Dragonfly

Who can fail to admire the fight of dragonflies as they dart around in search of insect prey, or as they flit above the water and drop their eggs?

Yet these aerial wonders have spent one or two years under water as creeping nymphs that lie in wait for prey, then seizing it with an extendible grasping jaw.

This life-cycle is typical of many insects, giving them the ability to occupy two utterly different ecological niches at different times. Unlike the dragonflies, order Odonata, many other orders of insects have an intermediate pupal stage, during which they transform into a an insect that hardly resembles the larva at all.

Damselflies belong to the insect order Odonata - sub-order Zygoptera, meaning twin-wings. Both pairs of wings are very similar, which is probably a primitive condition.

They fly rather weakly, and they look fragile, but they have survived for over 200 M years.

Their larvae live in water for about one year, before emerging and moulting for the last time. the larvae generally have three gills at the tip of the abdomen. They catch small prey.

The mating behaviours of both damselflies and dragonflies are very different from those of most insects.

Egg-laying often takes place while the male and female are still coupled. Sometimes the female even walks down a plant stem and goes right under the water, to lay an egg in the stem. All the time the male is balanced upright with the female's neck still clasped at the tip of his abdomen.

This is a male Calopteryx splendens. The female has dull brown wings. These damsels are found by slowly flowing streams, often in considerable numbers. The iridescent colours are produced by minute surface structures and not by pigments.

This is a much smaller damselfly, found at ponds and lakes. Some species, such as Ischnura elegans, are found in several colour variations. If you have a pond you may see some varieties which are unusual.

Dragonflies belong to the insect order Odonate - suborder Anisoptera, meaning different wings, the front and rear pairs being different.

Dragonflies are generally bigger and faster than damselflies.

Some dragonflies can fly non-stop from dawn until dusk, sixteen hours in some cases. They are the swifts and swallows of the insect world. During these flights they find all the food they need by seizing flying insects. When mature, they find a suitable pond, and, if male, they fend off rivals for their territory while waiting for a female. Some dragonflies are fiercely territorial, and may engage in short but spectacular dog-fights.

After the female has laid her eggs, either in pond plants or scattered over the water, they will soon hatch out into small larvae. Over a period of one, two, or possibly three, years, these larvae will catch many small fish, tadpoles and other creatures, using a an extending appendage called a mask. It is equipped with two curved spikes, which hold the prey while it is eaten.

The larvae, or nymphs, do not differ as much from the adults as caterpillars do from butterflies, and there is no pupal stage, but the transformation from the nymph to the adult instar is a magical event, especially if the colours are produced then and not days later. In some cases the body changes from almost transparent to a metallic colour in a few seconds. If you see a dragonfly in the process of emerging, don't walk away. Watch.

A cloud of new dragonflies rising from the margin of a pond in the early morning light is a fine spectacle.

The basic external design of dragonflies seems to have changed little in 200 M years, though none today reach the two foot, 60 cm, span of some ancient species.

This a Libellula depressa. In the first picture it has finished expanding its wings, but has not yet moved them into the final position. The position over the back is reminiscent of the may-flies, members of another ancient order of insects, the Ephemeroptera.

The wings are small and tightly crumpled when the insect emerges from its previous skin, but after pumping up its body, it expands the wings by pumping fluid into the veins.

Once the wings have hardened in their final shape, the veins serve to maintain the shape of the wings, and to control flexure in flight. The position and thickness of each vein contributes to the control of the wings that the dragonfly needs in order to make accurate attacks on smaller insects, and to avoid predators.

Each eye has many thousand tubular ommatidia, which produce an image composed of small hexagonally arranged pixels. The small size of these eyelets means that the insect can detect very small objects and very slow movements. Try to catch one and see what happens.

This species belongs to the group of dragonflies which are sometimes known as darters. Their method of hunting is to sit on a perch and wait for a likely prey. Some use a favourite perch so reliably that you can use this behaviour to get good photographs.

Sometimes, by putting your finger at exactly the right place, you can get the dragonfly to sit on it. It helps if your finger doesn't look too different from the perch.

The male Libellula depressa has a mainly pale blue abdomen, with yellow markings at the edge. These markings are visible in the pictures. The female is a pale yellowish brown. Many dragonflies do not get their final colours for some days, or even weeks, after emerging from the water.

This dragonfly was photographed in the French Jura.

In the left hand picture the wings are drying after expansion. In the right hand picture the dragonfly has acquired its final colour, and is ready to make its first flight. Unlike those species which take days to gain their colours, this one took only a minute or so. It was a magical transformation from pale buff to metallic bottle green. Such a colour needs no pigment - it relies on the structure of the surface. See Interference

Let's see if we can calculate something about a dragonfly's vision. Some species have over 20000 ommatidia in their eyes. Let's be conservative and assume that there are 10000 in each eye, and for simplicity that they point equally over a sphere.

Now we can calculate the solid angle that each one covers. The whole sphere is 4pi steradians, so one ommatidium covers 4pi/20000, or 0.0002pi.

Now we can work out the linear angle that each one covers. We have a circle of area 0.00002pi on a unit sphere.

If its diameter is d, then pi X d² = 0.0002pi.

So d = 0.014, and the angle is 0.014 radians, a little less than a degree.

If the dragonfly can detect a change in an image by one pixel, then at one metre it can detect a movement of 1.4 cm.

This calculation is extremely crude, and includes no allowance for possible ability to construct finer boundaries using several pixels. But it gives some idea of what might be possible.

As the dragonfly homes in on prey, the accuracy of the vision will improve. At 10 cm, our 1.4 cm becomes 1.4 mm, which is less than the range and span of the legs for catching.

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 this insect is tubular, which is advantageous for breathing, because the animal has no lungs. Air simply diffuses. So an insect can never be enormously big, because every part needs to be near the surface. A tube is just the right shape, therefore, 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 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 hooks. 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 pumped into the veins of the wings 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 spars of an 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 look 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 of some of the weight is carried by them directly.

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, having not developed proper wings. Who knows how their distant descendents 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 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 small objects.

Making reasonable assumptions, it is not difficult to calculate the smallest size and speed of what a dragonfly can see.

The spiny legs, used for seizing prey and taking it to the mouth, are hollow 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.

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.

Click here and here to find out more about iridescent colours.

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 just 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 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 as conditions change,

if only because all other species are changing, the multi-dimensional 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.