More than a safety line: jump-stabilizing silk of salticids

Methods for Silk Property Analyses across Structural Hierarchies and Scales

Silk from silkworms and spiders is an exceptionally important natural material, inspiring a range of new products and applications due to its high strength, elasticity, and toughness at low density, as well as its unique conductive and optical properties. Transgenic and recombinant technologies offer great promise for the scaled-up production of new silkworm- and spider-silk-inspired fibres. However, despite considerable effort, producing an artificial silk that recaptures the physico-chemical properties of naturally spun silk has thus far proven elusive. The mechanical, biochemical, and other properties of pre-and post-development fibres accordingly should be determined across scales and structural hierarchies whenever feasible. We have herein reviewed and made recommendations on some of those practices for measuring the bulk fibre properties; skin-core structures; and the primary, secondary, and tertiary structures of silk proteins and the properties of dopes and their proteins. We thereupon examine emerging methodologies and make assessments on how they might be utilized to realize the goal of developing high quality bio-inspired fibres.

A review on advances in the applications of spider silk in biomedical issues

Spider silk, as one of the hardest natural and biocompatible substances with extraordinary strength and flexibility, have become an ideal option in various areas of science and have made their path onto the biomedical industry. Despite its growing popularity, the difficulties in the extraction of silks from spiders and farming them have made it unaffordable and almost impossible for industrial scale. Biotechnology helped production of spider silks recombinantly in different hosts and obtaining diverse morphologies out of them based on different processing and assembly procedures. Herein, the characteristics of these morphologies and their advantages and disadvantages are summarized. A detailed view about applications of recombinant silks in skin regeneration and cartilage, tendon, bone, teeth, cardiovascular, and neural tissues engineering are brought out, where there is a need for strong scaffolds to support cell growth. Likewise, spider silk proteins have applications as conduit constructs, medical sutures, and 3D printer bioinks. Other characteristics of spider silks, such as low immunogenicity, hydrophobicity, homogeneity, and adjustability, have attracted much attention in drug and gene delivery. Finally, the challenges and obstacles ahead for industrializing the production of spider silk proteins in sufficient quantities in biomedicine, along with solutions to overcome these barriers, are discussed.

Rapid mid-jump production of high-performance silk by jumping spiders

Jumping spiders (Salticidae) do not rely on webs to capture their prey, but they do spin a silk dragline behind them as they move through their habitat. They also spin this dragline during jumps, continuously connecting them with the surface they leapt from. Because spiders cannot spin silk in advance, this silk must be spun at the same speed as the spider jumps - in effect, requiring spin speeds over ten times faster than typical. And while many spiders can move rapidly, for example when running or rappelling, previous research on silk has found that silk spinning rates in excess of walking and web-building speeds (∼2-20 mm/s) result in lower quality silk and even dragline failure¹. Here we report that, despite being spun at high speeds (∼500-700 mm/s; 100-140 body lengths/s), jump-spun salticid silk shows consistent, uniform structure as well as the high-performance qualities characteristic of silk spun by other spiders, including orb-weaving species, at low speeds². The toughness of this jump-spun silk (mean = 281.9 MJ/m³) even surpasses reported values for all but the toughest orb-web draglines². These results show that salticids are capable of spinning high-performance silk and are able to do so extremely rapidly under natural conditions.

Hanging by a thread: unusual nocturnal resting behaviour in a jumping spider

Background

For diurnal animals that heavily rely on vision, a nocturnal resting strategy that offers protection when vision is compromised, is crucial. We found a population of a common European jumping spider (Evarcha arcuata) that rests at night by suspending themselves from a single silk thread attached overhead to the vegetation, a strategy categorically unlike typical retreat-based resting in this group.

Results

In a comprehensive study, we collected the first quantitative field and qualitative observation data of this surprising behaviour and provide a detailed description. We tested aspects of site fidelity and disturbance response in the field to assess potential functions of suspended resting. Spiders of both sexes and all developmental stages engage in this nocturnal resting strategy. Interestingly, individual spiders are equally able to build typical silk retreats and thus actively choose between different strategies inviting questions about what factors underlie this behavioural choice.

Conclusions

Our preliminary data hint at a potential sensory switch from visual sensing during the day to silk-borne vibration sensing at night when vision is compromised. The described behaviour potentially is an effective anti-predator strategy either by acting as an early alarm system via vibration sensing or by bringing the animal out of reach for nocturnal predators. We propose tractable hypotheses to test an adaptive function of suspended resting. Further studies will shed light on the sensory challenges that animals face during resting phases and should target the mechanisms and strategies by which such challenges are overcome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00410-3.

