1. From the side, the web is a round sheet with tangled silk on both faces
The Joro spider (Trichonephila clavata) is an orb weaver, a spider that builds a round web. Wikipedia describes its web as having three layers: the neat orb in the middle, and an irregular tangle of silk in front of it and another behind it [3]. Researchers who study the genus call these tangles barrier webs [1]. Only the orb in the middle is the sticky trap that catches food. The tangles are not described as sticky, so they probably do other jobs, though what those jobs are is not fully settled.
The adult female is about 17 to 30 millimeters long (roughly 0.67 to 1.2 inches), and the male is much smaller, at 4 to 8 millimeters. Adults are most visible in autumn [3]. These size figures come from Wikipedia, a secondary source, so treat them as rough estimates.
2. The tangled silk may work as a shield against attackers
The best field observations come from a close relative of the Joro spider, Nephila clavipes, studied by Linden Higgins at the University of Vermont. She writes that the barrier silk is very stiff. She has watched large tropical damselflies attack young spiders and simply bounce off the barriers. She also once saw a tanager (a bird) attack a mature female and fail, because the bird could not fly through the barriers. But birds can fly through the barriers of a young spider [1].
These are observations of one researcher watching a different species, not an experiment on Joro spiders. They show that the barriers can stop some attackers some of the time. They do not show that this is why the spiders build them. Other ideas, such as an early warning when something touches the silk, have also been suggested, but they remain guesses.
3. The tangled silk may also keep the round web pulled tight
Higgins points to a second possible job. If you look closely, she writes, the barrier is connected to the spot where the spider sits. Maintaining tension in the web may be a second function of the barrier, she suggests [1]. A simple way to picture it is a tent: the guy ropes do not keep rain out, but they hold the cloth tight.
She also notes that insects flying through tangles of silk can become disoriented, which might help the spider catch more prey. That finding came from other researchers working on a different species [1]. Her wording is "may be," so this is a possibility, not a proven result for the Joro spider.
4. Inside the round web, a thread that other spiders remove is kept and used for repairs
The round sheet itself holds another surprise. Orb webs have sticky spiral threads that catch insects. Between them runs a second spiral that is not sticky. In most orb-weaving spiders, this non-sticky spiral is only a temporary scaffold, used while building and then removed. A 2025 paper found that in the Joro spider it stays as a permanent part of the web [2].
The paper reports that the non-sticky spiral forms strong, wrapped junctions where it crosses the other threads. This contrasts with the loose, droplet-based grip of the sticky spiral. The authors suggest these junctions help the web stay stable. When part of the web is torn, the spider repairs it by copying the loop pattern of the original web [2]. The spider seems to treat its web less like a net thrown away each time and more like a house that is mended. This is a finding about the round web, so it does not show what the barrier layers do.
5. The silk looks gold in sunlight, and nobody knows what that does for the spider
The Joro spider belongs to the group called golden orb weavers. Their silk is yellow, and in sunlight the web can look a rich gold [3]. Higgins writes that the silk of her study species is commonly yellow, and that a researcher named Cay Craig found spiders building at bright edges of clearings spin more intensely colored silk [1]. What the color is for, and why it differs from place to place, is a question these sources leave open. The Japanese dictionary explanation I first found says the silk turns golden as it ages through oxidation, but I could not confirm that in an English-language scientific source, so I leave it out.
6. How to check the three layers yourself, and what to read next
If you find a Joro spider's web, look at it from a safe distance. View it from the front, then from the side, and look for the loose silk in front of and behind the round sheet, without approaching or disturbing it. Do not touch the web or the spider. Wikipedia says orb weavers are not known to inflict medically significant bites [3], but there is no reason to bother it. If you can, sketch the web, then come back the next day and draw it again. Did a torn spot get mended?
For the original sources, the Higgins page is short and easy to read. The 2025 paper on the non-sticky spiral is written for researchers, but its abstract is a good place to start, and its figures show the junctions up close.