Spiders in the Garden

Back in the spring, I planted four small spruce trees along the western fence line of my back yard. They aren't as tall as the fence and so do not provide me any shade yet. But they are growing. 

It turns out that spruce trees are excellent habitat for spiders. I've identified at least two kinds that took up residence in them over the summer. One kind builds funnel webs tucked in between branches. You can see them lurking at the center of the funnel if you sneak up quietly and make sure your shadow doesn't fall on the trees first. The other kind, a common garden spider, builds vertical webs between the trees and between the fence and the trees. The distances are around 3 to 4 feet, nothing particularly difficult for these spiders. The spiders started out quite small, maybe the diameter of a pencil eraser, but they appear to have had a successful summer of hunting and several of them have leg spans that are easily over an inch in diameter now.

I check on the spiders every morning when I let the dogs out for a last potty before I head off to the lab. Over the course of the summer, I've collected some interesting observations about these spiders and their webs.

First of all, when they were small, the spiders often centered their webs in the middle of the two objects. It is rare now to find a centered web. Nearly all the webs are offset closer to the trees. My cursory online research suggests that the spiders are simply adapting to the more likely position of prey, mainly flies, midges, mosquitoes, and the like.

But here's a really interesting thing. When the spiders started building their vertical webs back in the early summer, they would build them at a variety of heights. However, the dogs walk between the trees--the trees get watered and the grass grows green and lush there so it's a favorite pooping area, and the lane between the trees and the fence was part of Harry's daily perimeter check. And I mow the lawn of course. So the lower webs would get broken on a daily basis. 

It only took about 2 to 3 weeks but the spiders stopped building webs below Azza's head height.

That's right. I think the spiders learned that low webs were not productive so they all shifted their webs to about 3 feet up and higher. The dogs walk between the trees with impunity. I can easily push the mower between the trees and not touch a web.

In case you think that I am insane (I might be but this is not the best illustration of it), here's another example. I walk the dogs very early in the morning. Places along the sidewalk where bushes lean in close to mailbox clusters would seem to be reasonable locations for spider webs. But I never encounter webs in those locations, and I think it is because those spots are too highly trafficked--the spiders in those areas also learned that, although it might look like a nice easy distance, those are actually crummy locations for webs. I do find spider webs when I veer from the sidewalk however.

There is spotty research on this but it does appear that spiders will change the location, shape, and size of their webs in response to prey availability and type as well as competition with other spiders. One group of researchers did a cost-benefit analysis on spiders that move their webs. I turned up some amusing older references which state that spiders do nothing more than execute a fixed program, like a computer runs a program, when building their webs. Manifest destiny! But newer perspectives suggest that spiders exercise a degree of free will, that they can evaluate past experience and change their web parameters, when web building. That latter view is more consistent with what I observed, and that is a far cry from a computer running a fixed program!

I'd also like to briefly mention some super freaky little spiders that live in my front flower bed. I've only seen them there because in the front flower bed are two plants, mint and some other flowering thing, that produce white flowers. These spiders are tiny and pure white. They don't build webs; instead, they stalk their prey. And they mostly capture bees that visit the flowers. Truly diabolical.