Role of legs and foot adhesion in salticid spiders jumping from smooth surfaces

Many spiders and insects can perform rapid jumps from smooth plant surfaces. Here, we investigate how jumping spiders (Pseudeuophrys lanigera and Sitticus pubescens) avoid slipping when accelerating. Both species differed in the relative contribution of leg pairs to the jump. P. lanigera accelerated mainly with their long third legs, whereas their short fourth legs detached earlier. In contrast, S. pubescens accelerated mainly with their long fourth legs, and their short third legs detached earlier. Because of the different orientation (fourth-leg tip pointing backward, third-leg tip pointing forward), the fourth-leg tarsus pushed, whereas the third-leg tarsus pulled. High-speed video recordings showed that pushing and pulling was achieved by different attachment structures. In P. lanigera, third-leg feet made surface contact with setae on their distal or lateral claw tuft, whereas fourth-leg feet engaged the proximal claw tuft, and the distal tuft was raised off the ground. S. pubescens showed the same division of labour between proximal and distal claw tuft for pushing and pulling, but the claw tuft contact lasted longer and was more visible in the fourth than in the third legs. Experimental ablation of claw tufts caused accelerating spiders to slip, confirming that adhesion is essential for jumps from smooth substrates.

Jump takeoff in a small jumping spider

Jumping in animals presents an interesting locomotory strategy as it requires the generation of large forces and accurate timing. Jumping in arachnids is further complicated by their semi-hydraulic locomotion system. Among arachnids, jumping spiders (Family Salticidae) are agile and dexterous jumpers. However, less is known about jumping in small salticid species. Here we used Habronattus conjunctus, a small jumping spider (body length ~ 4.5 mm) to examine its jumping performance and compare it to that of other jumping spiders and insects. We also explored how legs are used during the takeoff phase of jumps. Jumps were staged between two raised platforms. We analyzed jumping videos with DeepLabCut to track 21 points on the cephalothorax, abdomen, and legs. By analyzing leg liftoff and extension patterns, we found evidence that H. conjunctus primarily uses the third legs to power jumps. We also found that H. conjunctus jumps achieve lower takeoff speeds and accelerations than most other jumping arthropods, including other jumping spiders. Habronattus conjunctus takeoff time was similar to other jumping arthropods of the same body mass. We discuss the mechanical benefits and drawbacks of a semi-hydraulic system of locomotion and consider how small spiders may extract dexterous jumps from this locomotor system.

Jumping Locomotion Strategies: From Animals to Bioinspired Robots

Jumping is a locomotion strategy widely evolved in both invertebrates and vertebrates. In addition to terrestrial animals, several aquatic animals are also able to jump in their specific environments. In this paper, the state of the art of jumping robots has been systematically analyzed, based on their biological model, including invertebrates (e.g., jumping spiders, locusts, fleas, crickets, cockroaches, froghoppers and leafhoppers), vertebrates (e.g., frogs, galagoes, kangaroos, humans, dogs), as well as aquatic animals (e.g., both invertebrates and vertebrates, such as crabs, water-striders, and dolphins). The strategies adopted by animals and robots to control the jump (e.g., take-off angle, take-off direction, take-off velocity and take-off stability), aerial righting, land buffering, and resetting are concluded and compared. Based on this, the developmental trends of bioinspired jumping robots are predicted.

Low temperatures impact species distributions of jumping spiders across a desert elevational cline

Temperature is known to influence many aspects of organisms and is frequently linked to geographical species distributions. Despite the importance of a broad understanding of an animal's thermal biology, few studies incorporate more than one metric of thermal biology. Here we examined an elevational assemblage of Habronattus jumping spiders to measure different aspects of their thermal biology including thermal limits (CT_min, CT_max), thermal preference, V̇CO₂ as proxy for metabolic rate, locomotor behavior and warming tolerance. We used these data to test whether thermal biology helped explain how species were distributed across elevation. Habronattus had high CT_max values, which did not differ among species across the elevational gradient. The highest-elevation species had a lower CT_min than any other species. All species had a strong thermal preference around 37 °C. With respect to performance, one of the middle elevation species was significantly less temperature-sensitive in metabolic rate. Differences between species with respect to locomotion (jump distance) were likely driven by differences in mass, with no differences in thermal performance across elevation. We suggest that Habronattus distributions follow Brett's rule, a rule that predicts more geographical variation in cold tolerance than heat. Additionally, we suggest that physiological tolerances interact with biotic factors, particularly those related to courtship and mate choice to influence species distributions. Habronattus also had very high warming tolerance values (> 20 °C, on average). Taken together, these data suggest that Habronattus are resilient in the face of climate-change related shifts in temperature.

