EMBARGOED FOR RELEASE UNTIL OCTOBER 11, 2006
Contact:
Christine Guilfoy
Office: (301) 634-7253
cguilfoy@the-aps.org
Oxygen Concentration In The
Air Limits Beetle’s Size
virginia Beach,
Va (October 11, 2006) – The delicate lady bug in your garden could be
frighteningly large if only there was a greater concentration of oxygen in
the air, a new study concludes. The study adds support to the theory that
insects were much larger during the late Paleozoic period because they had a
much richer oxygen supply, said the study’s lead author Alexander Kaiser.
The study, “No giants today: tracheal oxygen supply to
the legs limits beetle size,’’ will be presented Oct. 10 at Comparative
Physiology 2006: Integrating Diversity. The conference will be held Oct.
8-11 in Virginia Beach. The research was carried out by Alexander Kaiser and
Michael C. Quinlan of Midwestern University, Glendale, Arizona; Jaco Klok,
J. Jake Socha and Wah-Keat Lee, Argonne National Laboratory, Argonne, IL;
and Jon F. Harrison, Arizona State University, Tempe, AZ. Harrison is the
senior author.
The Paleozoic period was a time of huge and abundant
plant life and rather large insects -- dragon flies had 2-foot wing spans,
for example. The air’s oxygen content was 35% during this period, compared
to the 20% we breathe now, Kaiser said. Researchers have long speculated
that the higher oxygen concentration allowed insects to grow much bigger.
This experiment was designed find out how much room the
trachea, which enables growth, takes up in the bodies of different-sized
beetles. The researchers also wanted to know whether trachea size increases
proportionately as the beetles get larger, and whether there is a limit to
the size a beetle could grow in the current atmosphere.
Tubes carry oxygen
First, a bit of background: Insects don’t breathe like
we do and don’t use blood to transport oxygen. They take in oxygen and expel
carbon dioxide through holes in their bodies called spiracles. These holes
connect to tubes, called trachea, Kaiser explained.
Whereas humans have one trachea, insects have a whole
tracheal system that transports oxygen to all areas of their bodies and
removes carbon dioxide. As the insect grows, tracheal tubes get longer to
reach peripheral tissue. In addition, the tubes must get wider or more
numerous to meet the additional oxygen demands of a larger body.
The insect can limit oxygen flow by closing its
spiracles. In fact, one reason insects are so hardy is that they can close
their spiracles and live off the oxygen they already have in their trachea.
Kaiser recalled a caterpillar that fell into a bucket of water in his lab.
When the creature was discovered the next day, lab workers thought it had
drowned. But when they removed its apparently lifeless little body from the
water, they were surprised to see it crawl away.
Trachea grows
disproportionately
The researchers used x-rays to compare the tracheal
dimensions of four species of beetles ranging in size from 3mm (Tribolium
castaneum, about one-tenth of an inch) to about 3.5 cm (Eleodes
obscura, about 1.5 inches).
They found that the trachea of the larger beetles take
up a greater proportion of their bodies, about 20% more than the increase in
their body size would predict, Kaiser said. This is because the tracheal
system is not only becoming longer to reach longer limbs, but the tubes
increase in diameter or number to handle the additional oxygen demands.
The disproportionate increase in trachea size reaches a
critical point at the opening where the leg and body meet, the researchers
found. This opening can get only so big, and limits the size of the trachea
that can run through it. When trachea size is limited, so is oxygen supply
and so is growth, Kaiser explained.
Using the disproportional increases they observed among
the beetles, the researchers calculated that beetles could not grow larger
than about 15 centimeters. And this is the size of the largest beetle known:
the long horned beetle, Titanus giganteus, which grows 15-17 cm,
Kaiser said.
And why wouldn’t the opening to the leg limit beetles
in the Paleozoic era? After all, beetles back then had the same bodily
architecture, but were much bigger. It is because when the oxygen
concentration in the atmosphere is high, the tracheal diameter could be
narrower and still deliver sufficient oxygen for a much larger insect,
Kaiser concluded.
* * *
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