The Function of Stabilimenta in Spider Webs
By Emma Clodfelter '21
BIOL 320: Evolution
Winner of the John Allen Award
I choose Emma’s paper because I was captivated by how she presented the problem on designs in spider webs. She was very successful at providing the necessary background information to understand the biological problem, including the currently proposed hypotheses and critiques for each one, leading to her proposed work.
– Paulina Mena
Perhaps the most unique trait of spiders is their silk. Despite other insects producing silk at some stage in their development, spiders remain distinctive due to both sexes using silk throughout their entire lifespan (Brunetta and Craig, 2010). The many species grouped together under the label “orb weaver” all spin a web as a means of trapping their prey (Rao, Cheng, and Herberstein 2006). Spiders all have spinnerets in their abdomens that are connected to many silk-producing glands (Brunetta and Craig, 2010). This allows the spider to utilize different kinds of silk for different purposes, such as being sticky to snare prey (Brunetta and Craig, 2010). It is thought that the many functions of their silk has allowed spiders to survive in so many different environments without radically altering their body shape or lifestyle (Brunetta and Craig, 2010). Spiders’ webs have become less visible over the course of their evolution, which makes the stabilimenta seem counterintuitive (Rao, et al. 2006).
The stabilimenta, or web decorations, are zig-zags of aciniform silk that form a conspicuous design, the function of which is unknown (Brunetta and Craig, 2010). Many hypotheses have been proposed, such as making the web visible to animals that may crash through it, attracting prey, and making the spider look larger to potential predators (Brunetta and Craig, 2010). Not all spiders use stabilimenta, and members of the same species may decorate their webs in different ways (Beccaloni, 2009). Because of how visible the web becomes when stabilimenta are present, there must be an advantage that would balance out this trade-off. The function may vary in different species, as well, which complicates the experiments and results.
The silk-producing glands use ducts to transfer the silk into the spinnerets (Brunetta and Craig, 2010). The spinnerets are covered with spigots, which are small tubes that expel the formed silk (Brunetta and Craig, 2010). The exact placement and number of spinnerets varies from species to species, and spiders are the only silk-spinning organisms with abdominal spinnerets (Brunetta and Craig, 2010). Spiders, with the exception of primitive species, produce six or more varieties of silk in their different glands, and each kind has its own function (Brunetta and Craig, 2010). The silk itself is formed by proteins, which can make the silk glue-like, woolly, strong, elastic, etc. in order to make a web that can withstand speeding insects, support the weight of the spider, trap the insects, and so on (Brunetta and Craig, 2010). The silk is so strong because it has a core-shell structure; the core is the strength-determining factor, and it is composed of the two spider silk proteins, hydrophobic spidroin I and hydrophilic spidroin II (Doblhofer, et al. 2015). Most of the spider silk genes come from the same gene family (Brunetta and Craig, 2010). The speed at which spiders spin their webs is affected by temperature—heat makes spiders work faster, and cold makes them slower (Vollrath, 2003). There is also evidence to suggest that the conditions of silk-spinning determine the strength of the silk, not just the genes, as silkworms can spin silk of nearly equal strength if they spin it in the same fashion as a spider (Vollrath, 2003).
Vertical orb weavers in the superfamily Araneoidea use the spiral in their web to catch their prey (Brunetta and Craig, 2010). The capture spiral is a combination of two silk proteins, the stretchy flagelliform silk and the sticky aggregate silk protein glue (Brunetta and Craig, 2010). Flagelliform silk is translucent, which makes it hard for insects to detect, and it can withstand the speed of flying insects when they hit the web (Brunetta and Craig, 2010). The aggregate silk is a glue that is applied to the web fibers as a fluid (Brunetta and Craig, 2010). The aggregate silk beads up and scatters light, which may help attract insects (Brunetta and Craig, 2010). Unlike other spider silks, the aggregate remains wet and absorbs water from the humidity in the air (Brunetta and Craig, 2010). Spiders that do not use aggregate silk glue use woolly cribellate silk, but this is less sticky than the aggregate silk and does not reflect any light (Brunetta and Craig, 2010). Aciniform silk, which the stabilimenta are composed of, are thought to be most similar to the original silk gland that appeared in the common ancestor of spiders (Brunetta and Craig, 2010). Females use it for the formation of egg sacs and wrapping up prey; males use it for depositing sperm before bringing it to the palpal organs (Brunetta and Craig, 2010). Aciniform silk is the strongest spider silk that has been tested, and it is also waterproof (Brunetta and Craig, 2010).
