They did not hunt humans because there were no humans to hunt, but insects of gargantuan proportions really did exist 300 million years ago.
Bloodthirsty cockroaches plot to destroy humans!
Scientists use fossil DNA to reconstruct gigantic man-eating ants!
Super-sized alien flies invade Earth!
Typical supermarket tabloid headlines? Perhaps. But for decades, frightening mythical images such as these have been prime fodder for monster movies and late night television. The images prey on our fascination and fear of insects.
The enormous insects depicted in bad B movies exist mostly in the realm of science fiction. However, insects of giant proportions really did exist 300 million years ago. They were not as big as dump trucks, but some insects achieved masses many times greater than those of their modern relatives.
The fossil evidence is abundant. Scientists know that dragonflies with wing spans as wide as a hawk’s and cockroaches big enough to take on house cats thrived during the Paleozoic era (245-570 million years ago). At the same time, mammoth millipedes longer than a human leg skittered across prehistoric soil.
Hundreds of different huge species evolved during the late Paleozoic era. The first dinosaurs appeared just about the time the giant insects disappeared.
These ancient giants fascinate Jon Harrison. A physiologist and professor of biology at Arizona State University, Harrison wants to know why giant insects evolved, and why they then disappeared.
The answer may lie in how insects breathe, according to research findings by Harrison and his colleagues. The ASU scientists are busy studying how the respiratory physiology of modern insects affects their body size.
Recent geologic findings opened a new window of thought on this issue. Some researchers are analyzing the composition of ancient soils. Their findings seem to comply with theoretical models. The findings indicate that there was a “pulse” in the concentration of environmental oxygen during the Paleozoic era.
In other words, there was much more oxygen in the atmosphere 300 million years ago than there is today. During this period, the oxygen concentration in the air reached 35 percent, almost double the present level of 21 percent. Oxygen concentration stayed high for about 100 million years, then dropped precipitously to about 15 percent.
Scientists think that the then-recent evolution of oxygen producing land plants caused this oxygen peak. Interestingly, the rise and fall of atmospheric oxygen also coincided with the evolution and extinction of giant insects.
Harrison’s colleagues include Robert Dudley from the University of Texas at Austin, and Jeffrey Graham of the Scripps Institution of Oceanography in La Jolla, Calif. They propose that the temporary overlap between the oxygen peak and the appearance of giant insects was more than just coincidence.
Other researchers had speculated that oxygen availability might limit the ultimate body size for insects. Harrison and his colleagues took the idea a step further. They hypothesized that high ambient oxygen could have permitted the existence of giant species. The demise of winged monsters and behemoth beetles 100 million years later may be explained partly by the simultaneous decrease in the air’s oxygen content.
Harrison says that the amount of available oxygen limits insect body size because of how the creatures’ respiratory systems are made. Instead of lungs, insects breathe with a network of tiny tubes called tracheae. Air enters the tubes through a row of holes along an insect’s abdomen. The air then diffuses down the blind-ended tracheae.
The distance oxygen can travel down the tracheae depends on its concentration in the air. If atmospheric oxygen is doubled, theory says that it should be able to make it twice as far.
According to Graham and Dudley, escalating Paleozoic oxygen levels may have helped speed oxygen transport in the longer tracheae of bigger insects. The environment itself could have opened the respiratory door for Paleozoic insects, allowing giant species to evolve.
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http://researchmag.asu.edu/stories/bugs.html