Death-trap or fortress - the two web designs of black widow spiders

I've written two news stories in this week's New Scientist. One is on the different tactics of four-year-old boys and girls as they compete for animal puppets. The other is on the webs spun by black widow spiders. The article on the venomous, evil, little critters is longer so I'm going to use this space to talk about the black widows instead...

Black widows are notorious for both the toxicity of their venom and the cannibalistic nature of their sex, but their webs are equally interesting and less well known.

The basic design - the "sheet-based" web - consists of a well-defined horizontal sheet of silk supported by some overhanging threads. Underneath the sheet, the spider spins "gum-footed threads" - vertical lines of silk with blobs of sticky glue at their base. These threads are incredibly taut and if an insect blunders into them, they break off from the ground, stick to the insect and catapult it into mid-air, where it struggles helplessly. The spider senses the vibrations through the sheet and goes in for the kill.

But Jacquelyn Zevenbergen at the University of Akron found that these webs are only used by hungry spiders. Over the course of a week, she fed several small Western black widows with daily crickets while starving an equal number of big ones. The result was a hundred or so spiders, all of the same size, but half of which were very hungry.

She found that the starving spiders mostly spun sheet-based webs, and did so twice as often as the sated spiders. They opted for a different design - the "tangle-based" web. As its name implies, this structure is a chaotic, three-dimensional network of non-stick strands. It's a mess to behold - no distinctive sheet, no gumfooted threads, just a mass of silk with the spider in the middle.

Fortress vs. fine-dining

Other spiders can change the size or shape of their webs, but the black widows go a step further, in a way that makes them unique. They change the entire architecture of the web, adding and deleting features that have specific functions. Tony Blackledge, who led the study, told me, "Their webs aren't simply more or less of the same thing."

The webs have different purposes too. When Zevenbergen let small groups of crickets loose in the spiders' enclosures, she found that the sheet webs were far deadlier traps than the tangle ones. Regardless of their own body condition, spiders that were allowed to hunt on the sheet webs spun by hungry spiders caught more crickets, and they caught them faster and more efficiently.

Partly, the sheet web's efficiency is due to the lethal gum-footed threads. The sheet also plays a role, for it acts as a better transmitter of the vibrations of struggling prey, and it allows the waiting spider great manoeuvrability.

Spider spins web controls spider

So why would a black widow choose to spin the less efficient tangle webs? The researchers offer two possible answers. The simplest one is that the tangles act like a fortress, providing the spider with better defences against predators. With their stomach full of cricket juice, they can afford to divert their resources to protection rather than predation.

But the more interesting theory is that the spiders could be using the webs to regulate their own behaviour. Zevenbergen found that even the well-fed spiders attempt to capture prey if they're placed on the sheet webs of hungry spiders and as we've already seen, they're more successful at it on these deadly constructions.

Blackledge suggests that black widows are somewhat incapable of ignoring the vibrations of struggling prey and would effectively overeat if they didn't change their web design. He says, "The tangle-based webs help to keep [them] from expressing capture behaviors when they don't need them." So black widows change the architecture of their lairs to stop themselves from overeating. It would be like me bricking up my kitchen doorway if I'm feeling fat.

Reference: ZEVENBERGEN, J., SCHNEIDER, N., BLACKLEDGE, T. (2008). Fine dining or fortress? Functional shifts in spider web architecture by the western black widow Latrodectus hesperus. Animal Behaviour DOI: 10.1016/j.anbehav.2008.05.008