When Giants Had Wings and Six Legs

[Danvarner@aol.com]

Hey! The world of prehistoric insects has its own Jim Cunningham! This is 
from today's New York Times. If you go to the link you can see a nice graphic 
of 
a life-size dragonfly giant. I still think a larva of one of these would be a 
nightmare! Kind of reminds me of my basement room when I was a kid. DV   
    

When Giants Had Wings and 6 Legs

February 3, 2004
By HENRY FOUNTAIN 





There was a time when giants roamed the Earth. 

No, not those giants, the dinosaurs that stomped and
slogged their way through the Mesozoic Era. These giants
crawled and crept, slithered and scurried, burrowed,
slinked, skittered and, above all, flitted and fluttered
millions of years before the dinosaurs arrived. 

They were the giant arthropods of the Carboniferous.


There were extra-large mayflies, supersized scorpions and
spiders the size of a healthy spider plant. There was an
array of giant flightless insects, and a five-foot-long
millipede-like creature, Arthropleura, that resembled a
tire tread rolled out flat. 

But perhaps the most remarkable of all were the giant
dragonflies, Meganeuropsis permiana and its cousins, with
wingspans that reached two and a half feet. They were the
largest insects that ever lived. 

These large species thrived about 300 million years ago,
when much of the land was lush and tropical and there was
an explosion of vascular plants (which later formed coal,
which is why the period is called the Carboniferous). But
the giant species were gone by the middle to late Permian,
some 50 million years later. 

Scientists have long suspected that atmospheric oxygen
played a central role in both the rise and fall of these
organisms. Recent research on the ancient climate by Dr.
Robert A. Berner, a Yale geologist, and others reinforces
the idea of a rise in oxygen concentration - to about 35
percent, compared with 21 percent now - during the
Carboniferous. Because of the way many arthropods get their
oxygen, directly through tiny air tubes that branch through
their tissues rather than indirectly through blood, higher
levels of the gas might have allowed bigger bugs to evolve.


But there are other possibilities - a lack of predators,
for example. Fundamentally, no one is certain why there
were giants. 

"It's been out there in the literature for a long time
without a causal mechanism," said Dr. Robert Dudley, a
professor at the University of California at Berkeley who
has studied the effects of elevated oxygen pressures on
modern insects. "This is a very imperfect science. There's
a very fragmented paleontological record." 

Dr. Jon F. Harrison, a professor at Arizona State who has
performed similar studies, said, "It's still in the realm
of speculation." 

While there has been much interesting research, he added,
"it doesn't prove anything yet." 

Some scientists argue that these large species may have
been nothing out of the ordinary, that, in effect, they may
not have been giants at all. 

Dr. David Grimaldi, a curator in the division of
invertebrate zoology at the American Museum of Natural
History and co-author of a forthcoming book on the
evolution of insects, noted that most Carboniferous insects
were of very similar size to those found today. But the
fossil record tends to be biased toward larger specimens
for the simple reason that they are easier to find. 

Though about a million insect species now exist, Dr.
Grimaldi added, over about 75 million years of the
Carboniferous, as species came and went, there were bound
to be many more. So the largest species may simply
represent the upper range of a far more diverse population.


"If you increase the sampling over millions of years, to
some extent you are bound to encounter some giants," Dr.
Grimaldi said. 

Still, the idea that there were bugs larger than anything
to be found today captures the imagination, particularly
the idea of a dragonfly with wings as wide as some hawks'
(though much less substantial), plucking smaller prey out
of the air as modern dragonflies do. 

For a long time, scientists believed that an insect of that
size must have been able only to glide, but most now
believe that the giant dragonflies actually flew. 

"It's pretty obvious that they were active fliers," said
Dr. Roy J. Beckemeyer, a retired aeronautical engineer in
Wichita, Kan., who has studied modern and fossil
dragonflies for a decade. Dr. Beckemeyer says he is
fortunate to live where he does because many of the best
fossil insect specimens come from deposits along ancient
bays in what are now Kansas and Oklahoma. 

One of his specialties in aeronautics was wing flutter, the
relationship between bending and twisting that in the worst
of circumstances can cause an airplane's wings to fall off.
Modern dragonflies, he said, bend and twist their wings,
giving them both loft and propulsion. 

There are similarities in the corrugated structure of
ancient and modern dragonfly wings, Dr. Beckemeyer said,
though in modern species the twisting occurs in the outer
half of the wing. "In ancient dragonflies, it appears there
was a more gradual twisting over the whole length," he
said. "It's likely that they didn't fly as fast." 

