Robustness in legged systems

As part of the graduate programme at the RVC (Royal Veterinary College), we give talks every year of our PhD in the Internal Seminar series. I was due to give mine today – but as I sit here snotting and coughing over my keyboard, I can’t help but feel that the people of the RVC will be glad that I had to cancel.

Yes, I have caught what seems like yet ANOTHER horrid cold. Snotty, coughy, feverish – the full works. And most upsettingly, I’ve lost my voice. So no seminar talk for me today. But instead, I thought I’d write about all the things I was going to say! It’s pretty much just an overview of my PhD so far and its aims, and as I keep promising a blog post on such things, it seemed like a good opportunity to do so!


So, first: the full title of my PhD – “Robustness in legged systems: an integrative approach.” It’s a nice title, because it pretty much left it up to me what ‘legged system’ I wanted to explore. As most of you will know, I chose the very-many-legged spider as my study animal. The “integrative approach” bit means that I bring together stuff from different disciplines to talk about robustness – in my case, biomechanics (spiders) and robotics!

Robustness is a tricky thing to define, both in layman’s terms and technically. Applying it to what I do, I tend to think of robustness as a measure of resistance to change, or an ability to cope with or adapt to change. For example, spiders have pretty robust locomotion (as do all legged animals) – if obstacles or bumpy ground are introduced to their environment, they have no problems running over it. Equally, if they lose a leg, they (generally) have no problems adapting their gait. And when talking about robustness in legged robots, these are the kinds of things I think about, too.

Why spiders? Apart from the fact that they’re awesome, you mean?! But really, there were several reasons for studying spiders…

  1. As a species, they live across all terrain types, so they’re highly adaptable.
  2. They can naturally lose legs in a sort of self-amputative process called autotomy, meaning that I can study how their gait adapts to missing legs. We know it must adapt somehow, because although some spiders can grow the legs back (depending on their age & species), sometimes they don’t grow back – so the spider must be able to move around for the rest of its life with less than eight legs. I’m super curious as to how they can do this, especially immediately after the leg has been lost.
  3. Spiders are pretty easy to get hold of! In the UK, wolf spiders are easy to spot in grassy areas in May/June, and garden spiders are abundant at the end of the summer in their massive webs. Always make your PhD life a bit easier if you can!!

Spider gait: Spiders move their legs in alternating tetrapods. This is quite difficult to explain in words, so here’s a picture to help me:

SpiderLegs2

The blue legs form one tetrapod, and the red legs form the second tetrapod. When a spider moves, its most basic gait is simply moving all the red legs together, and then all the blue legs together. See how they’re diagonally criss-crossed over the body so the centre of mass is constantly supported by the legs. This pattern was well described and discussed by Donald M. Wilson in his 1967 paper about tarantulas. In this paper, he also looked at the effects of losing two legs from the same tetrapod – how does the spider adapt its gait? Of course, in 1967 technology to quantify this was limited, and so the paper is more descriptive than data-heavy.

So the aims for my PhD, with the age of high-speed cameras and automated tracking software, were to look at this in more detail – how exactly do spiders change their gait when they lose legs from the same tetrapod? How do their legs rearrange (if at all)? How long does it take them to adapt – do they switch to their ‘new’ gait straight away, or is there an intermediate gait first? And, finally, how can we use this knowledge to improve our legged robotics?

What have I done so far? I’ve collected a large dataset from the common garden spider, Araneus diadematus. These spiders predominantly move around on webs but are also able to move over ground. I looked at how they adapt their gait to missing legs by inducing autotomy in two legs within one tetrapod. High-speed footage taken from above as the spider runs across a flat perspex arena allows me to track individual legs, producing XY coordinates. These can be used to work out what gait the spider is using.

Digitising the high-speed videos like this is proving to be an arduous task – spiders have a lot of legs! So I’m still in the process of doing this. But preliminary results from two of the spiders look like they might be doing some funky things with their front & hind legs… Watch this space!

What am I doing now? Currently I’m working on collecting a dataset from wolf spiders – repeating the same experiments as with the garden spiders, but with added Lego rough terrain! (See my previous post if you’re interested!) This is so I can directly compare the two species. I added the rough terrain with wolf spiders because they’re hunters, actively hunting their prey down on the ground, so they should be well adapted to moving over rough terrain, and I wanted to see how they manage it after autotomy. As well as this, I’m still digitising the garden spider work, and preparing for a poster presentation at the Society for Experimental Biology’s annual conference this summer.

