SUMMARY CHART ON GLIDING AND JUMPING GLIDCHRT.SAM
Jumping or gliding from small plant (to 50 cm)
This would be useful for escape, and the ability to land upright could
be significant, but the heights invoved are not sufficient for gliding.
For energy saving . . .?
Gliding from medium or high plants (from 50 cm to 20 m)
It has been proposed under the PAranotal Theory that insects would
climb up plants to eat, and then would "descend on enlarged paratergal
outgrowths" at a time when "the appearance of winged insects and high
vegetaion was almost synchronous." (Sharov, p. 111). Indeed, according
to the fossil record, by the midle Devonian, plants had reached 10-12
ft in height,(A. C. Seward, Plant Life Through the Ages, 1933), and
in the late Devonian plants with straight trunks, 20 to 40 ft in height
existed. By the time of winged insects, in the Pennsylvanian Carboniferous,
the height of certain lycopods had attained 100-125 ft. (C. L. and
M. A. Fenton, The Fossil Book, Doubleday, 1958).
However, we can raise a point. On plants where there was enough for
insects to eat moving both up and down, there would be no need to
jump off. There would only be an advantage in energy saving on tall
plants with bare trunks, certainly extant in the Paleozoic era. However,
once the insect dropped from one plant, it would have to climb up
another one, so it might be better to traverse the vegetation horizontally
through overlapping foliage, a generally common characteristic of
plants. The idea of energy saving by dropping or gliding from heights
seems to rely too much on widely spaced or solitary plants.
Gliding off plants to escape
It is generally proposed that gliding so as to land on the feet provides
quick escape. However, it seems this would only be significant on
small plants, up to 50 cm. From greater heights the suddenly increasing
distance would keep the gliding insect safe even if not landing on
the feet. If the pursueing predatory insect could jump as well, then
it, too, would have to land correctly to successfully capture the
former. A parallel, mutually reinforcing development, such as takes
place between prey and predator, host and parasite, would not be discountable,
in developing increasing attitude control in both insetcs, still
there would be serious obstacles. Lack of precision in attitude control,
aerial forces, obstructing vegetation, and the resulting seriously
decreased visibility would mean that the predator might drift or be
deflected from the target, and once on the ground, it would undoubtedly
lose sight of the victim. Simply jumping off, or gliding, under condition
where landing speeds are not injurious, appear sufficient to allow
an escape.
Sensitivity in attitude control in gliding
To change attitude through minor changes of body shape, thus altering
the angle of paratergal expansions, would no doubt require a sensitivity
to minor changes in attitude. But such ability is not primary in insects,
which in their normal surface locomotion may be at any angle to the
ground. Extant flying insects can, of course, judge and control attitude
changes in flight. For example, Diptera have vibrating hind wings
developed into stalked knobs, or halteres, which are gyroscopic instruments
sensing rotation of the body. An ability to sense rotation for a gliding
insect, with legs off the substrate, may evolve, but selective pressure
would be far less here than in the case of flight, where increased
speeds place far greater stress on controlling attitude.
Anatomic question of attitude control
We can allow that an insect would perhaps compress the thorax asymmetrically,
and this would change the planes of the paratergal expansions simultaneously
on both sides. Sufficient increase in lobe size and the ability to
control their angles could offer a measure of control. However, independent
mobility of lobes on each side would be a refinement, but this is
not posssible unless the lobes gain independence from the thorax by
Lobes large enough to glide, but not foldable
Lobes would increase in surface area to prolong gliding, but
there might be limits in cases other than nuptial display because
these lateral expansions could impede terrestrial or arboreal movement.
There would be a point in the devlopment of size, needing a compromise
between the requirements of normal locomotion and aerial gliding.
The ability to fold the proto-wings wuld be a solution here, a detail
adding to the compexity of flight evolution form gliding on lobes.
It may be said that large, near wing-sized gliding lobes would not
be a problem in locomotion if these emerged in a last instar, charged
solely with reproduction, similarly to extant Ephemeroptera, ants
and termites. Still, unless folding them was possible, an oversized
lobe may impede a prospective suitor at the worst moments. On the
positive side, large non-folding lobes, however, could help in keeping
rivals from access to a couple engaged in epigamic activity.
The need to land on the feet
As already mentioned above, this ability is only meaningful when involving
small heights, where gliding and attitude conmtrol are not pracical,
therefore we may concluded that this ability would not lhave good
reasons for developing.
Rebounding on landing
Except on surfaces most absorbent of impact, an insect that did not
glide -- very small ones float safely -- An insect that lands in
terrestrial jump, or from a jump or drop from a certain height will
tend to rebound when hitting the target, unless this is absorbent
of impact, such as soggy or matted plant material. This is another
reason why landing on feet is not practicable for any but a gliding
insect, which would have no great need for it.
Correct incidence
The degree of success for the glide an insect might perform depends
not only on the insect's structural aerodynamic characteristics, but
on its angle of incidence, or angle of body inclination as it begins
to fall. But considering that an escaping insect, up on a plant, may
be at any angle to the ground, we can ask whether the defvelopment
of attitude control in escaping has any point, if those who land from
an average height, whether on the feet or not, can survive the escape
as well as those that have controlled their landing attitude.
Gliding from plants as nuptial display
This activity would be applicable for nuptial display, especially
in the form of large numbers of one gender drifting down in the air,
presetning a large, moving visual display. Climbing back up would
not be necessary, whether a last instar moulting, from which the reproductive
form emerged, took place high up on the plant, and the insects droppd
only once, or if the insects have climbed up to drop only once, since
when we are talking of large swarms, there might be enough individuals
produced to prolong the display.
Aerial dispersal is perhaps the most likely adaptive function we can
imagiune for gliding from high plants.
I can't think of anything else to summarize
about dispersal