A focus on jumping spiders

This is probably my penultimate blog post of the summer holiday, and I’ve decided to show you a very small, common and humble type of creature found in most gardens all over the country. They are, however, a lot more interesting than most people think.
Here’s a summary video to kick off, but if you want to read into the detailed science behind all of the remarkable behaviour, keep reading, I think it’s really interesting.

I’d say that they are the only group of spiders that can feasibly be classed as cute – they’re small, their legs aren’t long, they’re not too hairy, and they have very big eyes.
In fact, their vision is very well developed. They need to be able to spot prey and gauge the distance away, so that they can jump accurately onto the insect.
Although you may think that with all four of those big, anterior (front) eyes, they must use binocular vision (or quadocular vision) to perceive depth. In fact, this is not the case. The eyes are too close together for that.
Neither do they use motion parallax (moving their heads, and seeing how quickly different objects move across their field of vision – just like an owl)Instead, they use image defocus. The spider’s retina contains four photo receptive layers (we only have one). The front two contain ultraviolet-sensitive cells, and the the deeper two contain green light-sensitive cells.
An image is focused by the lens onto the deepest green-sensitive layer of cells, where a clear image is produced. However, the image on the layer in front will be slightly blurred. By comparing these two images, the spider can determine how far away a prey item is. Scientists tested this by shining different coloured lights onto flies. The jumping spider only successfully jumped onto flies bathed in green light – it wasn’t able to accurately judge the distance to the other ones in an absence of green light.

 

However, they can also see other colours too. They don’t have red and blue-sensitive cones like we do, but instead have a layer of filters placed in front of the photoreceptors. This allows them to see in a third colour channel – so together with the ultraviolet and green receptor cells, this gives them the ability to see a wider colour spectrum than we can!

And let’s not forget their extra two pairs of eyes, the smaller ones that nobody notices round the back. These give it a 360 degree angle of vision! These posterior medial and posterior lateral eye pairs are more motion sensitive – so they can sense predators approaching. You can test this by wiggling you finger behind it, and it will most likely turn around and give it a hard stare.

If you are watching a spider, and you suddenly find that the spider is watching you, then it is probably a jumping spider. They are particularly human-aware, and very curious. They have also been known to be captivated by films played on screens.


So their eyesight is pretty good. But jumping is in the spider’s name, and is an even more significant feature than its eyesight.
Jumping spiders as a family can jump between 10 and 50 times their bodylength (!!!), which isn’t too far off fleas, who can jump 80 times their own height.
How do they jump so far? They don’t have very muscular legs like a grasshopper.
Instead, they use a hydraulic system whereby blood is pumped into the back legs at high pressure, which extends them very quickly, producing the power to catapult them forwards.
But what is even more spectacular, in my opinion, is how they can hunt on vertical walls or upside down on ceilings, and jump horizontally onto prey, defying gravity and landing back on the wall.

I’m not sure how this works, but from the very little I have managed to find on the internet it looks like they could use one of two techniques.
When they jump, they attach a line of silk on the wall behind them to act as a sort of safety rope.
It is possible that when they jump, they swing on this silk and it brings them in an arc back towards the surface.
This relies on the silk being exactly the right length, so the spider would have to stop producing it at the crucial moment mid-jump.
It is also known that spiders, such as jumping spiders, manage to walk up walls and ceilings in the first place because the tiny hairs on their legs employ Van der Waals forces. Although walls are neutrally charged, the hairs of spider’s legs are not. This charge causes the electrons in the wall to either be attracted towards the spider or repelled, depending on whether the spider’s charge is positive or negative. If the charge is negative, it will repel the negative electrons, leaving the positive nucleus closer and it will be attracted strongly to that. If the charge is positive, it will attract the electrons closer than the nucleus, and the spider will be strongly attracted to the electrons instead.

In this way, jumping spiders can produce an adhesive force of over 173 times their own body weight – so they’re very unlikely to fall off!
But is it enough to attract the spider back towards the wall when all of the tiny hairs, known as setules, have left contact with the wall completely?
I don’t know, but I would guess so. If anyone can confirm or deny this please let me know, comment below!

Male Jumping spiders are also famed for their fascinating dancing, which they use to grab the attention of the females. I’ve seen videos, and they are absolutely stunning. Scientists have placed them on special equipment that detects the vibrations they make through the ground when they dance, and it coincides very neatly with the dance moves, suggesting that it is part of the display as well.
Have a look at this video: https://www.youtube.com/watch?v=d_yYC5r8xMI
I’d absolutely love to film this behaviour for myself one day.

A final thing to remark upon is the jumping spider’s intelligence. Scientists, after a series of experiments, discovered that they can plan routes towards prey, featuring detours.
They can also see a prey item and memorise its location, before approaching it via detours around water and other obstacles while it remains hidden – in other words, they plan ahead; they think before they act.

So all in all, jumping spiders are pretty incredible creatures! I hope I’ve given you some food for thought next time you see one…

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