Biophysicist Bob Suter is off on a spider hunt.
If we're going to catch any spiders out here that are ballooning
they're going to be falling past us downward
because we're sitting still in the air and the spiders are falling down.
All they need to get airborne is an upward flow of air
and a single strand of silk.
Spiders are denser than air but they can gain height
if the air column is rising faster than gravity is pulling them down.
Human balloonists need lots of hot air to keep them aloft.
We're actually neutrally buoyant
which means we're at the same density as the air that's around us.
This whole system that we're in is of the same density.
So we're sitting still.
With respect to the air column, a spider is always falling through it.
Let's see if we got anything on here.
- You got him? - Yeah, I got him.
Size is an important factor
in determining a spider's chance of getting airborne.
The forces important in spider ballooning are gravity
because that acts on all objects no matter where they are,
and the drag force created by air flowing past the spider.
The hot wire anemometer measures the speed of the air currents.
The stream of smoke gives an indication
of the direction of the air flow.
A spider that stands on the tip of a twig with silk in the air above it
and is trying to balloon is going to become airborne
only when the velocity of the air is high enough in an upward direction
so that the drag the spider experiences
exceeds the pull of gravity.
Because air currents are unpredictable
and the next puff of wind could carry it up or down,
the spider has to choose the right moment for takeoff.
These graphs show velocity of the wind here, the angle of the wind
and the probability that a spider can get airborne under those conditions.
So the angle of the wind is what I got from the smoke.
The velocity of the wind came from that anemometer that I was using.
This shows that for all the variety of angles and variety of winds,
there's a very small proportion of the time
when the spider can actually get itself airborne.
If you look at this in more detail you can see how this works out
for very small spiders, medium-size spiders and larger spiders.
And what you see is that for the very large spider,
this is 1.6 milligrams,
becoming airborne is very difficult
and only happens in a very small fraction of the time
when conditions might be right.
For a medium spider and for a very small spider,
a tenth of a milligram conditions are much more frequently ideal
for getting airborne.
After takeoff, the direction they go depends entirely on the wind.
But balloonists of all sizes
have some control over the duration of their flight.
The way we're controlling our height right now is by burning propane
into the air above us which make us less dense so we go up.
We could vent some air or we can just let it cool down
and we'd go down,
adjusting how dense we are relative to the air around us.
Spiders can't adjust their density.
What they can do is vary their surface area to increase or decrease
the viscose drag.
Some of the work I've been doing
has shown that as they spread their legs out
they substantially increase the drag that they experience
so they fall more slowly.
And if they pull those legs in, that is crunch them up against their body,
they can fall more rapidly.
And falling spiders make subtle use of ballooning silk.
The length of silk that they let out
or reeling up the silk which they can do with their legs,
can decrease or increase the amount of silk that's out there
and therefore decrease or increase
the amount of drag that they're experiencing
but they're always falling.