Web Silk Research Articles

Do parasitized caterpillars protect their parasitoids from hyperparasitoids? A test of the ‘usurpation hypothesis’

Caterpillars that are attacked by some species of parasitoid wasps are known to survive for several days after the parasitoid larvae emerge and pupate. It has been argued that the behaviour of the parasitized larva is ‘usurped’ by the parasitoid and that it ‘guards’ the parasitoid cocoons against their own natural enemies such as hyperparasitoids (the ‘usurpation hypothesis'). We tested this hypothesis in the association involving a gregarious endoparasitoid, the wasp Cotesia glomerata; caterpillars of its host, the large cabbage white butterfly Pieris brassicae; and a pupal hyperparasitoid, the wasp Lysibia nana. In laboratory experiments, we presented cocoon broods of C. glomerata to single females of L. nana in arenas for 6 h. We tested several treatments for rates of primary parasitoid survival, including variation in the position of the caterpillar and the presence or absence of an additional silk web spun by parasitized caterpillars. Parasitized P. brassicae larvae survived longer than the period necessary for C. glomerata adults to emerge. Rates of parasitoid survival were, however, unaffected by the presence of a P. brassicae larva on the cocoon brood, although significantly more parasitoids emerged when the silk web was present. Analyses of the foraging behaviour of individual L. nana females in arenas, performed using Observer software, revealed that the wasps showed a greater tendency to leave cocoons when caterpillars and silk were present. The laboratory experiments only partially support the usurpation hypothesis. In nature, usurpation of the host of the primary parasitoid may be a more effective strategy against generalist predators than against more specialized and better-adapted hyperparasitoids.

The silk tuft web decorations of the orb-weaver Gasteracantha cancriformis : testing the prey attraction and the web advertisement hypotheses

AbstractSeveral orb-web spiders build conspicuous decorations in their webs. The prey attraction hypothesis proposes that decorations increase spider foraging success by attracting prey, and that attraction is linked to UV reflectance. Alternatively, the web advertisement hypothesis proposes that decorations are a signal that advertises the presence of the web to large animals. We tested both hypotheses for the web silk tufts of Gasteracantha cancriformis. Even though tufts are UV reflective, we did not find support for the prey attraction hypothesis. In the field, when webs with tufts painted black and control webs were compared, there were no differences in the number of prey captured, number of damaged areas in webs and type of prey captured. In the laboratory, Drosophila melanogaster did not demonstrate preference for tufted silk lines versus non-tufted silk lines. Our data also did not give support for the web advertisement hypothesis. The proportion of web destruction was similar between web with tufts painted black and control webs during four days of experimentation. Therefore, two of the most favoured hypotheses that attempt to explain decorations do not apply for web silk tufts in our study system. Instead we propose that silk tufts might be an aposematic signal.

Does the Giant Wood Spider Nephila pilipes Respond to Prey Variation by Altering Web or Silk Properties?

AbstractRecent studies demonstrated that orb‐weaving spiders may alter web architectures, the amount of silk in webs, or the protein composition of silks in response to variation in amount or type of prey. In this study, we conducted food manipulations to examine three mechanisms by which orb‐weaving spiders may adjust the performance of webs to variation in prey by altering the architectures of webs, making structural changes to the diameters of silk threads, and manipulating the material properties or amino acid composition of silk fibers. We fed Nephila pilipes two different types of prey, crickets or flies, and then compared orb structure and the chemical and physical properties of major ampullate (MA) silk between groups. Prey type did not affect orb structures in N. pilipes, except for mesh size. However, MA silk diameter and the stiffness of orbs constructed by spiders fed crickets were significantly greater than for the fly group. MA fibers forcibly silked from N. pilipes fed crickets was significantly thicker, but less stiff, than silk from spiders fed flies. Spiders in the cricket treatment also produced MA silk with slightly, but statistically significantly, more serine than silk from spiders in the fly treatment. Percentages of other major amino acids (proline, glycine, and glutamine) did not differ between treatments. This study demonstrated that orb‐weaving spiders can simultaneously alter some structural and material properties of MA silk, as well as the physical characteristics of webs, in response to different types of prey.