Some observations of orb weavers have led to the conclusion that species in the Cyclosa genus use the stabilimenta to camouflage their location in the web. These spiders litter their webs with debris and spin the stabilimenta in a spiral-like pattern, but this is only notable in these species, not in other spiders that use web decorations (Beccaloni, 2009). The problem with the hypothesis that the stabilimenta prevent birds from flying through the web is that spiders can rebuild their webs in a short amount of time. Although this does not disprove the hypothesis, it makes it seem less likely than the idea that the stabilimenta are used to attract prey. This hypothesis states that the stabilimenta attract prey by reflecting UV light, but this is not supported by the findings of Samuel Zschokke, whose research indicated that the stabilimenta are not any more reflective than the other types of silk (Zschokke, 2002). However, these results conflict with other research that suggests that the stabilimenta are the most reflective part of the web (Brunetta and Craig, 2010). Therefore, my hypothesis is that the stabilimenta do attract prey.
It is thought that the stabilimenta vaguely mimic flowering grasses, but the spiders do not use the same pattern repeatedly, which prevents their prey from learning to avoid their webs (Brunetta and Craig, 2010). Some researchers observed that when Argiope argentata spiders decorated their webs daily with different patterns, there were more insect hits than when the spider did not use decorations or used the same ones consistently (Brunetta and Craig, 2010). However, the stabilimenta have also been noted to attract wasps, so spiders that decorate frequently eat well, but do not live as long as their non-decorating counterparts, due to wasp predation and parasitizing (Brunetta and Craig, 2010).
These observations were made on only one species, so repeating their methods with many other species of orb weaver and comparing the results would provide more evidence to support the hypothesis of prey attraction. If the function of the stabilimenta is prey attraction for the majority of species that use it, then the expected results would coincide with what the researchers found when observing Argiope argentata. For their experiment, they photographed the spiders in their webs under UV light against a background of flowering grasses (Brunetta and Craig, 2010). They took note of all the stabilimenta patterns used by spiders and recorded damage to the web that indicated insect hits (Brunetta and Craig, 2010). After these initial observations, they randomly selected webs to transplant stabilimenta onto and changed the designs at random the next day (Brunetta and Craig, 2010). Then they marked and trained stingless bees—the prey of Argiope argentata—to forage in the particular sites with the webs and recorded the number of bee hits in the webs (Brunetta and Craig, 2010). They found that bees hit decorated webs more frequently, but after getting caught and escaping once, the bees learned the pattern and avoided the web, but would get caught again if the decoration pattern was changed (Brunetta and Craig, 2010).
An experiment that tested for the prey-attraction ability of web decorations found both that Thelacantha brevispina spiders were more likely to catch prey when the decorations were present, and that the combination of decorations and web barriers caused a decrease in predation rates by birds (Tseng, et al. 2011). These tuft decorations consist of silk, insect carcasses, eggs, and debris, and they may serve a similar function to, or may include, the stabilimenta (Tseng, et al. 2011). The experimenters designed three treatments for the spiderwebs: removal of the web decorations, removal of the web decorations and the barrier webs, and no treatment as a control group (Tseng, et al. 2011). They used video monitoring to keep track of the spiders’ prey-capture rates and predation rates (Tseng, et al. 2011). To find how the UV-reflectiveness of the decorations compared to the decorations concealed with paint, the researchers developed a photoreceptor sensitivity model with honeybees and blue tits to compare how insects and insect-eating birds viewed the treated and untreated webs (Tseng, et al. 2011). They found both that the paint did conceal the decorations, and that the untreated decorations were visible to insects and birds (Tseng, et al. 2011).
