EVOLUTIONARY SHIFTS IN THE SPECTRAL PROPERTIES OF SPIDER SILKS

Photoreflectance/scattering measurements of spider silks informed by standard optics

The silks of certain orb weaving spiders are emerging as high-quality optical materials. This motivates study of the optical properties of such silk and particularly the comparative optical properties of the silks of different species. Any differences in optical properties may impart biological advantage for a spider species and make the silks interesting for biomimetic prospecting as optical materials. A prior study of the reflectance of spider silks from 18 species reported results for three species of modern orb weaving spiders (Nephila clavipes, Argiope argentata and Micrathena Schreibersi) as having reduced reflectance in the UV range. (Modern in the context used here means more recently derived.) The reduced UV reflectance was interpreted as an adaptive advantage in making the silks less visible to insects. Herein, a standard, experimental technique for measuring the reflectance spectrum of diffuse surfaces, using commercially available equipment, has been applied to samples of the silks of four modern species of orb weaving spiders: Phonognatha graeffei, Eriophora transmarina, Nephila plumipes and Argiope keyserlingi. This is a different technique than used in the previous study. Three of the four silks measured have a reduced signal in the UV. By taking the form of the silks as optical elements into account, it is shown that this is attributable to a combination of wavelength-dependent absorption and scattering by the silks rather than differences in reflectance for the different silks. Phonognatha graeffei dragline silk emerges as a very interesting spider silk with a flat 'reflectance'/scattering spectrum which may indicate it is a low UV absorbing dielectric micro-fibre. Overall the measurement emerges as having the potential to compare the large numbers of silks from different species to prospect for those which have desirable optical properties.

Spider silk colour covaries with thermal properties but not protein structure

Understanding how and why animal secretions vary in property has important biomimetic implications as desirable properties might covary. Spider major ampullate (MA) silk, for instance, is a secretion earmarked for biomimetic applications, but many of its properties vary among and between species across environments. Here, we tested the hypothesis that MA silk colour, protein structure and thermal properties covary when protein uptake is manipulated in the spider Trichonephila plumipes. We collected silk from adult female spiders maintained on a protein-fed or protein-deprived diet. Based on spectrophotometric quantifications, we classified half the silks as 'bee visible' and the other half 'bee invisible'. Wide angle X-ray diffraction and differential scanning calorimetry were then used to assess the silk's protein structure and thermal properties, respectively. We found that although protein structures and thermal properties varied across our treatments only the thermal properties covaried with colour. This ultimately suggests that protein structure alone is not responsible for MA silk thermal properties, nor does it affect silk colours. We speculate that similar ecological factors act on silk colour and thermal properties, which should be uncovered to inform biomimetic programmes.

Experimental evidence that high humidity is an essential cue for web building in Pasilobus spiders

Spiders in the subfamily Cyrtarachninae, including bolas spiders, are moth specialists, and it has been suggested that these spiders initiate web-weaving under high humidity. Here we used Pasilobus hupingensis to experimentally test whether Cyrtarachninae spiders build webs exclusively under high humidity. The results showed that humidity, as well as temperature and prey feeding history, affected web-building probability, but humidity had a much stronger effect. Moreover, spiders never constructed webs at under <70% humidity. We suggest that a mechanical property in sticky materials derived from moth specialization; namely, unusually high, yet rapidly degrading stickiness, is likely to have promoted the evolution of plastic foraging behaviour that varies with humidity.

Nutrient deprivation induces property variations in spider gluey silk

Understanding the mechanisms facilitating property variability in biological adhesives may promote biomimetic innovations. Spider gluey silks such as the spiral threads in orb webs and the gumfoot threads in cobwebs, both of which comprise of an axial thread coated by glue, are biological adhesives that have variable physical and chemical properties. Studies show that the physical and chemical properties of orb web gluey threads change when spiders are deprived of food. It is, however, unknown whether gumfoot threads undergo similar property variations when under nutritional stress. Here we tested whether protein deprivation induces similar variations in spiral and gumfoot thread morphology and stickiness. We manipulated protein intake for the orb web spider Nephila clavipes and the cobweb spider Latrodectus hesperus and measured the diameter, glue droplet volume, number of droplets per mm, axial thread width, thread stickiness and adhesive energy of their gluey silks. We found that the gluey silks of both species were stickier when the spiders were deprived of protein than when the spiders were fed protein. In N. clavipes a concomitant increase in glue droplet volume was found. Load-extension curves showed that protein deprivation induced glue property variations independent of the axial thread extensions in both species. We predicted that changes in salt composition of the glues were primarily responsible for the changes in stickiness of the silks, although changes in axial thread properties might also contribute. We, additionally, showed that N. clavipes' glue changes color under protein deprivation, probably as a consequence of changes to its biochemical composition.