Effects of Abdominal Rotation on Jump Performance in the Ant Gigantiops destructor (Hymenoptera, Formicidae)

Jumping is an important form of locomotion, and animals employ a variety of mechanisms to increase jump performance. While jumping is common in insects generally, the ability to jump is rare among ants. An exception is the Neotropical ant Gigantiops destructor (Fabricius 1804) which is well known for jumping to capture prey or escape threats. Notably, this ant begins a jump by rotating its abdomen forward as it takes off from the ground. We tested the hypotheses that abdominal rotation is used to either provide thrust during takeoff or to stabilize rotational momentum during the initial airborne phase of the jump. We used high speed videography to characterize jumping performance of G. destructor workers jumping between two platforms. We then anesthetized the ants and used glue to prevent their abdomens from rotating during subsequent jumps, again characterizing jump performance after restraining the abdomen in this manner. Our results support the hypothesis that abdominal rotation provides additional thrust as the maximum distance, maximum height, and takeoff velocity of jumps were reduced by restricting the movement of the abdomen compared with the jumps of unmanipulated and control treatment ants. In contrast, the rotational stability of the ants while airborne did not appear to be affected. Changes in leg movements of restrained ants while airborne suggest that stability may be retained by using the legs to compensate for changes in the distribution of mass during jumps. This hypothesis warrants investigation in future studies on the jump kinematics of ants or other insects.

Energy and time optimal trajectories in exploratory jumps of the spider Phidippus regius

Jumping spiders are proficient jumpers that use jumps in a variety of behavioural contexts. We use high speed, high resolution video to measure the kinematics of a single regal jumping spider for a total of 15 different tasks based on a horizontal gap of 2–5 body lengths and vertical gap of +/−2 body lengths. For short range jumps, we show that low angled trajectories are used that minimise flight time. For longer jumps, take-off angles are steeper and closer to the optimum for minimum energy cost of transport. Comparison of jump performance against other arthropods shows that Phidippus regius is firmly in the group of animals that use dynamic muscle contraction for actuation as opposed to a stored energy catapult system. We find that the jump power requirements can be met from the estimated mass of leg muscle; hydraulic augmentation may be present but appears not to be energetically essential.

Hunting with sticky tape: functional shift in silk glands of araneophagous ground spiders (Gnaphosidae)

Foraging is one of the main evolutionary driving forces shaping the phenotype of organisms. In predators, a significant, though understudied, cost of foraging is the risk of being injured by struggling prey. Hunting spiders that feed on dangerous prey like ants or other spiders are an extreme example of dangerous feeding, risking their own life over a meal. Here, we describe an intriguing example of the use of attachment silk (piriform silk) for prey immobilization that comes with the costs of reduced silk anchorage function, increased piriform silk production and additional modifications of the extrusion structures (spigots) to prevent their clogging. We show that the piriform silk of gnaphosids is very stretchy and tough, which is an outstanding feat for a functional glue. This is gained by the combination of an elastic central fibre and a bi-layered glue coat consisting of aligned nanofibrils. This represents the first tensile test data on the ubiquitous piriform gland silk, adding an important puzzle piece to the mechanical catalogue of silken products in spiders.

Arachnid aloft: directed aerial descent in neotropical canopy spiders

The behaviour of directed aerial descent has been described for numerous taxa of wingless hexapods as they fall from the tropical rainforest canopy, but is not known in other terrestrial arthropods. Here, we describe similar controlled aerial behaviours for large arboreal spiders in the genus Selenops (Selenopidae). We dropped 59 such spiders from either canopy platforms or tree crowns in Panama and Peru; the majority (93%) directed their aerial trajectories towards and then landed upon nearby tree trunks. Following initial dorsoventral righting when necessary, falling spiders oriented themselves and then translated head-first towards targets; directional changes were correlated with bilaterally asymmetric motions of the anterolaterally extended forelegs. Aerial performance (i.e. the glide index) decreased with increasing body mass and wing loading, but not with projected surface area of the spider. Along with the occurrence of directed aerial descent in ants, jumping bristletails, and other wingless hexapods, this discovery of targeted gliding in selenopid spiders further indicates strong selective pressures against uncontrolled falls into the understory for arboreal taxa.

Composition and substrate-dependent strength of the silken attachment discs in spiders

Araneomorph spiders have evolved different silks with dissimilar material properties, serving different purposes. The two-compound pyriform secretion is used to glue silk threads to substrates or to other threads. It is applied in distinct patterns, called attachment discs. Although ubiquitously found in spider silk applications and hypothesized to be strong and versatile at low material consumption, the performance of attachment discs on different substrates remains unknown. Here, we analyse the detachment forces and fracture mechanics of the attachment discs spun by five different species on three different substrates, by pulling on the upstream part of the attached thread. Results show that although the adhesion of the pyriform glue is heavily affected by the substrate, even on Teflon it is frequently strong enough to hold the spider's weight. As plant surfaces are often difficult to wet, they are hypothesized to be the major driving force for evolution of the pyriform secretion.