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Boating spiders – Spiders: how they move and spin (4/5)

September 10, 2019


The features of spiders
that let them walk on water are primarily
that they can’t get wet. Secondarily
that they have hairs on them that also can’t get wet. That is the surface of both
at the molecular level are hydrophobic. That means
that water is repelled by them. Raft spiders are another species
that can walk on water. Their weight is supported
by two different forces. The surface tension is really
the primary one in this case. The other is buoyancy. Because when she makes a dimple
by pushing down on the water that acts just like a hull of a boat
– it’s supported some by the density
of the water trying to flow in. Each legs makes its own supporting
dimple in the water. So a floating spider
is like a tiny multi hulled boat. Now how about
how we’re being propelled? We’re heading for the shore –
let’s try not to go there. As I pull against these oars, what’s happening is that I’m pushing
water away from me backwards and the momentum
that I give to the water backwards is the same as the momentum
that I give to the boat forwards. That’s one of Newton’s laws. And actually when you look out there
at the end of an oar you can see
that it’s moving the water… backwards, the boat goes forwards. For fishing spiders
it’s not quite so simple. They have very little to push against because there’s almost no friction
between the spider and the water. Yet they still manage to row across
what is for them a very slippery surface. There have been a variety of models
of how that might happen. One is that as the spider’s leg
moves backwards, if it moves backwards fast enough there’s a wave that forms
on the leading edge of that leg. As with an oar, the leading edge is the side that’s
pushing against the water. Another possibility is that the leg
and the dimple that moves with it could themselves act like an oar
and behave the way an oar does. To test the wave theory,
Bob uses a leg from a dead spider attached to a sensitive force meter. Moving water
pushing against a fixed leg mimics a moving leg
pushing against still water. The end result is the same. As the water speeds up,
a wave starts to build on the side of the leg
that’s pushing against the water. It turns out a really interesting
thing about waves on water is that there are no waves
on the Earth on water if the thing causing the wave
is less than 20cms a second. That is waves just don’t go less
than 20cms a second on the Earth. Well, a quick experiment
measuring the force on the leg as the leg moves slowly
and then faster and faster and faster shows that there’s a continuous rise
in the force that’s generated from zero velocity to say,
40cms a second. A slow-moving leg still creates
a propulsive force. If waves were what mattered,
the spider couldn’t get underway unless it moved
its rowing legs backwards at more than 20cms a second. Metalised beads and a low-power laser reveal that the dimples
that support the spider are also the key to rowing. Just like an oar, a moving dimple
creates turbulence and changes the momentum
of the water around it. Any time you change
the velocity of the water you’re changing
the momentum that the water has. And a changing momentum
is the same thing as a force. The momentum the moving leg
gives to the water backwards is the same as the momentum that the
water gives to the spider forwards. It’s the viscose drag of the water
tugging at the leg and dimple that provides the resistance
the spider needs to propel itself forward. High-speed video reveals
that a rowing spider is like a four-oared boat. When a spider wants to start moving
by rowing it picks its legs up,
moves them forward, then pushes them
into the water surface, making this thing
that I keep calling a dimple. It’s that deep dimple then that
gets moved across the water surface and constitutes the same thing
as the paddle part of an oar, that is the wide part of an oar. There he goes. So now watch the dimples change
shape – push down, push down, go off screen. Let’s look at that one more time. And watch both sides now. Third set of legs now,
second set of legs next. So each time
it takes a stroke like that, it’s pushing its legs down
into the water just a bit, not enough to break through
the surface tension, just enough to make this dimple which is the equivalent of taking
something that’s long and skinny and pushing it down
into the water surface and making it into something
that’s oar-shaped because it goes
from being a skinny leg to being this dimple. It pushes the dimple back,
makes the spider go forward. But rowing still has its drawbacks. When a spider is rowing, then it’s got serious problems
as to how fast it can go partly because it’s touching the
water on part of the return stroke and partly because it can’t move
its legs very fast backwards and still have the dimple intact.

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