How Many Knees Does a Spider Have?

How Many Knees Does a Spider Have?

Female jumping spider, look at those legs! (Photo: Wiki Commons)

Spider Knees in the Know

I had a reader e-mail me for a 10 year old student who wanted to know exactly how many knees a spider has.  If you Google the answer you get a web page that is aimed at kids and has the wrong answer as the first choice. This always infuriates me because they offer an easy, off-the-cuff quick answer that requires absolutely no thought or research. I hate it when people play down to kids and don't do the work required to give an accurate answer. So here's my answer.

What is a knee? 

If you want to answer the question, "How many knees does a spider have?," first you need to know what a knee is right? In animal/human terms a knee is a joint whose focus is a bone called the patella. The knee joint connects the bones of the thigh (sing it with me now) or the femur,  to the bones of the lower leg which are the tibia and fibula.

Knee Joint (Photo: Wiki Commons)
So, in mammals a knee is a joint that is made out of bone and it connects the upper leg to the lower leg. On to spiders.

Let's Start With Spider Anatomy

This is where I think the other web pages have everything all wrong. Their answer to our original question was that spiders have 48 knees or "six joints on each" leg. Now you tell me, does this make any sense whatsoever?   How in the world can a living animal have 48 knees?  This would mean that the animal would have 48 sets of long bones in its leg and 48+ short bones too (double that if it has a tibia and fibula). This is just weird. Here's a case of not believing everything you read on the internet. Let's look at a diagram of spider leg anatomy.

Spider leg (Image: University of Kentucky)






In the diagram above you can see that a spider's leg has 7 major segments: 

  • Coxa-attaches the leg to the body
  • Trochanter-swivels
  • Femur-like your long bone, gives structure and support
  • Patella-the famed knee cap!
  • Tibia- the short leg bit
  • Metatarsus- like your foot bones in one long bit
  • Tarsus- like your toe bones in one
  • Claws- like your toe nails on steroids

Even if you are generous and call all the joints where leg segments meet "knees" there are 7 of them, not 6, which would mean that there are 56 total and not 48. This is still silly, because you can't tell me that a spider has a knee on its foot. Additionally, there is ONE accepted patellar region on each leg of a spider, and that sits between the femur and tibia. This means that a spider has exactly 8 knees.

Garden spider (Photo: Wiki Commons)
Interestingly, these eight knees are related to a very specific gene called the Dachshund gene (dac).   Scientists from Oxford Brookes University studied this and determined that:  "The acquisition of a novel function, or neofunctionalization, protects duplicated genes from redundancy and subsequent loss, and is a major force that drives adaptive evolution (Molecular Biology and Evolution, Volume 33, Issue 1, 1 January 2016, Pages 109–121)."   This is science-speak for how genes duplicate themselves to turn on and off body features, like adding a patella or knee-cap onto an embryonic spiderling in an egg. Cool eh? You can read the paper here.

Do Spiders Have Boney Knees? 

No, spiders don't have bones, they have a hard exoskeleton made up of material called chitin (kite-in) which is about the same as the stuff your fingernails and hair are made of. I've already written about how spider legs work, and their hydraulics if you want to know more, just click here. This means though, that if you're a purist and you require that for a knee to be "knee" it must be bone then spiders do not,  in fact,  have true bony knees. They have chitin-ey knees, which I'm sure do not knock together nearly as well.

In Defense of Spider Knees

The patellar region of a spider's knee has been heavily studied by researchers because they want to know how to model machines, rescue robots, and even ambulatory aids for humans based on spiders.   Spiders and humans aren't all that different. This, in part, is why blowing off an answer to a question like, "How many knees does a spider have?" upsets me, because it dismisses how important and similar spiders are to us and our own human nature. Please always remember to check websites for accuracy when you're looking for answers online.

Thanks to my readers for this great question, and keep them coming. If you have a question, or want to know more on a topic then please e-mail me: