# Consistency of Giant Spider Webs

Giant spider webs in fiction always seem very thin compared to the scale of the spider itself. (See: Hobbit 2) I would expect giant spider webs to be like cables, but that might not be the case.

My question is: Does the thickness and consistency of web scale linearly with the scale of the spider?

There must be some actual spiders that we could draw reasonable comparisons from.

Assume:

• Giant Spiders Exist
• I'm nowhere close to having enough information for an answer but here's a though. The weight of a spider grows with the cube of the size. The strength of a thread with the cross section of the thread (I think) and thus the square of the size. So if a spider is 4 times as big, the threads need to be 8 times as thick. At a spider 9 times the size, we're looking at a thread 27 times as thick. Commented Nov 17, 2014 at 15:57
• @overactor That would also depend on the size of the prey, but I would agree that the scaling wouldn't be quite "linear." Danny: What do you mean by "consistency?" The stickiness of the web? Commented Nov 17, 2014 at 16:00
• @Cragor I mean the thickness and overall appearance. Would it be braided like steel cable? Would it be a threaded mass of smaller strands? Commented Nov 17, 2014 at 16:02
• @overactor which, given these numbers are accurate, au.answers.yahoo.com/question/index?qid=20090228234222AAkTwcC, awesomely enough is still only around .027 - .108mm thick... Commented Nov 17, 2014 at 16:08
• A traditional web used to passively catch prey would be pretty useless if the strands were thick enough to be easily visible. And who says that the web needs to support the spider's weight? A large spider could like to eat a lot of small things while it hides in a corner. Commented Nov 17, 2014 at 20:46

Well the largest known web-spinning spider is the Nephila komaci. It gets to be about 5 inches and can spin webs up to about 3 ft. Otherwise you might notice most other large spiders don't spin webs. The Goliath Bird eater is the largest known living spider and it maxes out at about 12 in. and it's a hunter

Likely what is going on is that the silk needed to catch larger prey for the larger spiders becomes more easily visible to it's prey and thus is avoided. So REALLY large spiders with webs would be unlikely unless they learn to herd their prey or use them like fishing nets casting them out into the path. Most of the webbing you see is for the creepy factor and really doesn't have much based in reality. It appeals (claws) at our nature, to be caught in such a terrible helpless position.

Most likely a Giant spiders silk would be reserved for wrapping and storing it's prey if it would even have kept that ability as it evolved larger.

• Have a +1. Well stated, and I didn't know that about real-life large spiders. Commented Nov 17, 2014 at 16:14
• en.wikipedia.org/wiki/Darwin%27s_bark_spider Big web, big bugs? A web could be used for easy transportation up or down a slope, or across a body of water. Probably not for capturing the prey of a super spider, though, unless it is used to draw in larger carrion prey from the unappetizing smaller prey hung there as a lure. I can imagine a spider doing something like that, instead. The bark spider doesn't do it, but its web has the effect by capturing dozens of insects. Commented Nov 17, 2014 at 16:19
• Spiders use their webs for a lot more than getting prey stuck. They <a href="ox.ac.uk/news/… their webs musically</a> so that they can tell which string their prey hit and therefore where it is. Funnel web spiders create slippery webs that prey can't climb back out of. Trap door spiders just create a camouflage hinge and ambush prey. My favorite though hang in concealment, lure with pheromones and bola their prey. Commented Nov 17, 2014 at 19:20
• All great points! though most of those a spider wouldn't need them to be as thick as cables to do their job. Commented Nov 17, 2014 at 19:47
• Some spiders also use silk threads for flying. Commented Nov 18, 2014 at 10:44

I am going to compare a 50kg and a 1g spider with a silk radius of .003mm. The breaking stess would be about 1000MPa for an average spider. Lets say the giant spider manages to get this up to 2000MPa which is the strongest in the world.

Quick calculation shows that the 1g spider can hold about 3 times its body weight of a single thread of silk. How much would that need to be for a 50kg spider? Only .5mm

This calculation is enough to show that in theory a spider with thin strands (say 2-3 mm) would be possible.

In practice a spider often uses several strands of silk together. I have my doubts about the accuracy of the .003mm radius. Basically, however, this shows that a giant spider could still possibly use silk like a small one iff it were worth it. I don't believe that it is an evolutionarily favorable path which is why we don't see it. In your world, however, you could possibly make it work.

Please note that there are likely many nuances that would make this more difficult for a larger spider. I don't want to lift myself by such a small part of my body. The thin diameter likely increases mechanical strength more than bulk properties would imply. There is likely evaporation which occurs in the thin strands which would not occur in a thicker one. There is potential to focus stress on a small region of the silk. Nevertheless, spider silk's properties are incredible and, while they would need to be thicker for a giant spider, they would not really need to be THAT thick.

Spider silk has a tensile strength of 1.3 GPa, or 132,563 g/mm^2. A "giant" spider could be described as any spider larger than 175 g, which is the record for the world's largest real spider. A 175 g spider would need silk with a cross-sectional surface area of greater than 1,320 μm^2 (0.001 mm^2), or a diameter of 41 μm (0.041 mm). For a 62 kg human sized spider, the silk would have to be greater than 771.68 μm (0.77 mm). Shelob from The Hobbit was 2.44 m long and 5.18 m across. This would make her 700-1,500 kg, and would require silk 2.6-3.8 mm in diameter.

Just because you scale up the spider, does not mean you can scale up its silk glands the same way. Spider silk is essentially rapidly printed out strand of quickset glue, if you scale it up, there is a very big chance it will not come out as a strand, but as a puddle.

The more likely scenario is that a giant spider would have hundreds of smaller silk glands, and instead of a singe thick cable, it would produce a kind of a thick yarn or rope made of hundreds of regular sized strands twisted together.