contexts.

Spiders have rich pigmentary and structural colour palettes.

Elucidating the mechanisms of colour production in organisms is important for understanding how selection acts upon a variety of behaviours. Spiders provide many spectacular examples of colours used in courtship, predation, defence and thermoregulation, but are thought to lack many types of pigments common in other animals. Ommochromes, bilins and eumelanin have been identified in spiders, but not carotenoids or melanosomes. Here, we combined optical microscopy, refractive index matching, confocal Raman microspectroscopy and electron microscopy to investigate the basis of several types of colourful patches in spiders. We obtained four major results. First, we show that spiders use carotenoids to produce yellow, suggesting that such colours may be used for condition-dependent courtship signalling. Second, we established the Raman signature spectrum for ommochromes, facilitating the identification of ommochromes in a variety of organisms in the future. Third, we describe a potential new pigmentary-structural colour interaction that is unusual because of the use of long wavelength structural colour in combination with a slightly shorter wavelength pigment in the production of red. Finally, we present the first evidence for the presence of melanosomes in arthropods, using both scanning and transmission electron microscopy, overturning the assumption that melanosomes are a synapomorphy of vertebrates. Our research shows that spiders have a much richer colour production palette than previously thought, and this has implications for colour diversification and function in spiders and other arthropods.

Punctuated evolution of viscid silk in spider orb webs supported by mechanical behavior of wet cribellate silk.

The origin of viscid capture silk in orb webs, from cribellate silk-spinning ancestors, is a key innovation correlated with significant diversification of web-building spiders. Ancestral cribellate silk consists of dry nanofibrils surrounding a stiff, axial fiber that adheres to prey through van der Waals interactions, capillary forces, and physical entanglement. In contrast, viscid silk uses chemically adhesive aqueous glue coated onto a highly compliant and extensible flagelliform core silk. The extensibility of the flagelliform fiber accounts for half of the total work of adhesion for viscid silk and is enabled by water in the aqueous coating. Recent cDNA libraries revealed the expression of flagelliform silk proteins in cribellate orb-weaving spiders. We hypothesized that the presence of flagelliform proteins in cribellate silk could have allowed for a gradual shift in mechanical performance of cribellate axial silk, whose effect was masked by the dry nature of its adhesive. We measured supercontraction and mechanical performance of cribellate axial silk, in wet and dry states, for two species of cribellate orb web-weaving spiders to see if water enabled flagelliform silk-like performance. We found that compliance and extensibility of wet cribellate silk increased compared to dry state as expected. However, when compared to other silk types, the response to water was more similar to other web silks, like major and minor ampullate silk, than to viscid silk. These findings support the punctuated evolution of viscid silk mechanical performance.

Spiders do have melanin after all.

Melanin pigments are broadly distributed in nature - from bacteria to fungi to plants and animals. However, many previous attempts to identify melanins in spiders were unsuccessful, suggesting that these otherwise ubiquitous pigments were lost during spider evolution. Yet, spiders exhibit many dark colours similar to those produced by melanins in other organisms, and the low solubility of melanins makes isolation and characterization difficult. Therefore, whether melanins are truly absent or have simply not yet been detected is an open question. Raman spectroscopy provides a reliable way to detect melanins in situ, without the need for isolation. In this study, we document the presence of eumelanin in diverse species of spiders using confocal Raman microspectroscopy. Comparisons of spectra with theoretically calculated data falsify the previous hypothesis that dark colours are produced solely by ommochromes in spiders. Our data indicate that melanins are present in spiders and further supporting that they are present in most living organisms.

Composition and Function of Spider Glues Maintained During the Evolution of Cobwebs.

Capture silks are an interesting class of biological glues that help spiders subdue their prey. Viscid capture silk produced by the orb web spiders is a combination of hygroscopic salts that aid in water uptake and interact with adhesive glycoproteins to make them soft and sticky. The orb was a stepping stone to the evolution of new web types, but little is known about the adhesives in these webs. For instance, cobweb spiders evolved from orb-weaving ancestors and utilize glue in specialized sticky gumfoot threads rather than an elastic spiral. Early investigation suggests that gumfoot adhesives are quite different viscid glues because they lack a visible glycoprotein core, act as viscoelastic fluids rather than solids, and are largely invariant to humidity. Here, we use spectroscopic and staining methods to show that the gumfoot silk produced by Latrodectus hesperus (western black widow) is composed of hygroscopic organic salts and water insoluble glycoproteins, similar to viscid silk, in addition to a low concentration of spider coating peptides reported before. Our adhesion studies reveal that the organic salts play an important role in adhesion, similar to that seen in orb web spiders, but modulating function at much lower humidity. Our work shows more similarities in the viscid silk produced by orb web and cobweb spiders than previously anticipated and provide guidelines for developing synthetic adhesives that can work in dry to humid environments.

