Aircraft, bamboo, bones, bridges, drinking straws,
feathers, insects, love, pipes, submarines and tunnels
Damselflies and dragonflies have many features of a typical insect. One of these is a cuticle, or exo-skeleton, made of chitin. Like many natural materials, chitin is very versatile. It can be thick and rigid, or thin and flexible. It even penetrates inside the insect in the form of breathing tubes, which are inward developments of the cuticle.The abdomen of these insects is tubular, which is advantageous for breathing, because the animals have no lungs. Air simply diffuses, helped in some cases by movements of the abdomen. So an insect can never be enormously big, because every part needs to be near the surface. A tube is just the right shape for a creature that spends much time flying, and needs much oxygen.
The long tube, with a relatively large moment of inertia, may help to stabilize the insect against the motion of the four large wings.
| The wings
of dragonflies are based on tubes, arranged in a fine
network. More "advanced" insects have fewer veins, often very
few indeed, and they also tend to use only two wings instead of four, just as
aircraft evolved from biplanes to monoplanes.
Even those with four wings have often evolved to the point where one pair acts only as a cover for the other par. In butterflies the two pairs are linked in flight by their shape. Some moths have hooks for the same purpose. In flies, the rear pair of wings have evolved into oscillating halteres, which function in balancing, much as a gyro does.
The word "advanced" is not entirely suitable, for flies, dragonflies and ourselves all made it to the present, after descent from a common pair of individual creatures, which must have lived several hundred millions years ago. So in a sense, we are all equally successful - so far. Which will be the last to survive - the descendents of ourselves, flies, or dragonflies?
The dragonflies were around long before the flies, which in turn preceded people by many millions of years.
Returning to the tubes in the wings, these have two main functions. When the insect has emerged from the previous skin, the wings are tightly crumpled, and are very small. If you watch an emerging dragonfly, you can see fluid moving around the body, through the transparent cuticle. The fluid expands the body to its final shape, and it is then pumped into the veins of the wings to expand them. In the first picture the Libellula depressa has completed the expansion. In the second picture it is almost ready for flight, but it it will not get its body colour for some days.
The abdomen of the male is mainly pale blue when mature. That of the female is yellowish brown.
The main wing spars of a large airliner are usually in box form, which is very convenient for carrying the fuel, as well as for stiffening the wing. Carrying the engines and the fuel on the wings may seem wrong when the aircraft is on the ground, but in the air, the weight is better distributed than if the fuel and engines were carried by the fuselage. The bending moment in the wings is much less if some of the weight is carried by them directly, rather than entirely through the roots.
On the ground the wings have to take a reverse force at the roots.
People have sometimes tried to make aircraft with everything inside a thick wing, but such schemes have never been as successful as those based on division of function among specialist parts, such as fuselage, wings, tailplane, and fin.
All animals that are highly evolved for flying or gliding have separate organs for flying. Animals such as gliding frogs, squirrels and snakes have rather poor gliding angles, have not developed proper wings. But who knows how their distant descendants might look?
As in aircraft spars or box-sections, the other function of the veins in the wings of an is to stiffen the cantilever that each wing actually is. The stiffening is greater near the front, so that even a crude up and down flapping will tend to produce lift and forward motion, as the angle of incidence changes during each flapping cycle. In fact the motion of the wings is very complex, and the attachment to the thorax, and the musculature and structure of the thorax, all contribute to the ability to fly. Some damsels can fly towards a spider's web, collect the spiders' prey, and fly backwards with the booty.Most dragonflies can catch prey on the wing. They are superb fliers. Some can fly non-stop from dawn to dusk, catching all the food they need, and looking for a mate as well. Others only perch, and dart out when prey is seen. With up to 20000 ommatidia, or individual sensing tubes, in each eye, the dragonfly can sense quite slow movements and very small objects.
Making reasonable assumptions, it is not difficult to calculate the smallest size and speed of something a dragonfly can see.
The spiny legs, used for seizing prey and taking it to the mouth, are tubular like the body. As with all insect muscles, the leg muscles are attached to the exoskeleton. Living in a box or a tube may have limitations, but it is a successful mode of life, judging by the enormous number of insect species. The most numerous insect order in terms of species is the coleoptera, or beetles, which are especially well protected by wing cases and cuticle.
Where the insects lose out is in size - they can never become the type of giants that are sometimes seen in films. One reason is the method of breathing.Most dragonflies look very pale for a long time after their last moult, and may take many days to acquire their full colours. But an amazing thing happens with some which have iridescent colours - in the space of only minutes you can watch an almost transparent body become a metallic green, for example. These colours are not caused by pigment, but by interference of light at tiny structures on the surface.
|It's hard to look at something like
a dragonfly without thinking that there must be something special about a
creature that can fly non-stop for sixteen hours, find its own fuel, and
reproduce. Yet in a sense it is not a perfect flying machine. It
could fly just a little better if it didn't have to carry digestive organs, legs
and reproductive organs. But then it would die out. So it is not
quite perfect at any one job.
At the other extreme is a female termite, which is little more a huge bulk for making eggs. It really cannot do anything else, except control the workers. It relies on the workers it controls to sustain it. It is almost a perfect reproducing machine. What the species has done is effectively to export some of the work of its organs by using other individuals. Some plants and animals take this to the extreme of parasitism.
We can admire the flight skills of the dragonfly, but there must be thousands of other attributes which are equally well tuned. What is most perfectly tuned by evolution is not any one attribute, but the probability of reproduction. In a hugely dimensional probability space, each species lies near a local maximum of probability. Perhaps the peak is never obtained, because conditions change, if only because all other species are changing, and so the probability map is changing. So each species is a little behind perfection all the time. It would be utterly impossible to compute a prediction of the evolutionary path of a system of life-forms. In effect, life on earth is a giant Monte-Carlo program, continually trying and rejecting or using possible paths.
This aircraft, like many others, has a fuselage based on a tube, and wings based on box sections. In fact, since people have to be inside, almost all forms of transport use an exoskeleton, like insects, with the living animal inside. One famous car was even nicknamed "Beetle". The bicycle and the motorcycle are exceptional in that the user is outside the vehicle.They are more closely related to the horse, donkey and mule. Both types of bike use tubes in their construction. Trains and cars can go through mountains or under rivers and seas in tunnels, which are circular tubes, like submarines, to resist the pressure, in places where the rock is not strong enough.
However, in many applications, it is simpler for fabrication and assembly to use rectangular or trapezium sections, as in the towers of many suspension bridges.