What are the Differences in Spider Feet?
Spider Feet For Different Purposes
For Valentine's Day my friends at the Smithsonian featured a cute little spider called the pink toed tarantula. This got me thinking about spider feet. They don't really have toes like you and I do, (though the idea of spider tube socks with toes is funny) but they do have special modifications that help them get about. The feet of spiders can be lumped into two general categories based on how the spiders make their living. There are web building spiders and wandering spiders.
The demands of weaving a web and walking a continual tightrope, are very different from those of a spider that wanders, runs, and leaps on prey ( If you haven't read my post on spider legs you should check it out too).
The First thing to know about spider toes is that all spiders have hairy feet (which is why so many people find them creepy), but it's the arrangement and function of these hairs that varies.
Hunting spiders tend to have very fuzzy feet with special hairy pads on them. They can also have a pair small claws (called tarsial claws) which are used for gripping and holding. In between the claws you can find a dense tuft of hairs called the scopulae or scopula pads. These pads are used for gripping and holding. The hairs of the pads are covered in even more hairs called setules, which gives each hair the appearance of a bottle brush.
This "bottle brush" effect creates a massive amount of surface hair-ea (I couldn't pass up a pun) for gripping wet or dry surfaces. The setules adhere to surfaces through Van der Waals Force. Very simply, their hairs carry a molecular charge that help them stick to things. This is similar to how geckos stick and hold on to walls.
One study found that in jumping spiders there could be over 624,000 setules per hairy foot pad. For you math hounds this means they increase their surface area contact, through all 8 legs, to 1.06 x 10^11.
The main reason that Van der Waals Force works is because of the spider's tiny size. If they were the size of a VW bug, the spider would be too large to create this type of molecular attraction for sticking to things. The same is true for large spiders, like the zebra tarantula, which has overcome this difficult by having spinnerettes (for silk secretion) on its feet! It secretes silk from its toes, which it uses to help climb wall and hang on to slick surfaces. It leaves cute little footprints of silk behind.
WEB BUILDING SPIDERS
Web building spiders also have the characteristic hairy feet, for helping them climb, but they have three tarsial claws instead of two like many wandering spiders. See the image below which shows how the claws are arranged. There are two toothed claws above, and one smooth hook in the middle.
The two larger hooks, which are barbed, work well for walking and climbing surfaces while the smooth middle hook is used for web walking, trapping the strands of silk and sliding them along. Web building spiders also have curved hairs on the tips of their toes next to these claws which helps keep the silk from sticking to their feet. This is, in part, why spiders don't stick to their own webs. (You can read a great paper on this here). One common myth about spiders that spin webs is that they have oil on their legs that keeps them from sticking. This isn't true, and there is no scientific evidence to support the claim.
Here's a fun video from Mentalfloss about why spiders don't stick to their own webs:
Spiders have many amazing and unique adaptations and spider feet are no exception. One of my favorite classes to teach is on the differences between wandering spiders and web building spiders. If you're looking for ideas for your classroom you might try doing comparisons of the two groups. Spider feet are just one example of their differences. I would often start off classes by simply showing the pictures of the two types of feet, and then having students try to guess which is the web building spider and which is the hunting spider. They would then defend their choices using logic and reasoning to support their ideas. At the end of the class I would ask students to design their own spider and a part of the assignment would be to include the right type of feet for the type of spider they had chosen.
Even though it's still winter here in the Eastern US, Spring is on its way. Take the time to check out our hairy friends and try to get a good look at their toes, they are quite fascinating, and if you find one with toe-socks, I REALLY want to know about them!