Tangled in a sparse spider web: single origin of orb weavers and their spinning work unravelled by denser taxonomic sampling

In order to study the tempo and the mode of spider orb web evolution and diversification, we conducted a phylogenetic analysis using six genetic markers along with a comprehensive taxon sample. The present analyses are the first to recover the monophyly of orb-weaving spiders based solely on DNA sequence data and an extensive taxon sample. We present the first dated orb weaver phylogeny. Our results suggest that orb weavers appeared by the Middle Triassic and underwent a rapid diversification during the end of the Triassic and Early Jurassic. By the second half of the Jurassic, most of the extant orb-weaving families and web designs were already present. The processes that may have given origin to this diversification of lineages and web architectures are discussed. A combination of biotic factors, such as key innovations in web design and silk composition, as well as abiotic environmental changes, may have played important roles in the diversification of orb weavers. Our analyses also show that increased taxon sampling density in both ingroups and outgroups greatly improves phylogenetic accuracy even when extensive data are missing. This effect is particularly important when addition of character data improves gene overlap.

The effect of insect surface features on the adhesion of viscous capture threads spun by orb-weaving spiders

Spider orb-webs intercept a broad range of insects and their capture threads must adhere to a range of surface textures. In species of the Araneoidea clade, these capture threads are composed of viscid droplets whose size and spacing differ among species. To determine how droplet profile and insect surface texture interact, we measured the stickiness of viscous threads produced by four species using four insect surfaces that ranged from a smooth beetle elytra to the dorsal surface of a fly abdomen that was covered by large, widely spaced setae. The adhesion of threads to these surfaces differed by as much as 3.5-fold within a spider species and 2.1-fold for the same insect surface between spider species. However, 96% of these differences in stickiness was explained by four variables: the ratio of natural log of droplet volume to setal length, the natural log of droplet volume per mm of thread length, setal surface area, and the area of cuticle not excluded from thread contact by setae. Compared with previous measurements of primitive cribellar capture threads produced by orb weavers of the Deinopoidea clade,viscous threads performed more uniformly over the range of insect surfaces. They also held bug hemelytra, which were densely covered with fine setae, more securely, but held beetle elytra, fly wings and fly abdomens less securely than did viscous threads. Hemelytra may be held more securely because their setae more easily penetrate the viscous boundary layer to establish a greater area of interaction and, after having done so, offer more resistance as they are pulled through this layer. Finely textured surfaces may also have higher effective surface energies and therefore may interact more completely with viscous material.

Adhesive recruitment by the viscous capture threads of araneoid orb-weaving spiders

The sticky prey capture threads of orb-webs are critical to web performance. By retaining insects that strike the web, these spirally arrayed threads allow a spider time to locate and subdue prey. The viscous capture threads spun by modern orb-weaving spiders of the Araneoidea clade replaced the dry, fuzzy cribellar capture threads of the Deinopoidea and feature regularly spaced moist, adhesive droplets. The stickiness of a cribellar thread is limited by its tendency to peel from a surface after the adhesion generated at the edges of contact is exceeded. In this study we test the hypothesis that viscous thread overcomes this limitation by implementing a suspension bridge mechanism (SBM) that recruits the adhesion of multiple thread droplets. We do so by using contact plates of four widths to measure the stickiness of six species' viscous threads whose profiles range from small, closely spaced droplets to large, widely spaced droplets. The increased stickiness registered by an increased number of thread droplets supports the operation of a SBM. However, the accompanying decrease in mean per droplet adhesion shows that droplets interior to the edges of thread contact contribute successively less adhesion. Models developed from these data suggest that the suspension bridge mechanism is limited to a span of approximately 12 droplets.

