Insect wings are believed to have come from aquatic gills. Eventually the
animals became surface skimmers and the gills were no longer used as gils
but as sails. Marden and Kramer note an analogical situation in a modern insect.
"The evolution of flight in insects triggered an uparalleled
radiation and diversification such that flying insects comprise
approximately two-thirds of all species, yet a gap in the fossil
record obscures the origins of wings and flight. Among modern
insects, stoneflies are morphologically primitive for several
flight-related traits, which makes their locomotor behaviour and
physiology of particular interest. Here we show that Allocapnia
vivipara may exemplify a precursor to flight. They raise their
wings in response to wind, thereby sailing across water surfaces,
but they are incapable of flapping. Sailing performance improves
steadily with increasing wing size, and even the smallest wings
significantly increase sailing velocity compared to wingless
individuals. Performance during aerial gliding is less affected
by wing size, which suggests that sailing is a more plausible
setting for wing evolution. These results support the hypothesis
that insect wings evolved from articulated gill plates of aquatic
ancestors through an intermediate semi-aquatic stage."~James H.
Marden and Melissa G. Kramer, "Locomotor performance of insects
with Rudimentary Wings",Nature, Sept. 28, 1995, p. 332.
Just a couple of weeks ago, another article was published on two other
species of stoneflies. Kramer and Marden write:
"Insect wings appear to have evolved from articulated gills of
aquatic ancestors. Possible remnants of ntermediate stages in
this transition occur in modern stoneflies that use non-flying
aerodynamic locomotion to move across water surfaces. Such wing-
propelledmovement is a feasible setting for elaboration of wing
size and the development of a powerful flight motor (muscles,
neural patterns and wing articulations). However, the mechanical
requirements of surface locomotion are not as strict as those of
flying. Flight requires dynamic control of body orientation and
generation of sufficient lift to achieve full aerodynamic weight
support. Given these mechanical challenges, how might surface
skimming insects have made the final transition to true aerial
flight? Here we report that stoneflies of the genus
Leuctra(Leuctrinae; Nemouridea, Plecoptera) use a form of
locomotion, mechanically intermediate between skimming and
flying, thus demonstrating an advanced stage on the continuum of
two-dimensional surface locomotion.
"Adult L. hippopus and L. sibleyi emerge in spring when air
temperatures are highly variable. Although otherwise relatively
proficient flyers, on contact with water surface at air
temperatures below 13[deg] C, Leuctra stoneflies curve their bodies
into a sterotypical 'S'-shaped posture, raising their fore- and
middle legs while they flap their wings and thereby move across
the water. Maintaining a nearly vertical body orientation
permits wing motion over a dramatically increased arc (nearly
180[deg]) compared with surface skimmers that maintain water contact
with all six feet and for whom the underlying water restricts
wing-beat amplitude to 90-110 [deg]. Excessive forward or rearward
pitch is controlled by briefly touching the middle feet or tip of
the abdomen down on the water such that a stable, upright body
position can be maintained over many wing-beat cycles. The risk
of injury during attitude control on water surfaces is minimal
compared with that during gliding or crude flapping (the
traditional model of insect flight envolution). Hind-leg
skimming thus demonstrates how 'flight-ready' wing-beat
kinematics and the ability to produce lift, thrust, and the
rudiments of attitude control might have evolved in a relativley
safe setting before aerial flight in insects.
"Leuctra stoneflies using hind-leg skimming attain speeds of
about 1.4 times faster than a taxonomically diverse sample of
stone flies that maintain water contact continuously with all six
legs."
. . .
"Stoneflies tested at an air temperature of 12 [deg] C can
occasionally rise just above the water surface and fly. Velocity
measurements of eight such events show that breaking free of
water and flying results in another 1.5-fold increase in forward
speed compared with hind-leg skimming."~Melissa G. Kramer and
James H. Marden, "Almost Airborne," Nature, Jan. 30, 1997, pp
403-404.
This exhausts my knowledge.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm