X-ray images help explain limits to insect body size
ARGONNE, Ill. (Aug. 9, 2007) – Researchers at the U.S. Department of Energy's
Argonne National Laboratory have cast new light on why the giant insects that
lived millions of years ago disappeared.
In the late Paleozoic Era, with atmospheric oxygen levels reaching record
highs, some insects evolved into giants. When oxygen levels returned to lower
levels, the insect giants went extinct.
The basis of this gigantism is thought to lie in the insect respiratory system.
In contrast to vertebrates, where blood transports oxygen from the lung to
the cell, insects deliver oxygen directly through a network of blind-ending
tracheal tubes. As insects get bigger, this type of oxygen transport becomes
far less effective. But if the atmospheric oxygen levels increase, as they
did in the late Paleozoic, then longer tracheal tubes can work. This would
allow larger-sized insects—even giants—to evolve.
Recent research published in the journal Proceedings of the National Academy
of Science helps confirm the hypothesis that the tracheal system
actually limits how big insects can be. The research provides a specific explanation
for what limits size in beetles: the constriction leading to the legs.
A collaborative team of researchers from Argonne's Advanced
Photon Source (APS), Midwestern
University and Arizona
State University wanted to study how
beetles' tracheal systems change as their body sizes increase. The team took
advantage of richly detailed X-ray images they produced at the APS to examine
the dimensions of tracheal tubes in four beetle species, ranging in body mass
by a factor of 1,000.
Overall, they found that larger beetle species devote a disproportionately
greater fraction of their body to tracheal tubes than do smaller species.
The team focused in particular on the passageways that lead from the body
core to the head and to the legs. They reasoned that these orifices may be
bottlenecks for tracheal tubes, limiting how much oxygen can be delivered to
the extremities.
“We were surprised to find that the effect is most pronounced in the orifices
leading to the legs, where more and more of the space is taken up by tracheal
tubes in larger species,” said Alex Kaiser, biologist at Midwestern University.
They then examined the tracheal measurements of the four species to see if
they could predict the largest size of currently living beetles. The head data
predicted an unrealistically large, foot-long beetle. In contrast, the leg
data predicted a beetle that nicely matches the size of the largest living
beetle, Titanus giganteus .
“This study is a first step toward understanding what controls body size in
insects. It's the legs that count in the beetles studied here, but what matters
for the other hundreds of thousands of beetle species and millions of insect
species overall is still an open question,” said Jake Socha, Argonne biologist.
Funding for this work was supported by the National Science Foundation. Use
of the APS was supported by the U.S. Department of Energy, Office of Science,
Office of Basic
Energy Sciences.
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For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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