I'm pretty sure these are what are called stabilimenta, although I've only seen them look like a zig zag, rather than a zipper shape. There are a few theories for what their purpose is. They reflect UV rays, so that may attract insects, maybe for defense from wasps (I believe this the least), maybe as a bird deterrent, to prevent the web from getting destroyed by accident. Just like most things concerning spiders, no one really knows for sure.
nature_nate_17 on May 23rd, 2022 at 15:43 UTC »
the crack cocaine spider figured building webs was for suckas
Spartan2470 on May 23rd, 2022 at 16:57 UTC »
Here is a higher resolution version of this image.
Per /u/stone_hands here:
mdave52 on May 23rd, 2022 at 20:14 UTC »
Looks like he's trying to make a zipper for his fly.