Hornet cooling theory creates stir: Proposal of vespine heat pump stings insect researchers.
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Hornet cooling theory creates stir
Proposal of vespine heat pump stings insect researchers.
5 June 2003
MICHAEL HOPKIN
Hornets face air
temperatures of more
than 40 °C.
 Corbis
How does a busy hornet
keep cool? Its skin is an
electrical heat pump, claim
Israeli researchers - the
first suggestion of such a
mechanism in an animal.
The idea has stung insect
specialists. Hornets are
unlikely to have a heat
pump, and don't need one
anyway, they argue.
It's a bold proposal, admit
Jacob Ishay and colleagues
at Tel Aviv University, who
came up with it. But a
pump could answer a lot of
questions about how
hornets, wasps and even
other insects stop
themselves overheating
during a day's work.
Foraging in sunlight, the insects can face air temperatures of
more than 40 C, but have no sweat glands to help them
dispose of excess heat. The thorax, which contains a hornet's
flight muscles, can become hotter still. Physiologists think
that heat from here is dissipated through the body by a blood-
like fluid called haemolymph.
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ˇ Hornet cooling
theory creates stir
5 June 2003
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Hornet cooling theory creates stir: Proposal of vespine heat pump stings insect researchers.
Hot topic
The team used a heat-sensitive camera to photograph
oriental hornets (Vespa orientalis) flying round a nest near
their lab. They saw evidence that parts of the insects' bodies
are often several degrees cooler than their surroundings.
Removing heat from a surface that is already cooler than its
surroundings - as in a commercial refrigerator - requires the
right hardware, not to mention a source of energy. The
structure of the hornet's outer layer, called the cuticle, is well
suited to the task, says Ishay's team.
They suggest that the cuticle works as a thermoelectric heat
pump. Such devices use electrical energy to shunt heat the
wrong way - from cool to hot. The process relies on
thermocouples. These are junctions within electrical circuits
where conductors of different materials meet. If a voltage is
applied across one, heat moves from one side of it to the
other.
Commercial thermoelectric heat pumps have a stack of
thermocouples, usually several centimetres thick. They are
used to cool a huge range of substances, from water in the
office drinking fountain to the motor in a dentist's drill.
The hornet's cuticle has the same structure, albeit packed
into a thickness of a few thousandths of a millimetre, claims
Ishay's team. Generating power would not be a problem - as
well as the insect's metabolic energy, the cuticle itself
generates a voltage when exposed to sunlight or ultraviolet
rays, much like a man-made solar battery.
Sting in the tale
Other researchers have greeted the proposal with scepticism.
"It needs a lot more evidence," says Julian Vincent of the
University of Bath, UK, who studies insect cuticles.
Vincent suspects that the cuticle is not arranged in layers,
but is composed of ordered molecules that just look like
layers under the microscope. What's more, he disputes the
assertion that hornets are cooler than their surroundings.
Although the insects look cold in the thermal photographs,
the air around them could be colder still, he warns.
The team needs to nail down the details of how heat moves
between a hornet and its surroundings, agrees
thermodynamics expert Geoffrey Hammond, also at the
University of Bath. The hornets' apparent coolness in the
photographs could be due to their shiny surfaces, which may
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Hornet cooling theory creates stir: Proposal of vespine heat pump stings insect researchers.
reflect heat as well as light.
"There's a lot more work to be done," says David Bergman, a
member of the Tel Aviv team. The researchers now hope to
catch a hornet's heat pump in action and find out whether
similar pumps are present in the hornet's relatives, such as
wasps.
References
1. Ishay, J. S., Pertsis, V., Rave, E., Goren, A. & Bergman, D.
J. Natural thermoelectric heat pump in social wasps.
Physical Review Letters, 90, 218102, (2003). |Article|
 Nature News Service / Macmillan Magazines Ltd 2003
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