Web Silk Research Articles

Developmental changes in barrier web structure under different levels of predation risk inNephila clavipes (Araneae: Tetragnathidae)

Individuals of the orb-weaving spider Nephila clavipesbuild complex webs with a region used for prey capture, the orb, and tangle webs opposite either face, the barrier webs. Barrier webs have been hypothesized to serve a variety of functions, including predator defense, and the primary function of the barrier web should be reflected in the relative size of the barrier to the orb under varying conditions of foraging success and predation risk. To investigate the effects of predation pressure and foraging success on barrier web structure, I conducted a comparative study in three disjunct populations that differed in predation risk and foraging success. Although both the orb web and the barrier webs are silk, there was no indication of a foraging-defense trade-off. Barrier web structure did not change during seasonal shifts in orb web size related to changes in preycapture rate, and barrier web silk density and orb radius were positively correlated. The hypothesis that the construction of barrier webs is in part a response to predation pressure was supported. Barrier webs do deflect attacks by some predators, and barrier webs built by small spiders, suffering frequent predation attempts, had a higher silk density than barrier webs built by larger individuals. Additionally, barrier web complexity decreased at a later age in areas with higher predation risk.

METZNERIA PAUCIPUNCTELLA ZELLER (LEPIDOPTERA: GELECHIIDAE), A MOTH INTRODUCED AGAINST SPOTTED KNAPWEED: ITS FEEDING STRATEGY AND IMPACT ON TWO INTRODUCED UROPHORA SPP. (DIPTERA: TEPHRITIDAE)

AbstractMetzneria paucipunctella Zeller is a seed head moth introduced for biological control of spotted knapweed, Centaurea maculosa Lamarck, in the Pacific Northwest. A cage study was conducted in Montana in 1987 and 1988 to assess the feeding strategy of the M. paucipunctella larva and its impact on two seed head flies, Urophora spp., also introduced on spotted knapweed. Each moth larva destroyed an average of 8.13 knapweed seeds. Most of the seeds were destroyed prior to seed dispersal in early September but the remaining seeds were attacked in the spring of the following year. Each larva tied an average of 7.0 seeds with silk webbing which allowed for feeding long after normal seed dispersal. Seed survival in cages containing both the moth and the two fly species (4.71 seeds per head) was significantly less than in fly-only cages (9.75 seeds per head). The moth larvae caused extensive mortality to the two Urophora species, especially during the spring months. Moth-caused mortality to U. affinis larvae increased from 19% during June through October to 67% in May of the following year. Mortality to U. quadrifasciata also occurred but did not increase significantly in the spring. Data from 10 field sites showed very little association between M. paucipunctella and the two Urophora species which suggests that the unnatural conditions associated with the cages did not increase the incidence of joint moth and fly attack to individual seed heads. The moth is a valuable biocontrol agent as it is contributing significantly to the overall consumer pressure on spotted knapweed seed heads.

The natural history ofPolyrhachis (Myrmhopla) muelleriForel 1893 (Formicidae Formicinae), a weaver ant with mimetic larvae and an unusual nesting behaviour

Polyrhachis muelleri has an unusual nesting behaviour: while the other leaf-nesting species of Polyrhachis hide their nests by weaving together leaves or by incorporating detritus into the silk web, P. muelleri weaves pure silk nests on the lower surfaces of large moderately translucent leaves. Any dark contents of such nests are visible from above through the leaves or from below through the thin silk wall. A special adaptation against visually hunting predators has evolved: the larvae and pupae are green. Habitat selecton, plant specificity, competition, nestbuilding, and other aspects of the behavioural ecology of P. muelleri are discussed.

