1 - War Against The Six-Legs: The Discovery of insecticides and pesticides

Just about the greatest problem we all face now is our own numbers. We crowd the earth more thickly now than we ever have before and this is creating strains.

Before the invention of agriculture about 8500 B.C., man lived on the animals he could catch and kill and on the plants he could find that were good to eat. At that time, there weren't many human beings on Earth. One careful guess is that there were only eight million people on the whole planet. (That's about the population of New York City today. Imagine New Yorkers being the only people alive and that they were spread over the entire planet.)

The reason there were so few then was that there are only so many animals to be caught and only so many plants to be found. If, for some reason, there were suddenly more people, some of them would be sure to starve to death. The population would shrink again.

Once agriculture was developed, people deliberately grew large quantities of plants that could be eaten. There was more food to be found in one spot and more people could eat well. Population increased.

By the time of Julius Caesar, in 50 B.C., there were fifty million people living on agriculture around the shores of the Mediterranean Sea. Another fifty million were living in China and another fifty million in the rest of the world. The total for the world was 150 million but that was still less than the population of the United States alone today.

Population continued to increase and by 1600 A.D., it had reached 500 million.

After that, the increase became so rapid that we can speak of a "population explosion." New continents had been discovered with large tracts of land into which people could push and where they could begin to farm. The Industrial Revolution came and made it possible to farm more efficiently and ship food greater distances.

By 1800, the world population was 900 million; by 1900, it was 1,600,000,000. Now, it is about 3,500,000,000. Three and a half billion people are alive today.

In recent years, medical advances have placed many diseases under control. The death rate has dropped and with fewer people dying, population is increasing faster than ever. The world population doubled between 1900 and 1969, a period of sixty-nine years. It will double again, in all likelihood, between 1969 and 2009, a period of only forty years.

When the twenty-first century opens, and the youngsters of today are in middle life and raising a family, the world population will be something like 6,500,000,000. The United States alone will have a population of 330 million.

Naturally, this can't continue forever. There comes a point when the number of men, women, and children is too great to feed and take care of. If the numbers become too great, there will be famine and disease. Desperate, hungry men will fight and there will be wars and revolts.

With this in mind, many people are trying to discover ways of limiting the population by controlling the number of births. It seems to make sense that no more children should be born than we can feed and take care of. It is no act of kindness to bring a child into the world who must starve, or live a miserable, stunted life.

It is possible that kind and intelligent ways of controlling birth will be accepted and that human population will reach some reasonable level and stay there. It will take time for this to come to pass, however, and no matter what we do the figure of 6,500,000,000 will probably be reached. Even if it goes no higher, we will have to count on feeding and taking care of this number.

This will be difficult. At this very time, when the world population is only 3,500,000,000, we are having difficulty. Large sections of the world are poorly fed. There are perhaps 300 million children in the world who are so badly underfed that they may have suffered permanent brain damage and will therefore never be quite as intelligent and useful as they might have been if they had only received proper food. Nations such as India face famine and would have seen millions die already if it were not that the United States shipped them huge quantities of grain out of its own plentiful supplies. But American supplies are dwindling fast, and when they are gone, what will happen to nations like India?

There are no longer large empty spaces of good land which farmers can utilize. The fertile areas of the world are all in use. We have to try to find less easy solutions. We can bring water to dry areas. We can use chemicals to restore the fertility of soil which has been fading out after centuries of farming. We can use more fish from the ocean; and perhaps we can even grow plants in the sea.

Actually, mankind has been steadily increasing food production since World War II. The trouble is that this food increase has barely matched the population increase. Despite all the extra food, each individual today gets no more than he used to get twenty years ago. The percentage of hungry people in the world stays the same.

And as the population rises ever faster, it is important that the food supply increase ever faster also. It is important to feed the ever-increasing numbers of human beings until such time as the population can come under control.

One way of doing so, without having to increase the size of our farmlands one bit, would be to prevent any of our precious food from being eaten by creatures other than humans. Farmers are always on the watch for hawks that eat their chickens, coyotes that eat their lambs, crows that eat their corn.

These are creatures we can see and do something about. We can lay traps, or shoot, or set up scarecrows.

But hawks, and coyotes, and crows are nothing at all compared to an enemy that is much smaller, much more dangerous, and until very recently, almost impossible to fight.

These are the insects; the little buzzing, flying six-legged creatures that we find everywhere.

Insects are the most successful form of animal life on earth. There are nearly a million different kinds (or "species") of insects known, and perhaps another two million species exist that have not yet been discovered and described. This is far more than the total number of different species of all other animals put together.

