Prey Detection and Spider Web Architecture

Spider webs are one of the most interesting aspects of spider biology. Their structure can range from a seemingly simple orb web to the complex mass of silk that makes up a tangle web (often referred to as cobwebs), but all serve the same general purpose: capturing prey for the spider to consume. To aid in this endeavor, many spiders are able to alter aspects of their webs to better suit the environment. Spiders use a number of different variables to determine how best to construct their webs, but generally speaking the biggest factor affecting the size and shape of a spiderweb is the spider’s previous experience with prey capture in the area.

This week’s article by Kensuke Nakata is from 2007 and is titled Prey detection without successful capture affects spider’s orb-web building behaviour. This article initially caught my eye because rather than looking at how the web’s architecture was affected by the number of prey captured or how differences in the web influenced how many insects a spider was able to capture, it focused on whether spiders would change their web if they sensed prey but was not able to capture it.

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The trashline orb-weaver Cyclosa octotuberculata (Photo: opencage.info)

To explore this question, the author collected trashline orb-weavers (Cyclosa octotuberculata) and allowed them to rebuild their webs in a lab setting. Then, the spiders were separated into 4 groups which each received a different stimulus to respond to: 1) a fly was placed on the web and allowed to be captured and consumed, 2) a fly was placed on the web but removed before the spider was able to capture it, 3) a fly was placed on the web and removed 5 times, with a 30 minute buffer between each time a fly was introduced, and 4) nothing was placed on the web. Group 4 acted as the control so that there was a baseline of how the spider would alter its web if no prey was detected or captured.

Spider webs were photographed both before the tests and after the spiders rebuilt their webs following the test (most orb-weavers will eat their webs and reuse the silk to create a new web every few days). The pictures were then analyzed for changes in the total thread length (the more thread used, the more energy a spider has committed to foraging in its habitat), the size of the capture area (the size of the central spiral, which is made up of sticky silk that is used for trapping prey), and the mesh height (the amount of space between threads in the central spiral, which affects how well the web can trap prey).

spider-web-diagram
A basic orb web

The total thread length and capture area both increased for spiders that detected or captured their prey, while both web attributes decreased for spiders in the control group. This response is likely due to the spiders in the control group, which did not detect any prey, investing less energy in their webs since they perceived their current environment to have a low prey population. Of the groups that showed an increase in total thread length and capture area, the spiders that were able to capture their prey spun webs with the largest increases in these attributes. Spiders that only detected their prey but were not able to capture it showed a slight increase in both attributes, but not to the same degree as the fed spiders.

The author suggests that the difference between webs spun by the fed spiders and those made by spiders that only detected the prey is likely due to the partial nature of the information the spiders received. While the spiders were aware that something landed in their web, they had no way of knowing if the trapped insect was edible or not (C. octotuberculata, like many other orb-weavers, will reject certain insects such as bees due to the risk of injury) and so they increased their foraging efforts, but not to the same degree as spiders that were able to identify the prey and capture it.

The most interesting result in this experiment (at least to me) is how the change in mesh height differed between the groups. For spiders that detected one prey item, the mesh height increased significantly, meaning that the spirals of the web were more widely spaced. This differed from the other three groups, which showed either no change or even a decrease in the mesh height in the case of the spiders that detected five prey items.

The author suggests that this is a result of the spiders adapting to the specific conditions they are experiencing. Multiple failed capture attempts suggest two things to the spiders, 1) prey density in the area is high, and 2) the web is not retaining the prey long enough for the spider to capture them. As such, the spiders reduce the mesh height of their webs to increase the amount of time their prey is trapped, increasing their chances of a successful capture.

While this explains why there was a decrease in mesh height for spiders that detected multiple prey items, the author does not discuss possible reasons for the increase in mesh height for spiders that detected a single prey. I would put forward this as a possible explanation: spiders that only detected a single potential prey are tailoring their webs towards an environment with a lower prey density. The increased mesh height, along with the slight increase in capture area lead me to believe that these spiders were focusing on increasing the chance of a prey item landing in their web rather than ensuring that the prey would be unable to escape.

The main takeaway from this article is that spiders are able to use information from unsuccessful capture attempts to alter their web architecture to increase their chances of capturing prey. These changes in architecture are in response to information relating to both the density of prey in the environment and the ability of the web to capture said prey. This ability to alter their webs to increase capture rates is an important aspect of spider behavior that has major implications on how well spiders are able to respond to changes in their environment.

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