Why are spiders so fast?

[MikeCollins]

Now I know why when I go to crush the tiny little spider on my windowsill that it reacts like lightning and slips away from my feeble attempt on it’s life. It makes sense that it can send nerve impulses down along it’s legs from the central nervous system very fast; after all, it is so small.

However, why in the living heeeeeeell (you heard me) are tiny spiders so fast? I mean the way they hurl themselves through space you’d think they could even stand a chance of outrunning me.

I was thinking maybe it’s power:mass ratio working here. The idea that perhaps 70% of the spiders total volume is in it’s long, slender legs - therefore it can propel it’s tiny little body at very high speeds. But I’m not really sure about this - so I want it addressed.

[casdave]

Well I take it you must mean those little jumping spiders and the like.

They need to be fast, the jump is a defense mechanism.

[Mangetout]

@MikeCollins

I was thinking maybe it’s power:mass ratio working here. The idea that perhaps 70% of the spiders total volume is in it’s long, slender legs - therefore it can propel it’s tiny little body at very high speeds.

That’s pretty much on the money, but there’s also a power:scale ratio - the strength of a muscle increases roughly in proportion with the cross-sectional area (a square measure) - ignoring for the moment that spider legs are hydraulic, rather than muscular - whereas the mass increases in proportion with the volume. For a ridiculously hypothetical cuboid limb, halving the scale reduces the mass to one eighth of what it was, but only reduces the muscular power to one quarter (so in terms of scale strength, it is twice as powerful) - this is part of the reason ants can lift 100 times their body weight.

[Bosda_Di_Chi_of_Tricor]

It’s a preator, and all predators are capable of brief busts of blinding speed.

[Bryan_Ekers]

It’s because they’ve got radioactive blood.

I mean, duh.

[Just1Lurk]

Dammit, Bryan Ekers, I opened this thread just to post that reply. Nuts!

[Bryan_Ekers]

@Just1Lurk

Dammit, Bryan Ekers, I opened this thread just to post that reply. Nuts!

Well, technically, the presence of radioactive blood only explains their strength, not their speed.

[MikeCollins]

@Mangetout

That’s pretty much on the money, but there’s also a power:scale ratio - the strength of a muscle increases roughly in proportion with the cross-sectional area (a square measure) - ignoring for the moment that spider legs are hydraulic, rather than muscular - whereas the mass increases in proportion with the volume. For a ridiculously hypothetical cuboid limb, halving the scale reduces the mass to one eighth of what it was, but only reduces the muscular power to one quarter (so in terms of scale strength, it is twice as powerful) - this is part of the reason ants can lift 100 times their body weight.

Sorry, I don’t mean to drag this on - but what’s the difference (according to your model) given the fact that the spiders legs are hydraulic? How does this change things in light of the above explanation?

[Mangetout]

Well, it’s a very different method of operation, however, I suspect the same general principle applies; the smaller the scale, the greater the scale strength; this applies to materials as well as forces.

Halving the scale of a hydraulic piston reduces its mass by seven eighths, but it’s power only by three quarters, so if the first piston can lift ten times its own weight, a piston half the size can lift twenty times its own weight.

If you have excess strength, you can use it two ways; either as raw brute strength, or using levers etc, reduce it to moderate strength at much greater than normal scale speed.