The number of individual insects is incredible. In and above a single acre of moist soil there may be as many as four million insects of hundreds of different species. There may be as many as a billion billion (1,000,000,000,000,000,000) insects living in the world right now-over 300 million insects for each man, woman, and child alive.

Almost all the different species of insects are harmless to man. They are, indeed, useful in the scheme of life. Many insects serve as the food supply for the pleasant songbirds we all enjoy. Other insects help pollinate plants, and without that the plants would die.

Some insects are directly useful to man. The bee produces honey and wax, the silkworm produces silk, and certain scale insects produce a brilliant red dye. Some insects, such as locusts, are even eaten by men in some areas of the world. To be sure, there are some species of insects that are troublesome. Perhaps 3,000 species at most (out of a possible three million) are nuisances. These include the mosquitoes, flies, fleas, lice, wasps, hornets, weevils, cockroaches, carpet beetles, and so on.

As a result, people come to dislike "bugs" and get the urge to swat or crush anything with six legs that flies or crawls. This is wrong, though. We don't want really to wipe out all insects because a few are bothersome. Insects, as I said, are necessary to the scheme of life.

In fact, all the different species of creatures are useful to each other. Even killer animals are useful to the creatures they kill.

As an example, mountain lions kill deer. Now deer are pretty animals while mountain lions seem to be dangerous killers that deserve to be wiped out. It has happened that men have killed the mountain lions in some areas and freed the deer from the danger.

That does not do the deer a favour!

While the mountain lions were active they killed some deer but never very many. What's more, they usually killed old or sick deer, for the strong young ones had a better chance to get away. The mountain lions kept the numbers of deer down and there was that much more food for those that were left.

Once the mountain lions were gone, the deer population increased quickly. Even the old and sick had a chance to live. All the deer searched the countryside for food and in no time the area was stripped bare. Starvation gripped the herd and all became weak and sick. They began to die and in the end there were far fewer deer than there had been in the days when the mountain lions were active.

So you see, the deer depend for their life and health on the very animals that seem to be killing them.

The way in which different species of animals depend upon one another results in a "balance of nature." The numbers of any particular species stay about the same for long periods of time because of this balance. Even if the balance is temporarily upset, when one species grows unusually numerous or unusually rare, the food supplies drop, or increase, in. such a way that the proper number is restored.

The study of this balance of nature is called "ecology" and it has grown to be one of the branches of science that is of greatest interest to mankind, for we have badly upset the balance of nature and are upsetting it worse each year.

In the end, we might suffer as the deer suffer when the mountain lions are gone, and scientists are anxious to prevent this if possible. By studying the principles of ecology, they hope to learn how best to prevent it.

Actually, insects wouldn't have grown to be such nuisances, if mankind hadn't upset the balance of nature many thousands of years ago when he first developed agriculture. Once he began to plant barley, for instance, he saw to it that many acres of land produced hardly anything but barley, barley, barley. All the other plants that might have been growing on those acres he wiped out as much as possible. They were "weeds."

Animals that lived on those weeds were starved out. On the other hand, animals that lived on barley multiplied, for suddenly they had a huge food supply.

In this way, agriculture encouraged certain insects to multiply and what had been just a nuisance became a great danger. As an example, locusts may suddenly multiply and swarm down on fields in gigantic armies of billions. This happened frequently in ancient times and even the Bible describes such a locust plague in the book of Joel. Locusts would sweep across the fields, eating everything green. When they left, a barren waste would remain.

This would be a disaster, for large numbers of people would be depending upon those vanished crops. Widespread famine would be the result.

Nor could anything be done about it. People were completely helpless as they watched their food disappear. They might go out and try to kill locusts, but no matter how hard they worked at it, there would be ten thousand left alive for every one they killed.

Even today, although scientists have discovered ways of fighting insects, there is serious trouble in some places and at some times. This is especially true in the less-developed countries where scientific methods of fighting insects are least available-and where the population can least afford the loss.

In India, for instance, there is an insect called the "red cotton bug" which lives on the cotton plant. If cotton plants were growing wild, some of them might be affected by the bug, but the plants would be few in number and would be spread widely apart. The bugs would not have much to eat and would find it difficult to get from one plant to the other. The number of red cotton bugs would therefore remain small and the cotton plants themselves would be only slightly damaged. They would continue to grow quite well.

In large cotton fields, however, the bugs have a tremendous food supply, with one plant right on top of the other. The bugs increase in numbers, therefore, and become a huge horde. Each year, half of all the cotton grown in India is destroyed by them.