Even slow flight for an insect that big, however, requires
much muscular activity, which creates heat. Dr. Michael L.
May, an entomologist at Rutgers, was the first to show that
the ancient dragonflies must have had some way to dissipate
the extra heat. "If they didn't, they would have cooked
themselves," Dr. May said. 

Modern dragonflies, like other insects, pump a fluid called
hemolymph throughout their bodies. When they get too hot,
they can increase the flow of hemolymph to the abdomen,
taking the heat away from the tissues in much the same way
a car's cooling system carries heat from the engine. The
abdomen, which is long and skinny, can dissipate the extra
heat through convection. 

Although there is no direct evidence, Dr. May said it was
possible that ancient dragonflies had a similar system,
enabling them to fly for longer periods without
overheating. 

The lack of evidence - with fossils, generally only the
skeletal tissues are preserved - presents problems in
figuring out just how these large species were able to
exist. But more certainty surrounds the way the oxygen
content of the prehistoric atmosphere changed over millions
of years. 

One way Dr. Berner and his colleagues study oxygen levels
is by looking at a different element, carbon, in ancient
sedimentary rocks. 

"The guiding light in all this is the burial of carbon,"
Dr. Berner said. 

Photosynthesis takes carbon dioxide out of the atmosphere
and converts it to oxygen, which is released, and organic
matter, which is incorporated in the plant. Plants die and
decay and are buried, Dr. Berner said, "and for every
carbon you bury, you leave an oxygen behind." 

So during the Carboniferous, as plants spread on land,
there was less carbon dioxide and more oxygen. Dr. Berner
calculated that oxygen concentration reached its peak of
about 35 percent 300 million years ago. It declined
abruptly at the end of the Permian, about 250 million years
ago, a time of the greatest mass extinction in the planet's
history. (The cause of the decline, and of the extinctions,
is a subject of much debate.) 

To determine whether all that extra oxygen could have led
to giant dragonflies and the like, Dr. Harrison, Dr. Dudley
and others turn to modern insects. 

Insects "breathe" through holes in their bodies, called
spiracles, which are attached to hollow tubes, or tracheas.
The tracheas branch into smaller and smaller tubes, and the
oxygen diffuses through them, nourishing the extreme
reaches of the insect's body. 

At current oxygen levels, there is an overall length limit
of these tracheal tubes; beyond that, the oxygen level is
inadequate. This effectively limits insect size. 

One approach, Dr. Harrison said, involves determining
whether it is harder for larger insects to get oxygen. If
this is true, he added, higher oxygen levels are a benefit
to them, and it can be argued that larger insects have had
an evolutionary advantage in a high-oxygen atmosphere. 

But Dr. Harrison said most of his experiments with
grasshoppers and dragonflies do not really support the idea
that raising the oxygen level makes a difference. "You've
got all the oxygen you need already," he said. 

For one thing, he noted, larger insects do not breathe
through passive diffusion only. There is some pumping that
creates pressure differentials that cause air to actually
flow through the tubes, reaching farther than by diffusion
alone. 

Other research, however, has shown that there is some
effect of greater oxygen concentration on the size of an
organism. Studies of marine invertebrates, for example,
have found a correlation between larger species and colder,
more oxygen-rich waters: the more oxygen in the water,
essentially, the bigger the creatures get. 

Dr. Dudley and others have conducted experiments raising
fruit flies and other insects in oxygen-rich environments.
Some have shown size increases; others have not. 

Dr. Dudley has focused on pressure because, in addition to
having a higher concentration of oxygen, the Carboniferous
atmosphere would have had much more of the gas. "Plants
were pumping oxygen into the atmosphere," he said. The
amount of nitrogen would have been undiminished, so overall
pressure would have risen. 

Though the results have yet to be published, his
experiments with fruit flies raised under elevated
pressures show a 20 percent increase in body mass over five
generations. 

But why would more oxygen make for bigger insects? 

One
idea, Dr. Harrison said, is that oxygen may be a trigger
for molting. Before they shed their skin, Dr. Harrison
said, invertebrates generally double their weight. During
this period, their spiracles and tracheas are of
pre-molting size, but they could use much more oxygen to
grow. So the ancient atmosphere would have provided more
oxygen during molting, enabling greater growth. "That might
be a mechanism that would explain this," he said. 

Or it may not. For so much remains unknown about these
giants. There may be plenty to suggest that oxygen played a
role in their evolution, Dr. Dudley said, "but it's real
difficult to take it one step further." 

http://www.nytimes.com/2004/02/03/science/03INSE.html?ex=1076833096&ei=1&;
en=3dea097580530589