Thursday,22
June, 2000
Good Vibrations
In the searing heat of the
desert a sand scorpion scurries out from a beneath a rock. It
is in search of dinner and detects the presence of a juicy moth.
Sensing it's prey, it sits and waits for the moth to move into
it's range and then it strikes, gripping it with its claws and
injecting it with its venomous sting.
The sand scorpion is a patient predator. It is blind and recognises
that vibrations are the keys to a good meal. But how do they
feel the vibrations, and how do they work out which way they
have to go to get a good mouthful of moth? Professor Leo van
Hemmen from the Technical University of Munich explains to Richard
Black, from Science In Action, how these arachnid sense
that it's dinner time.
Along
with their relatives - spiders, ticks and mites - scorpions
are, for many, the stuff of nightmares. Although their venom
makes them a frightening prospect, in reality most are not dangerous
to man and death by stinging is relatively rare.
However, it is unlikely that the scorpion would be characterised
as a sensitive creature and yet as Hemmen explains, they do
rely heavily on their senses:
'When a prey is moving in the neighbourhood at a distance of
at most half a metre, the prey produces a surface wave that
ripples. After a while these ripples hit the sensors at the
tips of each of the scorpion's eight legs. The sensors are extremely
sensitive. Take a grain of sand, drop it in the direction of
the sand scorpion and it will notice it.'
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| 'Take
a grain of sand, drop it in the direction of the
sand scorpion and it will notice it'
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From Vibration to Action
Once the scorpion has detected the ripple it acts. But what
is happening in the scorpion's mind? Hemmen comments:
'Each sensor sends the so-called spike, to a specific neuron.
So in the brain, which is a very small structure, you have eight
neurons waiting for information from the eight legs. Each sensor
sends a spike, which lasts less than a millisecond, to its corresponding
neuron that is then activated. However at the same time an inhibitory
neuron is activated.'
Put simply the scorpion is working out which leg is nearer to
the prey. Hemmen continues:
'The wave passes the sensors and I look at the sensor working
opposite to the one working nearest to the prey - so the one
furthest away will be activated last. It sends a spike to the
brain. However since the inhibitory neuron from the leg on the
other side was activated first, it says, "Ha! Ha! You have to
shut your mouth, because I already have this information and
I am faster than you are!". What you have therefore is an ensemble
of eight neurons, which are now a voting committee. They starting
voting on what the scorpion is gong to do and the one who gets
most of the input will fire most.'
Once the prey is within the scorpions grip, dinner begins. Paralysing
venom is injected into the prey and digestive enzymes are secreted
onto it. The subject is then eaten in fluid form.
Scorpion Habitats
Whilst most people think that scorpions are desert dwellers,
many live in a range of habitats including savannahs, rain forests,
grasslands and caves. In fact scorpions have even been detected
in the snowy Andes Mountains of South America at a height of
over 12,000 feet.
So, can the surface that they operate on affect their sense
of vibration? Hemmen explains the acute nature of their senses
and how versatile the creature can be:
'A scorpion adapts very well to it's surrounding. With sand
it is quite remarkable, as a wave in sand is very slow, approximately
50 metres per second. The diameter of its legs, of the circle,
is about 5 centimetres and that is fine, because at 50 metres
per second, it makes a temporal difference of one millisecond.
What is so fascinating about these creatures is that they have
adapted themselves to a millisecond time scale.'
Sensitive To Touch
Whilst vibrations are the scorpion's primary source for sensing
prey, they are also sensitive to touch. The pedipalps (head
appendages) are covered in sensory hairs called trichobothria,
which are sensitive to airborne vibrations and the surface of
the legs and body are covered with hairs, known as setae, that
are sensitive to the direct touch.
The question is who or what would want to get near enough to
touch them?
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| Professor
Leo van Hemmen can be heard on Science In Action.
Find this on our interactive
radio... |
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| Scorpion
Facts |
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| There
are approximately 1,300 species of scorpion. The
largest of which is thought to be the "long
tailed" South African scorpion which can reach
over 8 inches in length. |
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| Sting
In The Tail |
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The
venom emitted from the scorpion is comprised of
a complex mix of toxins. It is used to stun prey,
however approximately 25 species have strong enough
venom to kill humans.
The most dangerous species can be found in North
Africa, India, Mexico, South America and the Middle
East. Death by stinging is still relatively rare,
but when it does occur it is usually caused by heart
or respiratory failure.
In
these countries, statistics indicate that children
and the elderly are most susceptible.
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