Even in the United States, we have trouble. An insect called the "boll weevil" feeds on the cotton plant in this country. We can fight the boll weevil better than the Indians can fight the cotton bug. Still, as a result of the boll weevil damage, each pound of cotton produced in the United States costs ten cents (about 10d.) more than it would if the boll weevil didn't exist.

The losses resulting from insect damage in the United States alone run to something like eight billion dollars each year. Man himself has also vastly increased in numbers since agriculture was developed. Before that, small groups of men hunted through wide stretches of forests. They offered only a small target for fleas and lice.

After the appearance of agriculture, farming communities were established. These were much larger than hunting bands, and in such communities, men lived huddled together. Fleas and lice multiplied and men had to do a great deal more scratching. Mosquitoes, too, gained a much larger food supply and increased in numbers.

You might think that insects like termites and boll weevils did real damage and that fleas and lice were just nuisances, but that is wrong. The insects that bite and sting human beings can be terrible dangers; and this was something that wasn't discovered until the opening of the twentieth century.

The discovery came in connection with yellow fever. This is a rapidly spreading disease that can kill vast numbers of people. Nowadays it is rarely heard of in the United States but in previous centuries, it would suddenly flare up in huge epidemics that would lay whole cities low. Twenty times in the history of the city of Philadelphia, yellow fever epidemics raged across it. New York had fifteen epidemics.

There seemed no way of preventing the epidemics. They struck out of nowhere and suddenly people were dying on every side. The United States military forces grew particularly interested in the problem in 1898.

That year they fought a short war with Spain. Most of the fighting took place in Cuba where few Americans were killed by Spanish guns, but many died of yellow fever. What people didn't understand was how the yellow fever passed from one person to another. Was it by infected clothing, by polluted water, or how?

In 1899, the American government sent to Cuba a team of doctors headed by Walter Reed. Their mission was to find out how yellow fever was spread. Yellow fever does not attack animals so the mission had to work with human beings, and that meant using themselves as guinea pigs.

They handled the clothing and bedding of people sick with yellow fever yet didn't come down with it themselves. Walter Reed remembered that a few people had advanced the notion some years before that mosquitoes might carry the disease. They would bite sick men and suck in infected blood, then pass the infection to the next person they bit.

Reed's group checked this. They introduced mosquito netting to keep mosquitoes away from certain houses. Sure enough, they found that people protected by mosquito netting usually didn't get the disease even when it was striking all around.

They went on to something more daring. They captured mosquitoes in rooms where there were men sick with yellow fever and then allowed those mosquitoes to bite them. Some of the group soon came down with yellow fever and one of them, Jesse William Lazear, died.

A mosquito bite is more than a nuisance, then. Mosquitoes of a certain species can pass on a deadly disease with their bite.

Yellow fever struck the United States again, for the last time, in 1904, with New Orleans the victim. But Reed had shown how to fight the disease. The mosquitoes were kept away with netting. The places where they bred were wiped out. As a result, yellow fever is no longer a serious danger in the United States. There hasn't been an epidemic in this country in over sixty years.

Another species of mosquito was found to spread the disease called malaria. Malaria isn't as dramatic as yellow fever. It isn't as rapid a killer. Besides, there is a drug, quinine (obtained from the bark of a South American tree), that, for centuries now, has been known to control the disease.

Even so, malaria is the most widespread disease in the world-or it was. As late as 1955, there were estimated to be no less than 250 million people in the world who were ill with malaria. Each year 2,500,000 people died of it. Those who didn't die were greatly weakened and couldn't work as healthy people could. Entire nations were greatly reduced in vigour and in the ability to help themselves because so many individuals among them were malarial. And all the result of mosquito bites.

Certain species of insects in Africa, called the "tsetse fly," spread sleeping sickness, a brain infection that usually ends in death. This disease spread into eastern Africa at the beginning of the twentieth century and between 1901 and 1906 it killed 200,000 people in Uganda. About two out of every three people in the affected areas died.

The disease also affects horses and cattle. It is the tsetse fly more than anything else-more than the heat, the jungle, or the large wild animals-that keeps sections of Africa from advancing.

Naturally, men were anxious to kill insects. Insects were starving mankind, eating his grain and fruits and fibres, too Insects were killing men with their infected bites. Men had to strike back.

One way was to poison insects. Suppose, for instance, you sprayed your crops with a solution of "Paris green," a deadly poison compound containing copper and arsenic.