Mechanical performance of spider silk is robust to nutrient-mediated changes in protein composition.

Spider major ampullate (MA) silk is sought after as a biomimetic because of its high strength and extensibility. While the secondary structures of MA silk proteins (spidroins) influences silk mechanics, structural variations induced by spinning processes have additional effects. Silk properties may be induced by spiders feeding on diets that vary in certain nutrients, thus providing researchers an opportunity to assess the interplay between spidroin chemistry and spinning processes on the performance of MA silk. Here, we determined the relative influence of spidroin expression and spinning processes on MA silk mechanics when Nephila pilipes were fed solutions with or without protein. We found that spidroin expression differed across treatments but that its influence on mechanics was minimal. Mechanical tests of supercontracted fibers and X-ray diffraction analyses revealed that increased alignment in the amorphous region and to a lesser extent in the crystalline region led to increased fiber strength and extensibility in spiders on protein rich diets.

Protein composition correlates with the mechanical properties of spider ( Argiope trifasciata ) dragline silk.

We investigated the natural variation in silk composition and mechanical performance of the orb-weaving spider Argiope trifasciata at multiple spatial and temporal scales in order to assess how protein composition contributes to the remarkable material properties of spider dragline silk. Major ampullate silk in orb-weaving spiders consists predominantly of two proteins (MaSp1 and MaSp2) with divergent amino acid compositions and functionally different microstructures. Adjusting the expression of these two proteins therefore provides spiders with a simple mechanism to alter the material properties of their silk. We first assessed the reliability and precision of the Waters AccQ-Tag amino acid composition analysis kit for determining the amino acid composition of small quantities of spider silk. We then tested how protein composition varied within single draglines, across draglines spun by the same spider on different days, and finally between spiders. Then, we correlated chemical composition with the material properties of dragline silk. Overall, we found that the chemical composition of major ampullate silk was in general homogeneous among individuals of the same population. Variation in chemical composition was not detectable within silk spun by a single spider on a single day. However, we found that variation within a single spider's silk across different days could, in rare instances, be greater than variation among individual spiders. Most of the variation in silk composition in our investigation resulted from a small number of outliers (three out of sixteen individuals) with a recent history of stress, suggesting stress affects silk production process in orb web spiders. Based on reported sequences for MaSp genes, we developed a gene expression model showing the covariation of the most abundant amino acids in major ampullate silk. Our gene expression model supports that dragline silk composition was mostly determined by the relative abundance of MaSp1 and MaSp2. Finally, we showed that silk composition (especially proline content) strongly correlated with some measures of mechanical performance, particularly how much fibers shrunk during supercontraction as well as their breaking strains. Our findings suggest that spiders are able to change the relative expression rates of different MaSp genes to produce silk fibers with different chemical compositions, and hence, different material properties.

Direct solvation of glycoproteins by salts in spider silk glues enhances adhesion and helps to explain the evolution of modern spider orb webs.

The evolutionary origin of modern viscid silk orb webs from ancient cribellate silk ancestors is associated with a 95% increase in diversity of orb-weaving spiders, and their dominance as predators of flying insects, yet the transition's mechanistic basis is an evolutionary puzzle. Ancient cribellate silk is a dry adhesive that functions through van der Waals interactions. Viscid threads adhere more effectively than cribellate threads because of the high extensibility of their axial silk fibers, recruitment of multiple glue droplets, and firm adhesion of the viscid glue droplets. Viscid silk's extensibility is permitted by the glue's high water content, so that organic and inorganic salts present in viscid glue droplets play an essential role in contributing to adhesion by sequestering the atmospheric water that plasticizes the axial silk fibers. Here, we provide direct molecular and macro-scale evidence to show that salts also cause adhesion by directly solvating the glycoproteins, regardless of water content, thus imparting viscoelasticity and allowing the glue droplets to establish good contact. This "dual role" of salts, plasticizing the axial silk indirectly through water sequestration and directly solvating the glycoproteins, provides a crucial link to the evolutionary transition from cribellate silk to viscid silk. In addition, salts also provide a simple mechanism for adhering even at the extremes of relative humidity, a feat eluding most synthetic adhesives.