Their results showed that there was no significant difference in prey-capture rates between treated and untreated webs, but in the 2010 study, the treated webs with concealed decorations had significantly lower prey-capture rates than the webs with visible decorations (Tseng, et al. 2011). Predation from wasps did not vary between the groups (Tseng, et al. 2011). However, this may be because the T. brevispina spiders have a thick, spiny cuticle as a means of protection from wasps, so their decorated barrier webs likely did not evolve as a defense against wasps (Tseng, et al. 2011). There were only six bird interactions, two of which did not involve an attack, and this only happened in the untreated control group, whose decorations and barrier were present (Tseng, et al. 2011). There were four bird attacks in total, three in the group without barrier webs or decorations, and one in the group without decorations only (Tseng, et al. 2011). There is still uncertainty whether the decorations were solely responsible for attracting prey, as they could not be separated from the barrier webs (Tseng, et al. 2011). However, when the barrier webs were present without decorations, they caught less prey (Tseng, et al. 2011).
Birds seemed to attack less when barrier webs with decorations were present, and although there was a small sample size of birds, this may suggest that the decorations make the barrier more visible to deter predators (Tseng, et al. 2011). The researchers did find that the decorations reflected blue light in addition to UV light, which is visible to birds and insects (Tseng, et al. 2011). The stabilimenta in other orb weaver webs conform to insect-form vision, but the tuft decorations do not, which suggests that they are imperfect mimics that nonetheless still function advantageously (Tseng, et al. 2011).
Another set of researchers studied the Argiope versicolor spider, an orb weaver, to see if predator-avoidance behaviors varied between adults and juveniles, and if these behaviors were stabilimentum-specific (Li, et al. 2003). Tactile stimulation and air movement were used as artificial stimuli to elicit any of four responses: shuttling, pumping, dropping, or shifting (Li, et al. 2003). Shifting involves moving away from the middle of the web, shuttling is when the spider shuffles between sides of the web, pumping is when the spiders pump their bodies while on the web, and dropping includes dropping out of the web to hide in the leaf litter below (Li, et al. 2003). These researchers believe that stabilimenta may aid in deterring predators because they are only found in diurnal orb-weavers that build their webs in open, visible areas (Li, et al. 2003). This does not mean that the stabilimenta cannot also attract prey, as it is possible that stabilimenta may have more than one function (Li, et al. 2003). The researchers also noted that all species in the genus Argiope that have been studied have been observed using stabilimenta (Li, et al. 2003). However, juveniles use a discoid stabilimenta pattern, whereas adults utilize cruciform stabilimenta (Li, et al. 2003).
The study found that juveniles in decorated webs had different responses than juveniles in undecorated webs (Li, et al. 2003). They shuttled more frequently, shifting was about the same, and fewer than 10% dropped (Li, et al. 2003). Adults with stabilimenta in their webs also responded to stimuli differently than adults with plain webs (Li, et al. 2003). They did not drop or shift as frequently, and they pumped more often (Li, et al. 2003). The researchers suspect that shuttling in juveniles is associated with the stabilimenta as a means of making the spider harder for predators to spot in the web (Li, et al. 2003). They also suspect that the discoid stabilimenta may block the juvenile from view or provide a barrier between it and the predator if the spider shuttles to the opposite side of the web (Li, et al. 2003). The researchers believe that since the adult A. versicolor build their webs in exposed areas, the wind loosens webs that lack cruciform stabilimenta, which may explain why pumping is more common on webs that have it (Li, et al. 2003). This study seems to find a connection between the stabilimenta and predator-avoidance rather than prey attraction, but this function may only be in some species.
An important factor in discovering the purpose of stabilimenta is whether it evolved once or multiple times independently, and whether the vertical orb weavers that use it are closely related or not. It is currently believed that the use of stabilimenta evolved independently at least nine times and has appeared in three families and twenty-two genera (Bruce, 2006). These families include Araneidae, Uloboridae and Tetragnathidae (Bruce, 2006). These families are not closely related to each other (Fernández, et al. 2018). Web decorations vary from being only stabilimenta to including debris such as egg sacs, plant matter, and dead insects, which may impact their function (Bruce, 2006). The pattern of stabilimenta can be described as belonging to one of six different categories: cruciate, linear, discoid, spiral, tufts and a silk mat (Bruce, 2006). As previously discussed, members of the same species may use more than one pattern throughout their lifetime, which is best observed in the webs of juveniles compared to the webs of adults (Bruce, 2006).
