Life on Earth

Locusts. In the eyes of man, one of the greatest plagues on earth.

But from a less human point of view, they are dramatically successful members

of a group that itself is the most numerous and varied kind of animal in the world.

The insects.

Like all insects, the locust's body is divided into three parts.

A head, a middle section and an abdomen that contains the digestive and reproductive organs.

The middle section is full of muscle and carries six legs

and usually a pair of wings.

Its skeleton is external, like a shell,

and it's made of chitin, a basically flexible material,

but one which can be hardened to make mouth parts tough enough to cut through leaves,

wood and even metal.

There may be as many as a million million locusts in a single swarm like this.

And these locusts are only one species.

Science has so far described and labelled nearly a million species of insects,

and there are probably two or three times as many still awaiting labels.

The very first insects evolved 300 million years ago.

From the beginning, many lived by eating plants,

but in one way at least the plants benefited from their presence.

They used them as messengers and recruited them with flowers.

Magnolias have flowers very like the first flowers developed by any plants.

They're relatively simple. They contain both male and female cells.

The male cells come from these structures around here, in the pollen,

and the female are buried at the base of this structure in the centre.

Clearly, there's a strong chance this flower might fertilise itself,

but there's a real advantage to be gained if the pollen can come from another plant

because that way, there's a greater chance of variation in the offspring.

And variation is the raw material of evolution.

It's here that the insects help the plants.

Beetles have probably fed on the spores of ferns and horsetails from early times.

So there can have been little difficulty in attracting them to the pollen in the first flowers.

Primitive moths also took to the habit very early.

Of course, if the insects ate all the pollen, that wouldn't help the plant,

but they're messy feeders, get grains all over them,

and these brush off onto other flowers and fertilise them.

So both plant and insect profit and the habit of pollen munching spread.

The plants produced more pollen than they required,

and all kinds of insects visited flowers to feast on it.

The sexual reproduction of flowering plants ensures the variation in the offspring

on which natural selection depends for evolution to take place.

The greater the insect traffic from flower to flower and plant to plant,

the greater the potential for variety and evolution.

In time, the first flowers increased the prizes on offer.

They produced sweet nectar,

and some insects turned their mouth parts into tubes

so they could probe deep into the flowers and sip it.

But such delectable rewards had to be advertised.

Some flowers became brilliantly coloured so they were conspicuous from a distance.

Some developed powerful perfumes to announce there was nectar on offer

and pollen to be transported.

The sheer beauty of flowers, their elegance of shape, their exquisite colours and patterns,

are an endless source of delight to us.

But flowers appeared on earth millions of years before man,

and they developed not to appeal to the human eye but to the eyes of insects.

These designs are far from arbitrary.

They are signals indicating where pollen and nectar can be found.

These patterns of dots and lines are as precise as instructions on an airfield,

showing the insect where to land and which way to taxi.

Many insects can see parts of the spectrum that are invisible to us, such as ultraviolet.

So if we photograph a flower with film sensitive to ultraviolet light,

we can get an insect-eye view of it, which is sometimes very different.

This meadow cranesbill seems to have faint lines on its petals,

but their ultraviolet markings are very distinct indeed.

Other plants have adopted a different tactic.

Instead of producing pollen in one place on a big flower,

they produce many tiny flowers in a showy bunch,

so wherever visiting insects go, there is pollen and nectar to be gathered.

Some have taken this design so far, they look like single flowers.

The yellow mass in the centre of this daisy is made up of several hundred small flowers,

each with its stamens and ovaries.

So it is to insects and their sensitive eyes that we owe so much beauty.

But there are many drab flowers: the hazel, for example.

It's obvious these must rely on a different way of transporting pollen. The wind.

The male flowers have to be large

to produce the great quantities of pollen needed for such a haphazard method.

But the female flower, with no need to advertise, is an inconspicuous little tuft.

Oak trees use a similar system

with male flowers that fill the atmosphere with pollen,

only a tiny proportion of which rains down onto the place where it serves its purpose:

on the female flower.

Some flowers use wind in a different way, to summon insects with perfume.

The arum lily's intoxicating scent attracts them just as it pleases us.

But some insects have different tastes from ours.

The stapelia smells of rotting flesh,

disgusting to us, but extremely attractive to flies that feed on carrion.

And when they arrive, they find flowers that tempt them still further,

for their petals resemble the wrinkled, decaying skin of a dead animal.

The amorphophallus of the jungles of the Far East relies almost entirely on smell.

The overpowering stench that comes from this huge bloom, as tall as a man,

resembles that of rotting fish mixed with burnt sugar.

Its European relative, the modest wild arum or cuckoo pint of English hedgerows,

also produces a faint unpleasant smell as well as warmth.

Having attracted small flies, it then traps them.

The lower part of the scent-producing rod secretes drops of oil.

Insect visitors lose their foothold and tumble past the slippery, downward-pointing hairs

into the lower chamber, where the flowers are.