Behavioral Attraction of Two Parasitoids, Venturia canescens and Bracon hebetor, to Silk Extracts of a Host Plodia interpunctella

Kairomonal activities of silk extracts of host Plodia interpunctella were determined by measuring the rates of behavioral responses of two parasitic wasps, Venturia canescens and Bracon hebetor. Silk of P. interpunctella larvae attracted both parasitic wasps but the cocoon silk of silkworm, Bombyx mori and the web silk of two-spotted spider mite, Tetranychus urticae did not. Silk components of P. interpunctella were extracted by using either hexane or methanol, and tested the rates of three serial responses of wasps; host location, antennal drumming and ovipositor probing behaviors. The patterns of each behavioral response were similar in two wasps. The rates of each response were increased at the higher concentrations of both extracts. Antennal drumming behavior was much more responsive to lower concentrations of both extracts than ovipositor probing behavior was. Furthermore, the rate of antennal drumming response was higher in hexane-extracts rather than methanol-extract in both wasps; V. canescens and B. hebetor for 20 and 17 times, respectively. However, ovipositor probing response was similar in two different extracts. Both extracts elicited 100% of antennal drumming response but ovipositor probing response was only 60 to 80% of all tested individuals. Our results were shown that silk extracts of host larvae elicited strong behavioral responses of two parasitic wasps and could be applied for practical application of parasitoids attraction in the biological control of agricultural pests.

Behavioral manipulation of host caterpillars by the primary parasitoid waspCotesia glomerata(L.) to construct defensive webs against hyperparasitism

AbstractMany parasitoids control the behavior of their hosts to achieve more preferable conditions. Decreasing predation pressure is a main aim of host manipulation. Some parasitoids control host behavior to escape from their enemies, whereas others manipulate hosts into constructing defensive structures as barriers against hyperparasitism. Larvae of the parasitoid waspCotesia glomerataform cocoon clusters after egression from the parasitized host caterpillar of the butterflyPieris brassicae. After the egression of parasitoids, the perforated host caterpillar lives for a short period and constructs a silk web that covers the cocoon cluster. We examined whether these silk webs protectC. glomeratacocoons against the hyperparasitoid waspTrichomalopsis apanteroctena. In cocoon clusters that were not covered by silk webs (“bare” clusters), only cocoons hidden beneath others avoided hyperparasitism. In covered cocoon clusters, both cocoons hidden beneath others and those with a space between them and the silk web avoided hyperparasitism, whereas cocoons that contacted the silk webs were parasitized. The frequency of cocoons that were hidden beneath others increased with the increasing number of cocoons in a cluster, but the defensive effect of cluster size was thought to be lower than that of silk webs. However, the rate of hyperparasitism did not differ between covered and bare clusters when we allowed the hyperparasitoids to attack the cocoon clusters in an experimental arena. This result was thought to have been caused by low oviposition frequency by these hyperparasitoids. As a result, silk webs did not guard the cocoons from hyperparasitoids in our experiments, but would protect cocoons under high hyperparasitism pressure by forming a space through which the ovipositors could not reach the cocoons.

Prey attraction as a possible function of discoid stabilimenta of juvenile orb-spinning spiders

Stabilimenta, the conspicuous, ultraviolet (UV)-reflecting, white silk structures around the hub of orb webs, are thought to increase a spider's foraging success by attracting prey (prey attraction hypothesis). No studies, however, have yet investigated the function of nonlinear and noncruciform stabilimenta and the stabilimenta of juvenile spiders. We examined whether the discoid stabilimenta spun by juvenile Argiope versicolor, an orb-spinning spider, function to attract prey. When we measured the UV reflectance of the web silks of A. versicolor juveniles, we found that the discoid stabilimentum silk, but not spiral silk or radial silk or the joint where the two types of silk met, reflected UV light. We then carried out two series of choice experiments in the laboratory to determine whether the webs with discoid stabilimenta attract more fruit flies than undecorated webs when UV+ white light (300–700 nm) is present. We also examined whether the discoid stabilimenta still attract insects when the UV section of the spectrum (300–400 nm) is eliminated (UV−). Webs with discoid stabilimenta attracted more fruit flies than the webs without stabilimenta in UV+ white light. However, in UV− light, fewer fruit flies flew to the webs, even when they were decorated with discoid stabilimenta. In a field experiment, significantly more insects were intercepted by webs with discoid stabilimenta than by webs without stabilimenta. We suggest that the discoid stabilimenta of A. versicolor juveniles act as a visual signal that attracts insects, supporting the prey attraction hypothesis.