Oviposition Response of Tetranychus urticae (Acari: Tetranychidae) to Direct Treatment and Residue of Pyrethroids on Soybean

The oviposition response of the twospotted spider mite (TSSM), Tetranychus urticae Koch, to direct treatment and residues of fenvalerate and permethrin on soybean was examined in the laboratory. Contact with pyrethroid residues (120 ppm) reduced eggs per mite-day for 72 h, then oviposition rates returned to normal. Most eggs (99%) on the control leaflets were oviposited directly on the leaf surface, while 83% and 78% of the eggs in the fenvalerate and permethrin treatments were suspended off the leaf surface onto trichomes, cage walls or in the silk webbing. Oviposition rate on untreated soybean foliage following five days on pyrethroid residues was essentially the same for all TSSM; however, the mites previously exposed to fenvalerate or permethrin residue still oviposited 85% and 37% of their eggs off the untreated leaf surface, respectively. A 120 ppm concentration of fenvalerate directly applied to TSSM resulted in 83% and 100% mortality after 24 h and 48 h and no oviposition. A similar direct exposure to permethrin (120 ppm) resulted in 54% mortality after 48 h, and a 58.5% reduction in eggs per mite-day. Direct exposure to a lower dosage rate (12 ppm) caused little mortality but significantly reduced the number of eggs produced in both fenvalerate and permethrin treatments for 48 h. Oviposition returned to normal after 72 h.

Alternative Foraging Modes of Orb Web Weaving Spiders

Forest-understory, orb weaving spiders display at least two, alternative, foraging modes. Large spiders build one large web per feeding period. Although their webs intercept more prey than webs built by small spiders, the biomass of captured prey relative to spider biomass is low. In contrast, small spiders build three to five webs per feeding period. The ratio of prey biomass intake relative to spider biomass, however, is high. Small spiders may be able to tolerate high rates of web loss because of their potentially high rate of prey intake. However, for large spiders, the high rates of web loss may not be tolerable because of the low rate of prey biomass intake. This work shows that orb weaving spiders forage differently and that different patterns of web use have resulted in at least two very different, but equally effective, means of prey capture. ORB SPINNING SPIDERS SPAN body weights over four orders of magnitude. Nevertheless, almost all studies on the foraging ecology of orb weavers has focused on large and medium sized spiders (Kajak 1965; Robinson & Robinson 1972, 1974; Uetz, et al. 1978; Kajak 1978; Eberhard 1979; Wise 1979, 1981; Olive 1980, 1981; Wise & Barata 1983; Spiller 1984). Small orb spinners, including the immature stadia of large web spinners, are more abundant than large spiders and form an important group of terrestrial predators in tropical forests. For web weaving spiders, web silk and the web spinning process are the highest costs associated with foraging (Peakall 1969). Therefore, web loss without prey capture may result in an important energetic loss for the spider. If prey are abundant, however, high rates of web loss may not significantly affect spider foraging effectiveness. Although large and small prey are captured in strong webs, only minute prey can be captured in the webs spun by small spiders. Nevertheless, in a variety of habitats, the most abundant insects are less than or equal to 4 mm in body length (Olive 1980; Greenstone 1984; Nentwig 1983; Shelly 1983, 1984; Wise & Barata 1983; Spiller 1984). In this paper I compare two aspects of spider foraging modes, rate of prey capture, and rate of web breakdown, among five species of tropical forest orb weavers whose size varies over three orders of magnitude. This work predicts that for small spiders prey are abundant and small spiders tolerate high rates of web breakdown. For large spiders, however, resources are less abundant and high rates of web loss are not tolerable. Prey capture at spider orb webs depends on web size, where the web is suspended, and the ability of the web to intercept prey (Kajak 1965, Colebourn 1974, Enders 1977, Brown 1981, Olive 1982, Craig 1987b). The probability of insect encounter at webs and the ability of webs to intercept prey are greatest just after the web is spun. As the web breaks down during the spider's foraging period, however, its effectiveness decreases. Web breakdown results from a variety of both biotic and abiotic factors. The most important biotic events affecting web breakdown and function are prey interceptions. At interception, the kinetic energy an insect transfers to a web is a function of the insect's mass and velocity (E = mv2/2). Most investigators have assumed that small insects are weak fliers (for example see Shelly 1983). Goldsmith et al. (1980), however, have shown that Anartepritchardi (Diptera: Cecydomiidae) is capable of rapid flight velocities up to 3.8 ms-'. Because the kinetic energy of flying insects increases as a function of velocity squared, insect flight speed has a much greater effect on insect kinetic energy than does insect size. Therefore, even an insect of low mass may possess a high kinetic energy when flying fast. This effect is most obvious when insect kinetic energy is calculated relative to insect size or the potential insect area that the web may intercept. Craig (1987b) calculated that the kinetic energy for the small flies (family Cecydomiidae) caught in webs spun by Epilineutes globosus and Leucauge globosa, to be close to the breaking energy of the webs' viscid silk. Hence, when flying fast, even fragile insects are capable of damaging and breaking through