The top ones are male, which are not mature. There's nothing here for the insects.

Below the male flowers are the female flowers.

The small flies, which may have visited other arums the previous day,

now inadvertently spread pollen on them.

But the insects can't escape. The oily hairs keep them imprisoned

and they have to remain there all night.

The next morning, the hairs, the bars of their prison, have shrivelled.

The female flowers have closed their stigmas so they can no longer be fertilised,

and secreted a tiny drop of honey as a reward.

But the male flowers have opened and shed pollen over the flies,

which are now free to look for another arum in which they may, inadvertently, spend the night.

Pollen taken from one species of flower and deposited on a different species is wasted.

So there's been a tendency in the insect-flower alliance for partnerships to develop

and for one species of flower to be intimately involved with one species of insect.

The nectar of some flowers is reserved

for those insects with exactly the right mouth parts and feeding manners,

and which will assiduously visit all the blooms of that species

that they can manage during the flowering season.

The salvia blossom only opens its doors when an insect of the weight and shape of a bee

lands on its flight deck, triggering the stamens to stamp pollen on top of its abdomen.

The flowers go on producing nectar, and a few days later their ovaries become mature.

When a bee comes to visit them this time, it's the stigma from the top of the ovary

that jerks downwards and collects the pollen.

This kind of relationship has led flowers away from the original circular designs like magnolias

to develop complicated constructions of triggers and levers,

delicately balanced platforms and slippery pits.

The bloom has now become a kind of obstacle course,

so visitors are not able to collect their rewards

without completing the essential service of transporting the pollen.

The most complicated mechanisms of all are those produced by orchids.

Even now, there are some we don't understand.

This one, the flying duck orchid from Australia, has the most extraordinary action as it opens,

but we don't know why it's shaped this way, why it moves like this

or on what insect it relies to carry its pollen.

This orchid attracts insects by sexual impersonation.

It gives off a perfume like that of a female ichneumon wasp.

When the male arrives, he finds something that smells and looks like a female.

At one end of the bloom, there's a mass of pollen stuck together into a horseshoe shape.

The ichneumon male copulates with the flower.

And the pollen mass is so placed that it fastens neatly onto his abdomen.

In fact, this orchid is totally dependent on one species of ichneumon wasp for pollination

and therefore reproduction.

The orchid can only survive as long as the ichneumon wasps do.

When the male insect copulates with the next flower, he delivers the pollen from the last.

The yucca plant of Central America

has a relationship with its insect partner that is so close

that now both insect and plant are completely dependent on one another.

The yucca's creamy blossoms are visited by tiny moths.

During the day, the moths spend a lot of time moving from flower to flower and inspecting them.

All are not at the same stage of development.

The stamens become mature first and split open, and it's these that the moth is looking for.

In the late afternoon, the female moth, having already mated,

is collecting pollen from suitable flowers.

She's now gathered the pollen into a tight ball

which she holds under her head as she searches

for other flowers which are in a different state of development.

This time, she's more interested in the central part of the flower,

and takes up a position alongside one of the ovaries, which have a green-tipped stigma.

Here she will stay for about 20 minutes.

Her egg-laying tube is deep at the bottom of the flower's ovary,

and she's laying her own eggs there.

Having finished laying, she separates pollen grains from the ball she's collected

and smears them into the stigma with mouth parts specially developed for the purpose.

Now she will repeat the entire procedure in other ovaries of the flower.

The egg-laying position again.

Again she will pollinate the flower.

First she removes a small amount of pollen from the ball she's holding.

By pollinating the flower, she serves not only the yucca but her own offspring,

for she ensures the eggs in the ovary below will develop

so that her caterpillars when they hatch will have a rich source of food immediately to hand.

But the caterpillars won't eat all the seeds.

The moths don't lay as many eggs as that. So when the yucca comes into fruit,

there are plenty of undamaged seeds to ensure that new plants will appear.

But the balance is a very delicate one.

If it went wrong, it could be disastrous for both plants and insect.

Without the moth, the yucca would not be pollinated.

Nothing else has those special mouth parts for pressing the pollen into the style.

And without the yucca, the moth's caterpillars would starve.

The seductive odours and beguiling shapes of flowers are so attractive to insects

that they find them virtually irresistible.

Other insects turn that to their advantage in a different way.

This ginger flower has petals that move.

It's one of the most extravagant designs of any insect.

For this, with flaps on its legs that match the petals of the flower, is a mantis.

The butterfly comes to sip nectar.

There are many different kinds of mantis, all marvellously camouflaged,

all voracious hunters.

The flesh of an insect is succulent, but first the mantis has to deal with the external skeleton,

the shell of chitin.

Chitin is dead material. It won't expand.

It's one of the few limitations to the insect body that is otherwise so versatile.

In order to grow, all insects have to shed their skin at regular intervals,

and this bug is just about to do so.