Stage-specific surface chemicals of Plodia interpunctella: 2-acyl-1,3-cyclohexanediones from larval mandibular glands serve as cuticular lipids

Cuticular lipid compositions of all life stages of the stored product moth Plodia interpunctella have been determined. Eggs and adults of P. interpunctella have cuticular lipids consisting solely of hydrocarbons. The composition of eggs and adult females is qualitatively nearly identical with ca. 86 hydrocarbons (11 n-alkanes, 39 monomethyl alkanes, 19 dimethyl alkanes, 11 trimethyl alkanes and 6 monoenes) except females lack the 2-methyl alkanes found in eggs. Adult males have a hydrocarbon composition qualitatively nearly identical to females with the exception that they lack the monoenes. Larval and pupal cuticular lipids are dominated by a mixture of ca. 20 previously described 2-acyl-1,3-cyclohexanediones, with only minute amounts of n-alkanes on the larvae and pupae. The 2-acyl-1,3-cyclohexanediones are continuously secreted onto their silk webbing and food particles by the paired mandibular glands found in all larvae. Extracts from dissected mandibular glands have a qualitatively identical composition to larval cuticular extracts. The pupal stage (which does not have mandibular glands) is enclosed in a silk cocoon also coated with 2-acyl-1,3-cyclohexanediones laid down while the wandering stage larvae spin the cocoon. The 2-acyl-1,3-cyclohexanediones have physical properties which closely mimic those of cuticular hydrocarbons, including melting point and boiling point range and hydrophobicity. This is the first report of an insect with a life stage that does not use conventional cuticular lipids for conservation of water.

Difference in Web Construction Behavior at Newly Occupied Web Sites Between Two Cyclosa Species

AbstractAnimals make decisions based on subjective assessments of their environment. To determine their future foraging activities, animals probably assess food availability from past foraging experiences. Thus, foraging also functions as a way for animals to collect information, with the uncertainty of an assessment decreasing as foraging activity increases. This suggests that different needs for a correct assessment may affect the investment made in foraging activities. Orb‐web spiders sometimes relocate their webs and relocation rate differs among species. After web relocation, several spider species have been reported to construct the first webs at newly occupied web sites using less silk than usual, possibly to avoid the risk of an overinvestment at sites where food availability has not been determined. Nevertheless, they may pay a cost, because of inadequate decision‐making, if webs constructed with less silk convey less information and increase the uncertainty of an assessment. We expect that stronger site tenacity necessitates a greater requirement for correct assessment of web site and the degree to which spiders reduce the amount of web silk in the first web after web relocation is smaller in species that use the same site longer. To test this hypothesis, we examined web construction in two orb‐web spiders, Cyclosa octotuberculata and C. argenteoalba. At the same time we found that these two species exhibit different web‐site tenacity, as C. octotuberculata does not relocate its webs as frequently as does C. argenteoalba. After artificially induced web relocation, C. argenteoalba constructed webs that were initially smaller and contained only about 2/3 of the silk in control webs that were constructed at the original site. In contrast, C. octotuberculata did not exhibit such decreases in web size or in the amount of web silk used. This result is consistent with our hypothesis.

