Why do spiders always make webs on car wing mirrors?

Thanks to Geoff Oxford from the British Arachnological society for the answer!

Will - It's a very common site, certainly here in the UK, to come out to the car in the morning and find the wing mirrors decorated with spiderwebs. But what is it about the wing mirror that's so irresistible to these spiders? I asked the Honorary Secretary of the British Arachnological Society, Geoff Oxford.

Geoff - The wing mirror has a structure which is ideal for web spinning spiders. The wing mirror sticks out, so you have angles there so that the foundation of the web can be scaffolded on. And also a wing mirror sticks out into the air as the car is driving along. And so there'll be almost a dead area behind the wing mirror where the wind will be much reduced compared to if the spider was anywhere else on the car.

Will - So wing mirrors are an excellent foundation and windbreak for the web, but what about the spider? How could it possibly survive such speeds?

Geoff - Well, the spider itself will not be in its web while the car is belting along at 70mph. The wing mirror housing has a lot of space in it. You've got the mirror itself, but the mirror itself has a gap round it so that you can adjust its gaze, as it were. And so the spider can get round the edge of the mirror itself and into the housing where there's plenty of room for it to set up home quite happily.

Will - There's a thought then. Your wing mirrors are most likely host to a spider or two. but the web itself isn't able to hide. So how does that fare?

Geoff - Silk is a remarkable substance. I mean, it's incredibly elastic, incredibly strong. And what's interesting is that it combines those two properties. Honed over millions of years of evolution so that the web of a spider is incredibly resilient to wind and rain and other forces acting on it

Will - Pretty well then. But were you to remove it, how much of a disservice are you doing to your hitchhiking spider?

Geoff - If you take the web away or if the web is destroyed because you are going over the speed limit, then the spider will just build another one. In fact, they rebuild their webs every day or so because the orb web, the common web that you see made on wing mirrors, traps insects because of glue droplets on the spiral of silk within the web. But of course, glue droplets are going to get un-gluey, as it were, very quickly because of pollen and dust in the air. And so orb web spiders take their webs down, eat the silk, recycle the amino acids in the silk, and build a new web every day or so. So if we do it for them, they just build another web.

Will - So maybe you don't feel too bad for having to remove the web, even if you are depriving the spider of a meal that day. Thanks very much to Geoff Oxford for the answer, and Simon for the question.