The final steps of my PhD are to learn from the spiders and apply it to robotics! I’m currently awaiting delivery of a T8X spider robot, which I can’t wait to get my hands on! This particular robot is marketed as a robot for everyone – it’s ‘plug and play’ so amateur roboticists can play straight from the box, but the programming is also accessible so you can hack into it to change different things – such as leg timings. This is what I’m hoping to be able to do – to design a new spider controller based on the data collected in actual spiders, that allows the robot to adapt its gait if legs are missing or damaged – to become robust to leg loss. This has applications in robots that are used in search & rescue situations, in the armed forces, and even perhaps in space exploration. Anything that lets the robot carry on its merry way without having to be taken in to a workshop to be fixed is a bonus!

So, that’s it! That’s where I’m at so far, and what’s left to do (eek!). Apologies to the people at the RVC who organise the seminar series – I’m not sure yet (at time of writing) whether my talk will be reallocated to another date, but thanks for understanding.

I genuinely am really looking forward to writing this up in my thesis!

Thanks to BBSRC for funding me, and to my supervisors for putting up with and getting me through my numerous blips so far!


If you enjoyed reading this, you might also like: Spiders and Lego

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Things I Don’t Know…

… about spiders.

Many months ago, the kind people over at Things We Don’t Know asked me to write a guest blog post for them. Due to unforeseen circumstances and being very busy on my internship, I unfortunately haven’t been able to write anything for them yet.

Things We Don’t Know (TWDK) focusses on, well, just that – explaining things that scientists don’t know (yet), in an accessible way for everyone to understand. So of course, my post will be on things we don’t know about spiders! I do intend to still write the post for TWDK, so this is going to be my way of getting my brain warmed up, thinking about some of the things I don’t know about spiders. Some answers may well be out there in the literature that I haven’t encountered yet, so please leave a comment if you know any answers! And some things I know are out there, and I just haven’t got round to reading about it yet. It’s a good exercise for me to figure out where the gaps in my knowledge lie, and what I should look up next! I will also try answer these questions in future blog posts, once I have the time to trawl through the literature again.

Things I Don’t Know About Spiders:

  1. How they make their webs. Now, this is something I know is out there – there’s a section in one of my textbooks, for a start. In my PhD so far I’ve been focussing on how spiders run across ground, so although webs are what people immediately associate with spiders, I haven’t learnt much about it yet! One thing that people (particularly my Grandad!) keep asking is how they start the web off – often there is one tiny strand of silk joining the web to a far-away tree or lamp post, and to be honest I don’t know how they do it – just jump I suppose!
    Araneus spider in a web (from pdphoto.org)

    Araneus spider in a web [from pdphoto.org]

  2. How they breathe. Again, this is something I know is out there, I just haven’t got round to look it up yet. After a very brief Google, it seems that different species breathe in different ways. I know there’s a species called a Diving Bell Spider that can live and breathe underwater for hours – now that’s impressive! I will look this up in detail soon, because again it’s a question that a lot of people ask me. 
    A pair of diving bell spiders underwater (L = female, R = male) [Credit: Norbert Schuller Baupi on Wikimedia Commons]

    A pair of diving bell spiders underwater (L = female, R = male) [Credit: Norbert Schuller Baupi on Wikimedia Commons]

  3. What exactly it is that makes people scared of them. Lots of people are arachnophobic. Hundreds of people. No arachnophobe I’ve met can really put their finger on why, though it seems to be a combination of their fast scuttling and sudden stops. It’s definitely something to do with their motion, though what exactly it is I don’t know! Perhaps an arachnophobe reading this could give me an answer? I’ve never been bothered by spiders so perhaps this is something I’ll never know…
    Jumping spider, Heliophanus sp. She's just here because I love jumping spiders. Seriously, how could you hate this little lady? Look at her big eyes and eyelashes. So cute. [Credit: Lukas Jonaitis]

    Jumping spider, Heliophanus sp. She’s just here because I love jumping spiders. Seriously, how could you hate this little lady? Look at her big eyes and eyelashes. So cute. [Credit: Lukas Jonaitis]

  4. How spiders move with missing legs. This fourth and final item in my list is of course the subject of my PhD. This question is loaded with lots of other questions, really – how they move in terms of their gait, their energetics, and how efficient they are at escaping predation and catching their own prey with missing legs. This has been touched on in the literature, particularly the latter questions looking at how they survive with missing legs, but what I’m really interested in is their biomechanics – how does the way they’re ‘built’ allow them to cope with such a drastic thing as losing a leg? How exactly can they still run, and run quickly? Do they spread their limbs out in time over the course of a stride cycle to compensate, or do they run in  the same way as always but with a gap? Do the answers to these questions depend on which leg is missing? 
    A Fishing Spider with two legs missing. [Credit: Flagstaffotos]

    A Fishing Spider with two legs missing. [Credit: Flagstaffotos]

As you can probably tell, my TWDK post is likely to be on this last point, as there are lots of things we don’t know about how spiders move without their full compliment of limbs!