A note on the web silk theft by Argyrodes cylindratus (THORELL) (Araneae: Theridiidae)

The web silk theft by Argyrodes cylindratus was observed. A female of A. cylindratus sneaked into the orb-web of Araneus ventricosus and consumed 17 threads of sticky spiral in 50 minutes. During silk theft sneaker did not steal insect prey ignored by the host.Although A. cylindratus is generally known as a kleptoparasite that steals prey caught or ignored by the host, it may sometimes steal some sticky silk from the host web.

The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders

Spider orb webs are dynamic, energy absorbing nets whose ability to intercept prey is dependent on both the mechnical properties of web design and the material properties of web silks. Variation in web designs reflects variation in spider web spinning behaviours and variation in web silks reflects variation in spider metabolic processes. Therefore, natural selection may affect web function (or prey capture) through two independent and alternative pathways. In this paper, I examine the ways in which architectural and material properties, singly and in concert, influence the ability of webs to absorb insect impact energy. These findings are evaluated in the context of the evolution of diverse aerial webs. Orb webs range along a continuum from high to low energy absorbing. No single feature of web architecture characterizes the amount of energy webs can absorb, but suites of characters indicate web function. In general, webs that intercept heavy and fast flying prey (high energy absorbing webs) are large, built by large spiders, suspended under high tension and characterized by a ratio of radii to spiral turns per web greater than one. In contrast, webs that intercept light and slow flying prey (low energy absorbing webs) are suspended under low tension, are small and are characterized by radial to spiral turn ratios that are less than one. The data suggest that for spiders building high energy absorbing webs, the orb architecture contributes much to web energy absorption. In contrast, for spiders that build low energy absorbing webs, orb architecture contributes little to enhance web energy absorption. Small or slow flying insects can be intercepted by web silks regardless of web design. Although there exists variation in the material properties of silk collected from high and low energy absorbing webs, only the diameter of web fibres varies predictably with silk energy absorption. Web fibre diameter and hence the amount of energy absorbed by web silks is an isometric function of spider size. The significance of these results lies in the apparent absence of selective advantage of orb architecture to low energy absorbing webs and the evolutionary trend to small spiders that build them. Where high energy absorption is not an exacting feature of web design, web architecture should not be tightly constrained to the orb. Assuming the primitive araneoid web design is the orb web, I propose that the evolution of alternative web building behaviours is a consequence of the general, phyletic trend to small size among araneoids. Araneoids that build webs of other than orb designs are able to use new habitats and resources not available to their ancestors.

The Significance of Spider Size to the Diversification of Spider-Web Architectures and Spider Reproductive Modes

The ability of spiders to withstand long periods of food deprivation and to respond rapidly through reproduction when food is available may be the most important physiological factors influencing araneoid evolution. For spiders, foraging effectiveness influences size at sexual maturation, rate of maturation, and reproductive output. I discuss the correlation between size changes, the diversification of web architecture (foraging tactics), and reproductive mode in araneoid spiders. I explore some of the constraints and potential for evolutionary diversification of web architectures and web silks. The data collected up to now suggest that scaling webs and web fibers is the simplest (but not the only) means of evolutionary differentiation among the kinetic properties of webs and, hence, among the classes of prey intercepted by orb webs. Although orb webs are built by spiders of all sizes, webs of other-than-orb design are built, in general, only by small spiders. Changes in web design allow derived araneoids to make use of new habitats not available to their ancestors. Spider reproductive output is a function of the intake of prey biomass. The evolution of alternative foraging tactics among spiders is correlated with the evolution of alternative reproductive modes. The present data suggest that small spiders reach reproductive maturity after fewer developmental stages than do large spiders, producing several egg sacs per spider. Large araneoids, in contrast, take longer to mature; after maturation, they fatten before they reproduce. Larger spiders tend to produce few egg sacs at the end of their lifetime, but each sac contains many eggs. Available data on the time to sexual maturity suggest that the evolutionary diversification of modes of spider foraging and reproduction have evolved through progenesis and, hence, through the reorganization of developmental pathways. The goal of this work is to begin to identify the ecological and developmental factors that have contributed to the evolution of the diverse web architectures and reproductive tactics of the araneoid spiders. This work does not prove or disprove the mechanisms or patterns through which web-weaving spiders have evolved, but it provides a new conceptual framework in which to study the evolution of web spinners.