Wet webs work better: humidity, supercontraction and the performance of spider orb webs.

Like many biomaterials, spider silk responds to water through softening and swelling. Major ampullate silk, the main structural element of most prey capture webs, also shrinks dramatically if unrestrained or develops high tension if restrained, a phenomenon called 'supercontraction'. While supercontraction has been investigated for over 30 years, its consequences for web performance remain controversial. Here, we measured prey capture performance of dry and wet (supercontracted) orb webs of Argiope and Nephila using small wood blocks as prey. Prey capture performance significantly increased at high humidity for Argiope while the improvement was less dramatic for Nephila. This difference is likely due to Argiope silk supercontracting more than Nephila silk. Web deflection, measured as the extension of the web upon prey impact, also increased at high humidity in Argiope, suggesting that silk softening upon supercontraction explains the improved performance of wet webs. These results strongly argue that supercontraction is not detrimental to web performance.

Mechanical performance of spider orb webs is tuned for high-speed prey.

Spiders in the Orbiculariae spin orb webs that dissipate the mechanical energy of their flying prey, bringing the insects to rest and retaining them long enough for the spider to attack and subdue their meals. Small prey are easily stopped by webs but provide little energetic gain. While larger prey offer substantial nourishment, they are also challenging to capture and can damage the web if they escape. We therefore hypothesized that spider orb webs exhibit properties that improve their probability of stopping larger insects while minimizing damage when the mechanical energy of those prey exceeds the web's capacity. Large insects are typically both heavier and faster flying than smaller prey, but speed plays a disproportionate role in determining total kinetic energy, so we predicted that orb webs may dissipate energy more effectively under faster impacts, independent of kinetic energy per se. We used high-speed video to visualize the impact of wooden pellets fired into orb webs to simulate prey strikes and tested how capture probability varied as a function of pellet size and speed. Capture probability was virtually nil above speeds of ~3 m s(-1). However, successful captures do not directly measure the maximum possible energy dissipation by orb webs because these events include lower-energy impacts that may not significantly challenge orb web performance. Therefore, we also compared the total kinetic energy removed from projectiles that escaped orb webs by breaking through the silk, asking whether more energy was removed at faster speeds. Over a range of speeds relevant to insect flight, the amount of energy absorbed by orb webs increases with the speed of prey (i.e. the rates at which webs are stretched). Orb webs therefore respond to faster - and hence higher kinetic energy - prey with better performance, suggesting adaptation to capture larger and faster flying insect prey. This speed-dependent toughness of a complex structure suggests the utility of the intrinsic toughness of spider silk and/or features of the macro-design of webs for high-velocity industrial or military applications, such as ballistic energy absorption.

Reconstructing web evolution and spider diversification in the molecular era.

The evolutionary diversification of spiders is attributed to spectacular innovations in silk. Spiders are unique in synthesizing many different kinds of silk, and using silk for a variety of ecological functions throughout their lives, particularly to make prey-catching webs. Here, we construct a broad higher-level phylogeny of spiders combining molecular data with traditional morphological and behavioral characters. We use this phylogeny to test the hypothesis that the spider orb web evolved only once. We then examine spider diversification in relation to different web architectures and silk use. We find strong support for a single origin of orb webs, implying a major shift in the spinning of capture silk and repeated loss or transformation of orb webs. We show that abandonment of costly cribellate capture silk correlates with the 2 major diversification events in spiders (1). Replacement of cribellate silk by aqueous silk glue may explain the greater diversity of modern orb-weaving spiders (Araneoidea) compared with cribellate orb-weaving spiders (Deinopoidea) (2). Within the "RTA clade," which is the sister group to orb-weaving spiders and contains half of all spider diversity, >90% of species richness is associated with repeated loss of cribellate silk and abandonment of prey capture webs. Accompanying cribellum loss in both groups is a release from substrate-constrained webs, whether by aerially suspended webs, or by abandoning webs altogether. These behavioral shifts in silk and web production by spiders thus likely played a key role in the dramatic evolutionary success and ecological dominance of spiders as predators of insects.

Molecular Changes in Spider Viscid Glue As a Function of Relative Humidity Revealed Using Infrared Spectroscopy.

Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.