ANCIENT ALTERNATIVE FOR ADHESION

It's one of the oldest scenes in the `Horror' repertoire; a spider's web brushing a passing face! But spiders and their webs have been around longer than humans have been afraid of them. Today, most spiders' webs are coated in a sticky glue to ensnare flies and other victims. But a few ancient species stuck to an older, drier alternative. They produce cribellar threads;thousands of minute filaments coating a supporting thread that snag insects by their setae, like Velcro hoops over loops. Remarkably, the silk also sticks to glass and metal sheer surfaces through van der Waals molecular forces. But not all dry webbed spiders produce the same types of microscopic fibril. Some species produce lumpy fibrils, while the most ancient member of the cribellar family, the rare Hypochilidae, produce smooth fibrils. Intrigued by the differences between both types of silk, Brent Opell and his student Anya Hawthorn noticed that the lumpy fibre's adhesive properties improved under damp conditions, while the smooth fibres didn't. Wondering whether the knobbly fibres picked up water and adhered through hygroscopic forces, the team modelled the fibre's adhesive qualities under dry and damp conditions(p. 3905).First they needed fresh spider silk, but Hypochilus refused to spin its smooth fibred webs in the lab, even though Hawthorn and Opell built them cosy cliffs to make their homes on. So the team headed south to the spider's home in North Carolina to gather the unusual silk. Fortunately, Hyptiotes was more cooperative, contented to construct their knobbly silk webs in plastics boxes in the lab.With a steady supply of web silk, the team began measuring both fibres'stickiness as the humidity rose. Although the `sticky-o-meter' was straightforward to use in the field, operating it inside a humidity-controlled chamber was tedious, but Hawthorn persevered. As the humidity rose, the knobbly silk became stickier, while Hypochilus' remained the same.But was the increased stickiness due to humidity, or some other physical property? Hawthorn and Opell modelled the effects of van der Waals and hygroscopic forces on the smooth and lumpy silks. After measuring the area of both fibres that pealed away from the sticky-o-meter's plate, and calculating the resulting van der Waals and hygroscopic forces, the team realised that the increased humidity was giving Hyptiotes the sticky edge.Which probably gives Hyptiotes the mechanical edge too. When the conditions are right, their silk becomes stickier by freeloading on the moist fibre's hygroscopic attachment. But if Hypochilus want to increase their threads' stickability, they have to coat their threads with more smooth fibrils, at a significant metabolic cost. Which probably explains why most dry webbed spiders have upgraded to hygroscopic knobbly fibres as a low-cost alternative adhesive, leaving the Hypochilidae as the sole surviving link to the spiders' silky past.

Palaeontology: spider-web silk from the Early Cretaceous.

The use of viscid silk in aerial webs as a means to capture prey was a key innovation of araneoid spiders and has contributed largely to their ecological success. Here I describe a single silk thread from a spider's web that bears glue droplets and has been preserved in Lebanese amber from the Early Cretaceous period for about 130 million years. This specimen not only demonstrates the antiquity of viscid silk and of the spider superfamily Araneoidea, but is also some 90 million years older than the oldest viscid spider thread previously reported in Baltic amber from the Eocene epoch.

ULTRAVIOLET REFLECTANCE OF SPIDERS ANDTHEIR WEBS

Abstract To determine the reflectance of spider webs and spiders under ultraviolet (UV) light, spiders and their webs were photographed under normal (white) light and under UV light. It was found that all silk in araneoid webs reflect slightly more UV light than white light; i.e., they had a positive UV-brightness. However, the often cited, particularly high UV-brightness of stabilimenta could not be confirmed. Spiders differed in their UV-brightness, with most spiders reflecting less UV light than white light. Based on the knowledge of the visual system of insects and invertebrates it is suggested that the main function of stabilimenta is predator defense. However, drawing a final conclusion requires more knowledge on the way potential predators and prey perceive spiders, spider webs and stabilimenta.