Orb-web visibility: the influence of insect flight behaviour and visual physiology on the evolution of web designs within the Araneoidea

Ecological differentiation among spiders that build orb-webs depends on the ability of webs both to absorb the kinetic energy of prey impact and to intercept prey (web visibility). Analyses of Diptera flying near webs show that Drosophila melanogaster Meigen avoids high-fibre-density orb-webs sooner than they avoid low-fibre-density webs. From analyses of the flight potential and visual capabilities of D. melanogaster, Trichoporosopon digitatum (Rondani) and Culex quinquesfasciatus Say, it seems surprising that orb-webs capture any prey at all. The thin diameter and refractive properties of web silks give spider webs a translucent quality that greatly reduces visibility. Web suspension in low- and diffuse-light environments, as well as against visually confused backgrounds, decreases one of the most important visual cues for prey, contrast between the web and its background. Among spiders that capture prey which are characterized by specialized visual and flight capabilities, oscillating webs and three-dimensional webs may enhance web capture effectiveness. Web oscillation and the static distortion of the plane of some orb-webs allow webs to ‘escape’ from the range of visual resolution of potential prey.

Mechanism of secretion of the contents of the mandibular glands of Plodia interpunctella larvae

ABSTRACT. Larvae of Plodia interpunctella deposit droplets of mandibular gland secretion onto silk filaments connecting particles within the substrate they inhabit, and on to the substrate itself. Active participation of the mouthparts is necessary for both the formation and deposition of these droplets along the filaments and occurs as the silk is spun. Deposition of secretion directly on the substratum does not involve the mouthparts and appears to result from contact between the lower surface of the head and the substratum. Larvae from which the mandibular glands have been removed by excision neither produce droplets on silk nor deposit them on the substratum. Those which are incapable of spinning silk due to cauterization of the spinneret are still able to deposit secretion on the substratum, however. Deposition of secretion on silk webbing does not occur when the setae which regulate formation of droplets on filaments are removed by shaving. Droplets are not characteristic of a particular larval instar but are produced by larvae of all instars as a normal function which does not depend upon intraspecific encounters. Four other Lepidopteran pests, Ephestia elutella, Ephestia cautella, Anagasta kuehniella and Antigasta catolaunalis, were also found to produce droplets of mandibular gland secretion in a similar manner to larvae of P. interpunctella.

The energetics of web-building in spiders

1. 1. The cost of web-building and the standard metabolism of a sheet web-building wolf spider, Sosippus janus Brady, were measured at 25°C. 2. 2. The cost of activity is about 18% of the total cost of the web, the remainder being web silk cost. 3. 3. The cost of activity in sheet web-building was compared with a literature value for an orb-weaver and found to be similar. An average value is proposed as useful in estimating the cost of activity for other species of web-building spiders, given the weight of the web.

Laser Holography: Its Application to the Study of the Behavior of Insecticide Particles2

Laser holography was used to study the characteristics and behavior of insecticide aerosols. It allowed direct 3-dimensional stop-action study of spray particles as they impinged onto the larva of the western spruce bud worm, Choristoneura occidentalis Freeman, and its silk webbing. The setae of this insect were effective impingement structures for particles ranging from submicron to 20µ diameter. Spray droplets coalesced on the tip and migrated to the base, both with and against the spray current and against gravity. Among 5 solvents studied there was significant evaporation of relatively nonvolatile solvents, particularly when the droplets were below about 40µ diameter.