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2005-02-23 | |
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Why Do Insects Stop 'Breathing'? To Avoid Damage From Too Much
Oxygen, Say Researchers, Challenging Previous Theories
A new study investigating the respiratory system of
insects may have solved a mystery that has intrigued physiologists
for decades: why insects routinely stop breathing for minutes at a
time.
Challenging previous theories, researchers at UC Irvine and
Humboldt University propose that insects such as grasshoppers,
moths, butterflies, some types of fruit flies, beetles and bugs
close off their respiratory systems periodically to keep out excess
oxygen, thus preventing damage to their tissues.
Timothy Bradley, professor of ecology and evolutionary biology at
UCI, and Stefan Hetz, assistant professor of physiology at Humboldt
University, Germany, report their findings in the Feb. 3 issue of
Nature.
The insect respiratory system is designed to accommodate
occasions when the insect is active. For example, a grasshopper is
most active when it flies. When the grasshopper is inactive and
resting, however, it continues to breathe in oxygen at the same high
volume it uses while flying. The result is excess internal oxygen
that can cause oxidative damage – the destruction of biomaterial due
to excess oxygen – to tissues. To protect their bodies, insects like
grasshoppers discontinue breathing.
“We propose that most insects stop breathing in order to lower
their internal oxygen concentration to physiologically safe levels,
and that they then substantially reduce gas exchange to maintain the
oxygen at these safe values,” Bradley said. “This hypothesis
explains the respiratory pattern of insects in different
environments in ways that previous models can’t.”
Two previous models for explaining why insects punctuate their
breathing with periods of closure are (1) such discontinuous
breathing reduces water loss and (2) it enables insects to rid their
bodies of carbon dioxide, respiration’s byproduct, when the insects
are underground. As is true for miners, insects, while underground,
are faced with high-carbon-dioxide and low-oxygen amounts,
necessitating a better ventilation system. While oxygen is essential
for their cells to produce energy, the removal of carbon dioxide
from their bodies is equally important to prevent its toxic buildup
in tissues.
“Even in our own case, our bodies have to supply oxygen to our
tissues, but they must also keep out excess oxygen to prevent
oxidative damage to the tissues. This damage is closely related to
aging. Hence, perhaps, the many anti-oxidative creams flooding the
market to combat aging. The concentration of oxygen in the air we
breathe is toxic to us. Indeed, fruit flies, which have been studied
closely for decades, die sooner from aging in a high-oxygen
environment.”
Insects take in oxygen through spiracles – tubes connected to
openings in their sides. In their study, the researchers inserted
fine tubes into the spiracles of a moth to measure not only how much
carbon dioxide the moth released but also the concentration of
oxygen in its trachea, the series of tubes that carry air directly
to cells for gas exchange. Using a respirometer (an instrument for
measuring respiration that consists of a chamber with a flow-through
air system), they monitored the moth’s breathing pattern. The
chamber, which housed the moth being studied, was filled first with
air that had been freed of carbon dioxide. Next, a device measured
when and how much carbon dioxide originated from the insect.
Bradley explained that insects typically maintain 4-5 kilopascals
of oxygen in their respiratory systems, 4-5 times lower than the
normal oxygen concentration in the atmosphere. In a normal
oxygen-concentration environment, the insect breathes for a period
of time and releases a burst of carbon dioxide. It then closes its
respiratory system, blocking off more intake of oxygen, to maintain
the internal oxygen concentration at 4-5 kilopascals, the right
oxygen concentration for its body. In a low-oxygen environment, the
insect opens its respiratory system for longer periods of time; when
it closes the system, it does so for only a very short time. In a
stream of air with high oxygen on the other hand, the respiratory
system opens briefly and then closes firmly for a long time. “In
other words, insects are actively keeping oxygen out and doing it in
a way that shows they know how to measure oxygen,” Bradley said.
“Their behavior indicates they are regulating oxygen.”
Bradley and Hetz will next work on developing a comprehensive
model of insect breathing. “We’d like to expand our present model to
explain all aspects of insect breathing. And we’d like to examine
insects that do not show this pattern – for example, desert beetles.
While we have shown that insects monitor oxygen precisely, we do not
know how that happens or which tissues and cells are involved. Once
we’re equipped with a better knowledge of the insect breathing
pattern it could shed light on when pesticides should be used to
control insects – crucial for the agricultural industry.”
The research was funded by a grant to Bradley from the National
Science Foundation.
About the University of California, Irvine: The University of
California, Irvine is a top-ranked public university dedicated to
research, scholarship and community service. Founded in 1965, UCI is
among the fastest-growing University of California campuses, with
more than 24,000 undergraduate and graduate students and about 1,400
faculty members. The second-largest employer in dynamic Orange
County, UCI contributes an annual economic impact of $3 billion.
Editor's Note: The original news release can be found here.
This story has been adapted from a news release issued by
University Of California, Irvine.
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