General arthropod question

[Warren Beattie]

Sorry this isn't about tarantulas in particular.

A friend and I are having a... 'discussion' about arthropod limbs.
His view is that arthropods have 6+ limbs, because due to their generally small size, they wouldn't have time to check themselves if they had any less and tripped. He used the choice of six legs in robotics (for stability) as his reasoning.

I'm less than convinced; but still, I'm not sure what the reason is (if any) for arthropods having more limbs than vertebrates. Can anybody more learned shed some light?

[Eleanor]

That is, in fact, an interesting biological question which may get a better response from a biology teacher/lecturer. Find one and ask them. I don't know the answer (if anyone does), but I can make some guesses.

Firstly, your friends first guess (you will be pleased to hear) is definitely off-track. Small size won''t make you any more likely to trip! Comparisons with robotics and stability may be more sensible, but remember one thing: we still haven't managed to create any form of artificial intelligence which can control a 'body' with limbs anywhere near as effectivily as an animal brain can, even a simple arthropod brain. One of the (many) limitations with robots at the moment is the complexity of the responses which need to be programmed in to make them function with fewer legs. Brains still beat computers (or programmers!) on this. New Scientist had something on robotics and legs recently which you may find interesting.

The answer is probably in the evolution of arthropods. To give a very simplistic view of things, the ancestors of arthropods (and of us, too) had a segmented body structure. Each segment would intially have been a repeat of the previous segment, and longer and larger bodies would have developed as the segments repeated. As these animals evolved some segments would develop differently - some in the middle would have legs. More or fewer legs could easily be evolved by growing more or fewer of these segments, so a huge amount of variation could occur, with one simple evolutionarly step! Just look at the segments on millipedes - each is a repeat of the other. This is why so many groups of arthropods differ so much in their limbs - it was easy to do this in the early stages, and still have a functional body. What the evolutionary pressures were to cause different groups to settle on different numbers I don't know.

The more legs you have, the more stable you are. Fewer legs are less 'costly' to grow, in terms of energy, so it is a balance of advantages and disadvantages. But I believe that the more interesting question is why vertebrates have so few limbs, and not the other way around. Perhaps it is because our limbs are so complex and costly to grow, that the loss of stability we suffer from in having so few of them is worth the gain in not having to grow more.

Then again, maybe I'm talking total rubbish. In fact, I think I probably am, so I'll shut up now.

[Michael Olsinia]

Perhaps extra legs are needed in inverts to support their body weight, as they lack bones?

[Richard Gallon]

As suggested it's more about evolutionary history.

All land vertebrates evolved from a common ancestor with 4 limbs. That's why you don't find multi-limbed vertebrates - it's just not in their genetic make-up. For a vertebrate to aquire additional limbs, significant skeletal alterations would be required (and spontanious mutations along these lines are likely to be fatal).

Arthopods on the other hand are a more diverse group than vertebrates, with limbs associated with body segmentation (rather than with an internal skeleton). For an arthropod to aquire additional legs it's much easier - just add a body segment (a spontaniously aquired extra segment/limbs wouldn't need a radical overhaul of an internal skeleton).

Richard