Stoneflies as model organisms for studying the
evolution of insect flight
How flight evolved in insects is of particular interest for two
reasons. Firstly, acquisition of flight triggered a relatively
rapid speciation and radiation that resulted in insects becoming
the most speciose life form on the planet (approximately two-thirds
of all described species). Secondly, flight is a foremost example
of one of the primary challenges for Darwinian evolution, which
is to determine transitions in function and selective advantage
for intermediate stages during evolution of complex suites of
interdependent anatomical, physiological, and behavioral features
(i.e. how do complex mechanisms evolve in a stepwise fashion?).
The fossil record and a variety of other evidence indicates that
insect wings most likely evolved from movable gill plates that
aquatic forms used for ventillation and/or swimming. We have recently
discovered that a group of morphologically primitive aquatic insects
(stoneflies) uses a form of locomotion that is intermediate between
swimming and flying. These stoneflies cannot fly, but instead
use their wings as sails (non-flapping variations in wing posture)
or as propellers (flapping) to skim across water surfaces in a
planar fashion. For a wing-flapping species, we have shown that
surface-skimming performance improves steadily with incremental
increases in wing size, muscle size, and muscle power output.
Surface-skimming is possible even when wing area is reduced by
as much as 80%, and when muscle power output is minimal. Thus,
we have concluded that this form of locomotion would have allowed
directional selection, beginning with forms possessing only gills,
towards fully flying forms. Our present goal is to determine whether
surface skimming locomotion represents retention of a primitive
state (the "missing link hypothesis") or is an evolutionary
digression from flying ancestors (the "penguin hypothesis").
To distinguish between these two alternatives, we are constructing
a molecular phylogeny for stonefly families/subfamilies using
18S DNA sequence data, onto which we will map data regarding 1)
how the various taxa locomote, and 2) functionally relevant aspects
of wing design.
Publications resulting from this project to date:
Marden, J.H. and M.G. Kramer. 1994. Surface-skimming stoneflies:
a possible intermediate stage in insect flight evolution. Science
266, 427-430. (abstract)
Marden, J.H. 1995. Flying lessons from a flightless insect. Natural
History 104 (2), 4-8
Marden, J.H. and M.G. Kramer. 1995. Locomotor performance of insects
with rudimentary wings: sailing on water versus gliding in air.
Nature 377, 332-334. (abstract & cover)
Marden, J.H. How insects learned to fly. The Sciences 35, 26-30.
Marden, J.H. and M.G. Kramer. 1995. Plecopteran surface-skimming
and insect flight evolution - reply. Science 270, 1685.
Kramer, M.G. and J.H. Marden. 1996. Almost airborne. Nature, 385,
403-404. Acknowledgements