Four or five years ago, before a copse of little ash trees overran it, the low place between the softball field and the tennis court in the Unity park was a house of horrors. At that time a strip of thick, tufty grass, willow herb, New England and bushy asters, milkweed, touch-me-nots, goldenrod and assorted other brush grew behind the outfield fence. You could walk right past it without noticing that thin, glinting cables were connecting the leaves and stems of practically every plant. Spiders had taken over the world.

The easiest ones to spot were the black-and-yellow garden spiders, aka Argiope aurantia, because the females are large and sit right in the center of their often expansive orb, or spiral-shaped, webs. Also abundant were banded garden spiders, Argiope trifasciata, and here and there were cross spiders, Araneus diadematus. Less conspicuous were the arabesque orb weavers, Neoscona arabesca — smaller, usually a reddish tan color with ornate markings, and skittish. They dash to shelter under a leaf or stem joint when they detect looming movement by, say, a gigantic bird or human.

If you got down and peered through the grass jungle, orb web after orb web appeared, sometimes diminishing in a sort of painter’s perspective into the leaf-tangled distance and creating a deadly obstacle course for gnats and dragonflies. Even if you missed the black-and-yellow female herself patiently waiting, your eye might be drawn to the thick white zigzag of silk at the center of her web.

This additional zigzag is called the stabilimentum, and different species of orb weavers construct it in different forms. There’s an up-and-down, or linear pattern of squiggles, like the adult garden spiders make; a cross shape; a roughly circular weave; a thick spiral strand curling around the center; tufts of silk placed along the web radii; and also debris, where prey remains or egg sacs are left scattered around the web.

So what does the stabilimentum do? The scientists don’t agree, and even its technical name is not quite accurate.

The term “stabilimentum” was devised early on by arachnologists who thought the decorations — as they are also, rather interestingly, referred to — were engineering features that helped stabilize the webs. But studies over the last few decades of ways this might work indicate it’s unlikely the stabilimenta serve any stabilizing purpose.

It might be an anti-predator device. Tufts or debris, for example, may act as a decoy away from the edible spider. Or silkwork at the center may create background camouflage that makes the spider’s size, shape or location on the web uncertain to a predator. It may also do the opposite, making the web more conspicuous so that birds, for example, spot it before crashing through and damaging it. And for some spiders, the stabilimentum provides shade from the hot sun, though most researchers think this is probably a benefit the spiders noticed after the stabilimenta were part of the routine. None of these explanations is conclusively supported by studies.

Another possibility is that a stabilimentum improves the catch rate. How this works is also debated, but it involves the fact that many spiders and bugs can see wavelengths of light in the ultraviolet end of the light spectrum that we can’t. Some studies have shown that spider silks reflect UV light better than they do white light, and the decorations often reflect it even more. This would make the silk appear darker to an insect than it does to us, and so the web would blend into the background vegetation and be harder for the wandering insect to avoid. This UV reflectivity could partly explain why stabilimenta are apparently not built in night webs, and why spiders that normally spin a stabilimentum catch more bugs than they do when they leave it out.

Other factors that are not well studied may be involved, too. Different orb weavers spin webs at different orientations to the compass and angles to vertical. Argiope florida, for example, which lives in the Southeast, builds its web due east-west at a 19-degree incline to vertical. Is this done to improve the catch? Probably, but, like the impact of UV reflectivity, this theory is not conclusively supported by evidence.

The arachnologists routinely refer to the stabilimenta as “decorations,” apparently forgetting sometimes that it’s a metaphor, not a literal description of spider behavior. Most orb weavers have poor eyesight, as spiders go, so while zigzag centers suggest to the human eye a whole other dimension of psychological reality, the spiders don’t have the neural works to recognize it. As far as anyone knows. So the implication that some sort of arachnid aesthetics is at work is also unstable, scientifically speaking. The most you might hypothesize about a stabilimentum’s attractiveness to spiders is that it could provide a larger surface area for female pheromones at mating time. But so far, one arachnologist told me, there’s no evidence this is actually happening.

Maybe different stabilimenta serve several of these purposes at once. No one knows for sure. I don’t know, either, why the bottom suddenly dropped out of the orb-weaver population at the park one summer. It happened after one of those springs when the snow was still 2 feet deep in mid-April, so maybe the prolonged cold caused an arachnid catastrophe that killed off the spring crop of spiderlings. Just an idea, no firm evidence. When the ash trees took over, the willow herb, asters and goldenrod got crowded out, too, and the horror show had packed up and left. Much to the relief of the dragonflies. Probably. Maybe.

Dana Wilde lives in Troy. You can contact him at [email protected] Backyard Naturalist appears the second and fourth Thursdays each month.

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