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Thursday, Jan. 30, 2003
Insects simply a breath apart
By ROWAN HOOPER
Insects are the most numerous, diverse and successful group of animals in the history of the planet. They are found in almost every environment, and range from the minute (less than a millimeter long for the feather-winged beetle) to the large (more than 15 cm for the South American longhorn beetle). But the longhorn is nothing compared to the monster insects that lived in the Paleozoic era, which ended about 250 million years ago.
One species of fossil dragonfly, Meganeura monyi, had a wingspan of almost 1 meter. Toward the end of the Paleozoic there was a jump in the levels of oxygen in the atmosphere, and this, some scientists believe, allowed insects to reach giant proportions.
This is how they did it. Rather than a fluid "blood" that carries oxygen to the tissues, as in vertebrates, insects take in oxygen by a process known as diffusion. Their bodies are punctured with small holes called spiracles. Air diffuses through the holes, down narrow tubes called trachea and directly on to the muscles, cells and tissues that need oxygen.
But diffusion is a slow, inefficient process. It limits the ultimate size of insects because if they get too big, they won't be able to get oxygen to their tissues fast enough to function. For this reason, in most insects no point in the body is more than about 5 mm from the surface.
In the Paleozoic era, high levels of oxygen in the air (35 percent compared to about 21 percent now) enhanced the efficiency of simple diffusion and freed insects from being limited to small bodies. And so the huge insects evolved.
But scientists at The Field Museum of Natural History in Chicago have discovered that insects don't just rely on diffusion to breathe. Using an advanced kind of X-ray, they found that the tracheal tubes of the wood beetle (Platynus decentis) rhythmically compress and expand just like lungs in humans.
"The discovery of this fundamental aspect of respiratory biology for insects could revolutionize the field of insect physiology," said Mark Westneat, associate curator of zoology at The Field Museum and lead author of a paper on insect breathing published last month in Science.
Tracheal compression was not found in all types of insects studied, but in those where it was identified compression patterns varied within individuals and between species. The three species most closely studied (the wood beetle, house cricket and carpenter ant) exchange up to 50 percent of the air in their main tracheal tubes approximately every second. This is similar to the air exchange of a person doing moderate exercise.
Up until now, it was impossible to see movement inside living insects. This problem has been solved by using a synchrotron, which generates one of the strongest X-ray beams in the world, to obtain X-ray videos of living, breathing insects. "This is the first time anyone has applied this technology to study living insects," said Wah-Keat Lee, a physicist at the Argonne National Laboratory, Ill., and a coauthor of the Science paper.
A synchrotron is a large, circular, particle accelerator. The one at Argonne, called the Advanced Photon Source, has a circumference of about 1 km and accelerates electrons almost to the speed of light. Doing this generates radiation, including X-rays that are more than 1 billion times as intense as a conventional X-ray source. With synchrotron radiation, structures once impervious to scientific investigation can now be precisely analyzed.
Lee stumbled onto the analytical power of the synchrotron about two years ago, when he placed a dead ant in the path of the X-ray beam and was amazed to see incredibly detailed images of the ant's internal organs. The new work opens up the possibility of developing a powerful new technique for studying how living animals function.
To this end, Westneat, Lee and colleagues plan to aim the synchrotron at the jaws of insects to see how they chew. "Most of the 12 moving parts in an insect's jaw mechanism are internal, so our inability to see inside living, moving insects has prevented us from understanding how these parts work together," Westneat said.
In the future, Westneat envisions using synchrotron X-ray videos to study a wide variety of animal functions, biomechanics and movements.
The work is important because new discoveries about animal function can have broad implications. For example, active tracheal breathing in the head and thorax among insects may have played an important role in the evolution of terrestrial locomotion and flight in insects, and be a prerequisite for oxygen delivery to complex sensory systems such as the brain, the authors say.
New information about how insects function is not only intrinsically valuable, but it also might provide insights into human health. Studying how larval fish move their backbones, for example, could shed light on how to treat spinal-cord injuries in humans. Likewise, studying the walls of blood vessels in mice and the tiny hearts in beetles (a beetle has 8 to 10 hearts) could shed light on how to treat high blood pressure.
"Basic principles of mammal, fish or insect physiology and function could have important implications for health care," Westneat said. "We intend to develop this novel technique for a range of applications that will greatly improve our knowledge of how tiny animals live and function."
Rowan Hooper welcomes comments at email@example.com