State-determinate foraging decisions and web architecture in the spider Dictyna volucripes (Araneae Dictynidae)

Foraging behaviors result from dynamic trade-offs made by organisms, incorporating factors such as past foraging success, reproductive effort, and predation risk. But, decision-making by animals occurs with incomplete information about the environment. We examined the relationship of web architecture and foraging decisions in the tangle web-building spider Dictyna volucripes Keyserling, a common spider in North American fields. Tangle webs are constructed over many days, which increases the total material and energetic investment in webs compared to orb-webs that are rebuilt daily. This reduces the profitability of changing web sites for tangle web spiders, making decisions about allocation of resources to foraging within single patches critical. We studied how foraging success affected investment in webs for High Prey spiders, fed two Drosophila daily, and Low Prey spiders, fed half that amount, over a 9-day period. We found no difference in the total area of silk added to webs. Increase in web area was proportional to initial web size in High Prey spiders, all of which increased webs by approximately 50%, despite substantial variation in initial sizes of webs. But, some Low Prey spiders with small initial webs increased web size by 200–300% while others, with large initial webs, invested proportionately little silk in webs. We also compared the effects of web architecture on prey capture of High and Low Prey spiders, under standardized prey density. There was no difference between High and Low Prey spiders in the number of Drosophila captured. We found a significant positive correlation between sizes of webs and prey capture in High Prey spiders but not in Low Prey spiders. We present the hypothesis that web investment by D. volucripes may incorporate information about both previous foraging effort and foraging success.

Signal conflict in spider webs driven by predators and prey

Variation in the sensory physiologies of organisms can bias the receptions of signals, driving the direction of signal evolution. Sensory drive in the evolution of signals may be particularly important for organisms that confront trade–offs in signal design between the need for conspicuousness to allow effective transfer of information and the need for crypsis of the signal to unintended receivers. Several genera of orb–weaving spiders include conspicuous silk designs, stabilimenta, in the centre of their webs. Stabilimenta can be highly visible signals to predators, warning them of the presence of a noxious, sticky silk web. However, stabilimenta can also be used by prey as a signal in avoidance of webs, creating a trade–off in signal visibility. I argue that the derived spectral properties of stabilimentum silk have resulted in part from this conflict. The innate colour preferences of insects, their ability to learn colours, and the spectral properties of flowers all suggest that the reflectance spectra of stabilimenta renders them relatively cryptic to many insect prey, while maintaining their visibility to vertebrate predators.

Calculation of Capture Thread Length in Orb Webs: Evaluation of New Formulae

This study provides ecologists and ethologists with an accurate tool to investigate the web investment of orb web spiders in the field. A crucial foraging investment for orb web spiders is the length of silk invested in each web. However, the different types of orb web silks have different costs for the spiders and must be treated separately. As the viscid silk material used to construct the capturing spiral in webs is more costly to produce and more likely to become limited than other types of silk, our study focuses on the length of these capture threads. We evaluate 3 formulae used to calculate capture thread length, by comparing their estimates with the actual total capture thread length measured from photographs of Nuctenea sclopetaria Clerck webs. The formulae used are from the literature and newly proposed ones. The results suggest the use of a newly proposed formula that not only provides agreement with the actual thread length but also is efficient and convenient for field studies.

Mitochondrial cytochrome oxidase I in tetranychid mites: a comparison between molecular phylogeny and changes of morphological and life history traits

AbstractSpider mites, Tetranychidae, represent one of the most cosmopolitan and economically important groups of terrestrial arthropods; however, many aspects of their evolutionary relationships remain uncertain. We sequenced part of the mitochondrial cytochrome oxidase subunit I (COI) gene in 20 species of phytophagous mites belonging to nine genera and two families (Tetranychidae and Tenuipalpidae), including several agricultural pests. As eported in insects, the sequences were extremely rich in A + T (75% on average), especially in the third codon position (95%). However, one of the genera we studied had a significantly lower A + T content (69% on average, 78% in the third codon position), showing that base composition can change substantially over short periods of time. Most interspecific differences were transversions and their number increased steadily with the number of non-synonymous differences, while the number of transitions remained constant. The phylogeny based on COI sequences was inferred using the maximum likelihood method. The results are compatible as a whole with the traditional classification based on morphological characters, but call for some minor taxonomic revisions. Some morphological characters and life history traits (mode of reproduction, adaptation to the host plant) were also analysed within this phylogenetic framework. At the family level, one can see a trend towards thelytoky becoming rarer compared to the general mode of reproduction of the group, arrhenotoky. There is also an evolutionary tendency towards a more complex mode of life, with the production of silk webs and correlated changes of the locomotion apparatus. However, in the Tetranychidae there seems to have been convergent evolution of these morphological characters together with independent development of a common adaptation to this mode of life in different genera.