Insects from the grazing food web favoured the evolutionary habitat shift to bright environments in araneoid spiders

The Araneoidea comprises a diverse group of web-building spiders, and part of this diversity is believed attributable to habitat expansion to bright environments. We clarified the fitness-related advantages of living in such environments by examining prey availability and the growth rates of 10 species in three families inhabiting grassland (bright) and forest understory (dim) habitats. Spiders in the grassland habitat captured more prey, derived mainly from the grazing food web, than those in the forest-floor environment, and this difference was manifested in their growth rate. Independent contrasts indicated that increased utilization of insects from the grazing food web led to an evolutionary increase in adult body size. These results suggest that the shift to bright environments enabled araneoid spiders to evolve diverse life-history traits, including rapid growth and large size, which were not possible in dim environments.

Unraveling the mechanical properties of composite silk threads spun by cribellate orb-weaving spiders

Orb-web weaving spiders depend upon the mechanical performance of capture threads to absorb the energy of flying prey. Most orb-weavers spin wet capture threads with core fibers of flagelliform silk. These threads are extremely compliant and extensible due to the folding of their constituent proteins into molecular nanosprings and hydration by a surrounding coating of aqueous glue. In contrast, other orb-weavers use cribellate capture threads, which are composite structures consisting of core fibers of pseudoflagelliform silk surrounded by a matrix of fine dry cribellar fibrils. Based on phylogenetic evidence, cribellate capture threads predate the use of viscid capture threads. To better characterize how pseudoflagelliform and cribellar fibrils function, we investigated the mechanical performance of cribellate capture threads for three genera of spiders (Deinopis, Hyptiotes and Uloborus). These taxa spin very diverse web architectures, ranging from complete orbs to evolutionarily reduced triangle webs and cast nets. We found that the pseudoflagelliform core fibers of these webs were stiffer and stronger, but also less extensible, than flagelliform silk. However,cribellate capture threads achieved overall high extensibilities because the surrounding cribellar fibrils contributed substantially to the tensile performance of threads long after the core pseudoflagelliform fibers ruptured. In the case of Deinopis capture threads, up to 90% of the total work performed could be attributed to these fibrils. These findings yield insight into the evolutionary transition from cribellate to viscid capture threads.

Colourful orb-weaving spiders, Nephila pilipes, through a bee's eyes

Many orb-weaving spiders in the tropics forage in open sites during the day and some of them have both bright and dark colourations. The conspicuous UV-reflective colour markings of these spiders have been reported to be attractive to visually oriented prey and thus could increase the spiders'foraging success. Using a combination of field and laboratory studies, we examine whether or not the body colouration of orb-weaving spiders exhibits optical properties that are attractive to insect prey from the viewpoint of insect visual physiology. We compared the prey interception rates and colour contrasts of the typical and melanic morphs of the giant wood spider, Nephila pilipes. Results of the field study showed that the typical morph caught significantly more insects than the melanic morph. Colour contrasts calculated from spectral reflectances of the background and body surface of spiders showed that the brightly coloured body parts of the typical morph exhibited rather high values, but those of the dark body parts were below the discrimination threshold. The differential colour contrasts of body parts generated a visual signal unlike that of a spider but rather like certain forms of food resources. On the other hand, the melanic morphs did not have bright colouration and the colour contrasts of every part of the body were significantly higher than the threshold, making the contour of spiders quite clear to bees.

The function significance of silk decorations of orb-web spiders: a critical review of the empirical evidence

A number of taxonomically diverse species of araneoid spiders adorn their orb-webs with conspicuous silk structures, called decorations or stabilimenta. The function of these decorations remains controversial and several explanations have been suggested. These include: (1) stabilising and strengthening the web; (2) hiding and concealing the spider from predators; (3) preventing web damage by larger animals, such as birds; (4) increasing foraging success; or (5) providing a sunshield. Additionally, they may have no specific function and are a consequence of stress or silk regulation. This review evaluates the strength of these explanations based on the evidence. The foraging function has received most supporting evidence, derived from both correlative field studies and experimental manipulations. This contrasts with the evidence provided for other functional explanations, which have not been tested as extensively. A phylogenetic analysis of the different decoration patterns suggests that the different types of decorations are as evolutionary labile as the decorations themselves: the analysis shows little homology and numerous convergences and independent gains. Therefore, it is possible that different types of decorations have different functions, and this can only be resolved by improved species phylogenies, and a combination of experimental and ultimately comparative analyses.