GIANT INSECTS:

(sfx: dramatic music)
     IT WAS 300 MILLION YEARS AGO, THE CLIMATE WAS MILD, THE FIRST ANIMALS
WERE MAKING THEIR WAY ONTO LAND, AND GIANT DRAGONFLIES RULED THE SKIES.
WHAT HAPPENED TO THEM? 
     GRAHAM:  "as a young kid I had been to museums of huge dragon flies that were about 2.5 wing
spans. the body. . . close o almost he size of a wrist. . . and by contrast insexts today don't get nearly that
large." 
     WHY DRAGON FLIES AND OTHER INSECTS USED TO BE SO LARGE, FASCINATED
JEFF GRAHAM AT THE SCRIPPS INSTITUTION OF OCEANOGRAPHY FOR YEARS.  HE
EXPLAINS THAT TODAY INSECTS HAVE TO BE SMALL, BECAUSE OF HOW THEY
BREATHE.  THEY DON'T HAVE LUNGS LIKE WE DO.  INSTEAD THEY'RE MUCH MORE LIKE
SPONGES. THE OXYGEN JUST SEEPS INTO THEIR BODIES.  IF THEY WERE TOO BIG THE
OXYGEN WOULDN'T MAKE IT ALL THE WAY INTO THE MIDDLE. BASED ON THE FACT
THAT OUR ATMOSPHERE CONTAINS 21% OXYGEN, SCIENTISTS HAVE CALCULATED
THAT A DRAGONFLY COULDN'T BE WIDER THAN A CENTIMETER.  BUT GRAHAM
DISCOVERED THAT PALEONTOLOGISTS BELIEVE OXYGEN LEVELS IN THE ATMOSPHERE
WERE ONCE MUCH HIGHER THAN THEY ARE TODAY.
     GRAHAM: "the oxygen levels . . . could have reached about 30% oxygen, that is about 1.5 times
as much oxygen tas there is presently. so this was very exciting to me and. . . we began to formulate this
hypothesis of how the oxygen. . . could have had an influence on animal life and the giant dragon flies is
an exciting ramification of that. "(240;30s)
     ABOUT 30 MILLION YEARS AGO THE OXYGEN LEVEL DROPPED DRAMATICALLY
AND MANY CREATURES MOST LIKELY DIED OFF, WHILE OTHERS ADAPTED TO THE NEW
CONDITIONS.  LUCKY FOR US, THE DRAGON FLY EVOLVED AND BECAME SMALLER, OR
MOSQUITOS WOULD BE THE LEAST OF OUR INSECT PROBLEMS.