The Argiope species have been studied most heavily so far, which would make it beneficial to study other species of orb weavers (Bruce, 2006). The genus Allocyclosa has been studied as well, and it seemed to demonstrate predator-avoidance with its stabilimenta (Bruce, 2006). As for the prey-attraction hypothesis, the idea that the stabilimenta imitate UV-reflective floral guides or gaps in vegetation has not been directly tested (Bruce, 2006). Therefore, this would be a prime topic of study in testing my hypothesis. Other pieces of supporting evidence have already been found, such as juvenile Argiope versicolor with decorated webs catching more Drosophila flies, but only when UV light was present, indicating that the stabilimenta are reflective (Bruce, 2006). Another study that involved the removal of web decorations showed that the spiders caught less prey without the stabilimenta, further supporting the idea that web decorations attract prey (Bruce, 2006). The main criticism of these studies is that they often compare the webs against artificial, colored backgrounds rather than natural backgrounds, which may alter the way the reflectiveness of the stabilimenta is perceived (Bruce, 2006). Decorated webs also tend to be smaller than undecorated ones, so it is best to compare decorated webs as a control with webs whose stabilimenta have been removed for the experiment (Bruce, 2006).
Materials and Methods:
Therefore, I would like to test the proposed mechanisms of prey-attraction with a natural background. My species of interest would be Argiope aurantia, as it is well-known and part of the Araneidae family. Although it would be of value to choose a species from a different genus, more research needs to be done in many species to determine habitat, predators, diet, and more to aid in determining the function of their use of stabilimenta, as it likely varies between species due to environmental context. By choosing a well-known and previously-studied genus, I can easily compare my results to other studies. This may also provide insight into whether species within the same genus use stabilimenta for the same purpose or not, which will allow researchers to see how different selective pressures affected the function of stabilimenta.
For my study, I would like to find the difference between the amount of prey captured in decorated webs and undecorated webs, as well as measure the reflectiveness of both types of webs, in order to test my hypothesis that the stabilimenta attract prey with UV light reflection. For ease of comparing my results to other studies, I will do a similar experiment to what Craig described, but instead of transplanting web decorations onto webs, I will remove the decorations from a sample of webs and compare them to untreated, decorated webs (Brunetta and Craig, 2010). I would make sure to measure the amount of UV reflectiveness against the natural backgrounds the webs are already up against. The A. aurantia spiders build their webs in a variety of places, but are most commonly found in dense vegetation (Enders, 1973). They also tend to choose areas that are not heavily shaded, but are protected from the wind (Enders, 1973). The spiders show no preference for which plant species they built their web between (Enders, 1973). A. aurantia mostly feed on insects belonging to orders Odonata and Hymenoptera, but they will still catch orders Coleoptera, Hemiptera, Lepidoptera, Orthoptera, and Diptera (Howell and Ellender, 1984). Adult A. aurantia will commonly feed on bees from the family Apidae, specifically honeybees and bumblebees (Howell and Ellender, 1984). The odonates they usually catch are dragonflies and damselflies (Howell and Ellender, 1984).
Because Odonata and Hymenoptera are these spiders’ most important prey, I will focus on how they both see the stabilimenta. Using a photoreceptor sensitivity model derived for honeybees, just as another study did (Tseng, et al. 2011), I will transport the portion of the web with the stabilimenta to a lab and use a spectrometer to measure the reflectiveness. I will also use a combination of video footage and careful inspection of the insect remnants in the web to identify the species caught. This experiment will carry on from the start of June to the end of August in order to take advantage of the summer activity of insects and spiders. It will also cover a variety of habitats that A. aurantia is found in, as the surrounding vegetation may impact how the stabilimenta are perceived by the prey. The spiders in the experiment will be left in their natural habitat.
If I reject the null hypothesis, then I should find that the spiders with their stabilimenta removed will consistently catch less prey than spiders with their stabilimenta intact. The significance of this would be determined using a two-sample t-test. If I fail to reject my null hypothesis, then I would expect for there to be no significant difference between rates of prey capture between the stabilimenta-absent webs and stabilimenta-present webs. I must also keep in mind that if I reject the null hypothesis, this does not mean that every species that utilizes stabilimenta is using it for the same purpose. More research will need to be done with other species in other genera before the scientific community is ready to reach a consensus. However, more studies should shed light on this biological mystery and increase our understanding of orb weaver spiders.
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