This has been a really useful exercise in figuring out the things I need to read up on, particularly because people ask me lots of questions once they know I study spiders, and I feel like a let-down when I can’t answer them! I will write posts on the answers to #1 and #2 when I get back into PhD-land and have a bit more time to read about them. And I know that during the remainder of my PhD I know that more questions will emerge than answers…

Thanks for reading – if you have any answers, or any questions of your own, just leave a comment!

Spiders self-amputate legs after wasp stings

Firstly, once again I find myself apologising for not writing more regularly. I WILL become better at this! I’ve been somewhat engrossed in writing up a report for my dreaded first year PhD appraisal, but on the way I have been fortunate enough to stumble across some truly remarkable literature on spiders and their amazing abilities – plenty of ammunition for future posts!

So what’s this you say? Spiders getting stung by wasps? Well it turns out that when venomous insects get trapped in spiders’ webs, in their struggle to get away they can end up stinging the spider. Although spiders are protected by their outer exoskeleton, if the insect is lucky enough (or cold and calculated enough) to aim their sting for the vulnerable joint membrane between the joints on the spiders’ legs, the sting can penetrate and venom is injected.

In this particular study I was reading (Eisner and Camazine, 1983), the authors saw this happen out in the field, with orb-weaving spiders (Argiope spp.) being stung by a small ambush bug, Phymata fasciata. This bug can give noticeable stings to humans, too. On seeing the struggle between spider and bug come to a halt, they noticed that the spider had been stung – and it immediately autotomised it’s leg.

Argiope aurantia

Argiope aurantia

Autotomy is a really remarkable ability that most spider species possess. Essentially, they are able to voluntarily ‘shed’ or amputate their own legs, as a defence mechanism if they’re grabbed by a predator, or if they get stuck in one of their moult cycles. It’s similar to the way that some lizards can pop their tail off if you grab them by it – sacrificing a limb in order to escape and survive. Sometimes spiders can even grow the legs back – but I think I’ll save that for another post!

So these spiders that had been stung by the ambush bug got rid of the injured leg, presumably to stop the venom spreading to the rest of the body and killing the spider. But, the authors asked, is this because of the physical puncture of the sting, or because of the injected venom? So they went to the lab and set up a series of tests. Firstly, they compared the reactions of spiders that had been stung by the bug, versus those that were punctured in the same part of the leg by a sterilised pin. Turns out that the pin-punctured ones didn’t shed their legs (well, only 1 out of 8 spiders autotomised the leg, compared to 10 out of 10 stung by the bug). It seemed then that it was indeed the venom that caused the spider to do this, so the researchers were curious as to what venoms, or components of venom, induced this reaction.

Wasp (top) and honeybee (bottom) venoms were used.

Wasp (top) and honeybee (bottom) venoms were used.

They selected wasp and honeybee venoms to test on the spiders, and four major components of these kinds of venom: serotonin, histamine, phospholipase A2, and mellitin. They also used a few inactive components of venom, such as dopamine and adrenaline. Along with the bee and wasp venoms themselves, these were all injected into legs of spiders, at concentrations comparable to those found in venom, and compared to control injections of sodium chloride and saline. [Here I have a bit of an issue with the way the experiments were carried out – they didn’t use separate spiders for each venom, they used separate legs, using up to four legs per spider – which I think makes the results less reliable. It doesn’t say what order the substances were injected in, and I have a theory that spiders may be reluctant to shed legs after they’ve lost a certain amount – so I think it would have been better to use separate spiders per injection.]

The results showed that the bee and wasp venoms caused spiders to autotomise their legs in approx. 70+% of cases, as well over 48% of cases in the active venom components. I think this is a really cool adaptation – how many of us have been stung by wasps or bees and just wished we could lop off the offending limb or finger to stop the pain?! And the amazing thing is, spiders shed the leg just seconds after being stung. Spiders really are quite incredible!