Written by Joe Ballenger

Your Name: Mark
Your Bug Question: I have spiders in and around my house.  I eventually get around to removing their webs from hard to reach places like living room chandelier.  It takes so long that i doubt I ever bother the same spider twice.  Yet after a year or so the spiders seem to know not to build webs on the chandelier.  It is as if they could be passing their learning along to other spiders.  That seems unlikely, but I cannot come up with a better explanation.  Can you explain what is happening?

I found this question interesting, because it’s really two questions in one. The first, is whether disturbance influences where spiders build webs. The second, and more interesting, is whether this information can be transferred between different spiders of the same species.

Spiders like to build their webs in locations which promise a lot of food, without the hassle of rebuilding the web. Image credit: Puamella, via Flickr. License info: CC-BY-SA-2.0

The answers to these questions depends on the ID of the spiders in question, and there are several species of web-building spiders which can be found in houses. The most common species found in homes, Pholcus phalangoides, is the one I’ll focus on. Not only is it the most common, I think it gives the best answer to the question because of it’s life history.

Pholcus phalangoides, more commonly known as the ‘cellar spider’, is actually a very interesting spider. It’s semi social, with juveniles sometimes cooperating to build webs. It’s also a pirate, frequently taking over the webs of other spider species after eating the occupants.

So let’s explore this idea, because it’s interesting territory.

Life History of Cellar Spiders

Pholcus phalangoides, the spider most commonly associated with cobwebs inside homes. Image Credit: Paul Sullivan, via Flickr. License info: CC-BY-ND-2.0

I think the first thing we should do is talk about how this spider lives, because that will guide the answer to this question.

Cellar spiders like to live in cool, dry, undisturbed environments. Their natural habitats are caves, and cave-like structures in the wild. They’re very hardy, and don’t need a whole lot in the way of food or water. They are very well adapted to human dwellings, and feed on the occasional insects which stumble into our homes.

I wasn’t able to find a whole lot about the life history of this species. Most of the work that’s been done on this spider is about sexual competition, and social behavior. I did find some papers indicating that development takes a little under 5 months, depending on food availability.

Most spiders are relatively short lived, at least compared to tarantulas. The typical lifespan of a non-tarantula spider is about a year, which kind of fits with the narrative of the question.

Do Social Spiders Communicate?

Sociality in insects is…complicated. Some insects are highly social, and others aren’t. In highly social insects, information is transmitted between individuals. Honeybees, for example, are particularly good at this. They can communicate everything from the health status of the queen, to the location of food.

Spiders are different, though. A lot of spider sociality seems to be self-organizing cooperation. They tolerate each other, help spin and maintain the web, and help coordinate to capture prey. Each of these activities does require some amount of communication, but nothing as complex as what we see in the video above.

There’s also a lot of antagonism in spider colonies, which you see in less developed insect societies. Spiders in colonies will fight between themselves over feeding sites on the prey, and even food sources…behaviors you don’t see in honeybee colonies. You see this somewhat in wasp colonies, but it’s not as pronounced as it is in spider colonies.

Cellar spiders, when they decide to live in groups, seem to be very loose-knit. They’ll feed communally, and build webs, but there isn’t really any evidence of communication between groups of different spiders.

As an aside, a lot of social spider species do display some very complicated behaviors. Some of the behaviors in the literature seem to be a lot more complicated than simple self-organization, and I wouldn’t be surprised if spider communication is more complicated than is thought at the current time. However, the behavior of cellar spider colonies in particular does seem to be explained very well by self-organized cooperation.

Do Spiders Choose Their Nest Sites Based on Disturbance?

Every arthropod species which has been examined seems to have some capacity for learning, although that capacity may be dependent on the group’s habits. Jumping spiders, which routinely tackle prey that’s large and dangerous, seem to be good learners. No studies on learning seem to have been performed on anything closely related to cellar spiders, but I’d be surprised if they didn’t have the ability to change their behavior in response to their environment.

Spiders like to build their webs in specific habitats, and they will change where they build their webs based on how often it’s destroyed. Silk is very expensive to make, and building a web takes time. If a spider isn’t in their web, they’re also at risk of being eaten. If you’re a spider, it pays to be very careful when choosing a nest site. Spiders choose their nest sites very carefully, and things like food availability and disturbance are a large part of the decision about where to build a home.

The Bottom Line

Here’s what I think is happening, based on what I know about cellar spiders, spider colonies and arthropod learning in general.

First, cellar spiders will take over old webs. By removing the webs, Mark is depriving spiders of already-made resources and making it more difficult for them to colonize new areas. Younger spiders may prefer to hang out in ready-made webs, and webless areas may be less attractive to the spiders.

As Mark is knocking the webs down, it’s also possible the spiders are learning which environments in the house are most often disturbed through the process of trial-and-error. Although this sort of habitat searching hasn’t been documented in cellar spiders, it has been documented in a wide range of web-building spiders. I think it’s very likely it happens in cellar spiders as well.

New cellar spiders occasionally come into the house, through birth or immigration, and wouldn’t have knowledge of which areas are disturbed less frequently. Because they’re not familiar with the habitat, naive spiders may attempt to build webs in the highly disturbed environments. It’s also possible that older spiders are pushing younger spiders out of the webs, and they’re looking for places to build new webs because of a lack of unoccupied webs.

So there are at least two possible explanations for what’s going on here. The first, cellar spiders preferring pre-webbed areas, is what’s most strongly supported by the literature. The second explanation, spiders learning that some areas are more disturbed than others, is inferred from habits of other web building spiders. Personally, I think both options are playing roles here.

I’m not sure that communication between individuals is needed to explain the sort of sporadic web-building behavior that Mark is observing.

Works Cited