Structural engineering of an orb-spider's web

THE silk of spiders first evolved 400 million years ago and orb webs emerged 180 million years ago1, 2; present-day spiders' webs are structures efficiently engineered by nature. The planar orb web of the garden spider Araneus diadematus has evolved with the prime function of capturing fast-moving and, on a relative scale, massive insects. We have now analysed the structural engineering of a complete web, using computer modelling, to incorporate the measured time-dependent stress–strain characteristics of the two chief types of web silk. With this model we unexpectedly discovered that aerodynamic damping plays a crucial role in prey capture, an observation that we confirmed in laboratory experiments on real webs.

Design and Prey Capture Ability of Webs of the Spiders Nephila clavata and Argiope bruennichii.

The relationship between design and prey capture ability of webs was examined in the two orb-weaving spiders Nephila clavata and Argiope bruennichii. We collected unstressed webs of both species by framing them with large paper mounts, and exposed them at the same microhabitat. Webs of Nephila tended to capture more prey than those of Argiope, due to the larger number of sticky threads. However, the proportion of relatively large prey (2-6mm) was greater in Argiope, probably due to the larger amount of adhesive material attached to the web silk. We conducted another experiment to assess the retention ability of webs for large prey (20-25mm) by putting it on the web. The amount of time the prey remained trapped in the web was similar between the species, though the variation was large. We consider that the web of Nephila functions to capture large prey as well as very small prey.

High-resolution images of cobweb threads of black widow spider Lactradectus mactans by Atomic-Force Microscopy

Its unique mechanical properties make spider silk well-suited to the capture of prey. Orb webs capture prey by dissipating kinetic energy of prey; therefore, high strain and low resilience are important properties of the silk. Cobwebs capture prey with thread breakage and entanglement; therefore, other properties may be more important. Studies reveal that orb web silks consists of proteins arranged in anti-parallel beta sheets. X-ray diffractions reveal orderly crystal regions interspersed with amorphous regions. Silks of different functions from the same web consist of similar proportions of amino acids, yet differ significantly in mechanical properties. It has been suggested that the amorphous regions play a role in determining mechanical properties of spider web. Given these differences in types of orb web silk threads from a single web, we expect to find different structural properties in the cobweb. We present our progress in measuring the structural properties of the cobweb of the black widow spider, Lactradectus mactans, at the molecular level. Our initial results consist of high resolution atomic force microscope (AFM) images of the thread.

Trail and arena marking by caterpillars ofArchips cerasivoranus (lepidoptera: Tortricidae)

The activity ofArchips cerasivoranus caterpillars is largely limited to their colonial silk web and trails. Silk pulled directly from the spinnerets of caterpillars and wound onto paper strips to form artificial trails elicited locomotion from the larvae. Trails made from extracts of silk and silk glands also elicited locomotion. These and other observations reported here indicate that the caterpillars are responsive to a water-soluble pheromone that is a component of the silk strand. Marker pheromones appear not to be secreted from other regions of the body, as has been reported for some other trail-following caterpillars.

Specialization in the choice and use of spider silk in the nest of the chaffinch (Fringilla coelebs) (Aves, Fringillidae)

The chaffinch, Fringilla coelebs, is one of a large number of birds from many families that use silk in the construction of their nests. Thirty-eight chaffinch nests collected from around the UK were examined to determine the nature and role of silk in nest construction. A regular survey of web, retreat and cocoon silk availability was made at a study site close to Glasgow, Scotland, over a 12 month period. The only spider web silk found in the nests was of the type produced by cribellate spiders. The majority of silk in nests, however, was spider cocoons, but there was no correlation between the amount of cocoon and web silk used. Nests with more lichen decoration contained more silk, and cocoon silk was particularly associated with the attachment of lichen. Nest construction at the study site took place from late April to mid-May. When nest building began, the availability of suitable web silk had doubled from its winter (lowest) level; however, its abundance continued to rise sharply until the end of M...