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Evolution of Insect Flight


Pterosaurs, birds and bats took to the air from evolutionary runways that
scientists believe they understand fairly well, but insects began flying so much
longer ago that details of their stepwise conquest of flight remain obscure.

Scientists at Pennsylvania State University hypothesize, however, that a
present-day flightless insect called the stone-fly may be closely related to
ancestral insects that first learned to fly more than 330 million years ago.

Last February, Dr. James H. Marden, a biologist at Pennsylvania State
University, and Melissa G. Kramer, his student, began studying the behavior and
biology of stoneflies - the immature nymphs of which are familiar to many
fishermen as delicacies for trout.

The nymphs begin life in river or pond water and then develop primitive wings
enabling them to skim across water at high speed without actually taking to the
air. Marden and Ms. Kramer have concluded that the humble ancestor of such
expert fliers as mosquitoes and wasps may have been very much like the

The stoneflies living in Canada and the northern United States, which belong to
a primitive species called Taeniopteryx burksi, breed and mature in cold water
and come to the surface for their skimming trip to shore in February and March.
To study them, a scientist must work quickly, since the life span of a stone-fly
is only about two weeks.

The adult stonefly has waterproof hair on its feet, and after reaching the
surface of the water, it supports itself by coasting on the water's surface
meniscus layer. To hasten its trip to the shore, the insect spreads its four
feeble wings and flaps vigorously, using aerodynamic thrust to scoot across the
water at speeds up to 2 feet per second. This, Marden said, appears to be the
only time in its life the stonefly normally uses its wings.

In a series of experiments Marden described in a report published in the current
issue of the journal Science, he found that although stoneflies in the wild,
where ambient temperatures were recorded as ranging between 32 degrees and 53.6
degrees Fahrenheit, are completely flightless, their flying ability improves
when they are warmed up in a laboratory.

Even when warm, the insects never voluntarily take flight from a horizontal
surface, but if they crawl to the edge of a table and drop over the side they
will fly for a few yards before settling to the ground. Several specimens tested
by the Penn State scientists actually gained a little altitude under their own
power after being launched by hand, but none remained in the air for more than a
few seconds.

Stoneflies are interesting, Marden said in an interview, because so little is
known of the specific changes insects underwent in the remote past as they
gained the ability to fly. The stonefly's faltering efforts to use its wings may
approximate a transitional stage of evolution that occurred some 350 million
years ago, when swimming insects first became fliers.

The study of insect evolution is hampered by a gigantic gap in the fossil
record. Although fossils of early nonflying insects have been found in
sediments dating from the Devonian period nearly 400 million years ago, no
insect fossils have turned up from the following 75-million-year period.

Marden said that fossil insects reappear in strata 325 million years old, but by
then they had evolved greatly, and their increased diversity suggests that at
least some species had left the water to colonize land. Many of the fossils of
that period look like present-day insects, including grasshoppers.

Stoneflies lack some features that are important for true fliers, Marden said.
They have relatively weak wing muscles, and their thoracic cuticle plates are
not fused together to create a rigid external skeleton. Rigidity, he said, is
needed to provide strong, inflexible attachment points for an insect's wing
muscles if it is to be capable of powered flight - a much more demanding
activity than skimming or gliding.

If the stonefly is similar to the first protofliers, he said, this would argue
against a widely held hypothesis that animal flight begins with gliding, from
which powered flight eventually develops. Stoneflies never glide, even though
they are on the verge of flying.

Although the stonefly may have evolved to its present form in a progressive
direction from primitive swimming insects, it is possible, Marden believes, that
its evolution was digressive - that its ancestors were true fliers that evolved
into nonflying skimmers.

Skimming requires much less energy than true flight, as demonstrated by a new
family of skimming "wing-in-ground-effect" flightless aircraft developed during
the last decade in Russia, China and Germany. These aircraft never rise more
than a few feet above the ground or water, but their stubby wings support them
on an air cushion that eliminates the drag of surface friction.

"Stoneflies seem to have found an ecological niche in any case," Marden said.

Whether the evolutionary pathway of the stonefly was progressive or digressive
makes little difference to the insect, he said, but to an entomologist, the
direction is important.

"By mapping behavioral characters and morphology of stoneflies, we hope
eventually to infer the direction by which evolution carried them to their
present stage of development," Marden said.