Paris green did not affect the plants. The plants lived on carbon dioxide in the air and on certain minerals which they absorbed from the soil. If there was some poison on their leaves, that made no difference.

Any insect trying to feed on the leaves that were coated with Paris green would, however, die at once. Insects simply could not live on sprayed plants and the plants grew large and ripe without being bothered. Paris green was an "insecticide," a word meaning "insect-killer."

(Nowadays, the word is used less often because insects are not the only kind of creature we want to kill. There are also worms and snails, mice and rats, even rabbits-all of which become serious problems if they grow too numerous. They are all lumped together as "pests" and any chemical used to kill any of them is a "pesticide." In this chapter, though, I will be talking chiefly about insects and I will continue to use the word insecticide.)

Paris green and other mineral insecticides have their drawbacks. For one thing, they are just as poisonous to human beings as they are to insects. Foods which have been sprayed with these solutions must be carefully washed, or they could be deadly.

And, of course, plants are washed, naturally, by rain. The rain tends to remove some of the mineral poison and drip it down to the soil. Little by little, the soil accumulates copper, arsenic, and other elements which will reach the roots of the plants eventually. There they do affect plants and the soil will after a while become poisonous to them.

What's more, such mineral insecticides can't be used on human beings themselves. Sometimes it would be most useful if we could use them so, to destroy insects that live directly on people.

Mosquitoes and flies may bite people and annoy them (or sometimes transmit diseases that kill them) but at least they don't actually live on people. If we want to attack them, we can keep them off by netting, spray the places where they land with poison, or find the stagnant pools or garbage where they breed and either remove or spray them.

But what about the fleas and lice that live in human clothing or hair? In many parts of the world even today there are no automatic washers in which clothes can be washed every couple of days. There isn't even a supply of soap or of clean running water. The poorer people have very little in the way of clothing and- if there is a cold season they must simply wear the same clothes all winter long.

Naturally, the fleas and lice in that clothing have a happy hunting ground all winter long. This was all the more true if people were forced to crowd into small dirty hovels or tenements. If anyone happened not to have fleas and lice, he quickly caught them from others.

This could be extremely serious because typhus, a disease always present among the poor, every once in a while became epidemic and spread everywhere. It was most likely to be found among poor, dirty people huddled together on ships, for instance, or in jails. It was particularly dangerous during wars when many thousands of soldiers might be penned up in a besieged town or in lines of trenches or in prisoners' camps.

When thousands of Irish emigrated to America after the potato blight brought famine to Ireland in the 1840s, half of them sickened with typhus on the way here. In World War I, typhus did more damage among the miserable soldiers in eastern and south-eastern Europe than the guns did.

The little country of Serbia drove back the armies of much larger Austria-Hungary several times in 1914 and 1915, but then typhus struck and crippled the small nation. The Austrians dared not invade while the epidemic was raging but afterwards they marched in and what was left of the Serbian army could not stop them.

By the time of World War I, however, doctors knew very well what was causing the spread of typhus. They had learned that from a French physician, Charles Nicolle, who, in 1903, had been appointed director of a medical institute in Tunis in North Africa. (Tunis belonged to France at the time.)

Tunis was riddled with typhus but Nicolle noticed a very curious thing. The disease was infectious only outside the hospital, not inside. Doctors visiting patients in their homes caught typhus. Medical attendants who admitted patients into the hospital caught it. But once the patients were in the hospital, they stopped being infectious, even though they might

be sicker than ever. Doctors and nurses who tended typhus patients inside the hospital never caught typhus themselves. Nicolle decided that something happened at the moment that patients entered the hospital that changed everything. For one thing, the patient had removed the clothes he was wearing and took a bath. The clothes were got rid of and the infectiousness disappeared.

By that time the word was about that mosquitoes spread yellow fever and malaria, so it didn't seem hard to believe that maybe typhus fever was spread by the lice in the dirty clothes.

Nicolle worked with animals, first with chimpanzees, and then with guinea pigs, and he proved his case completely. Typhus would spread by a louse bite, not otherwise.

Nor is typhus the only disease to be spread by such body insects. There is a dreaded disease called "plague." In the fourteenth century, it spread all across Europe and killed one out of every three human beings on the continent. It was called "the Black Death" then.

This disease is spread by fleas. The fleas that are most dangerous live on rats and wherever the rats spread, so do the fleas. When a flea bites a sick rat, then jumps on a human being and bites him, it is usually all up with the human.

These are hard diseases to conquer. Rats are difficult creatures to get rid of. Even today they infest American slums and are a downright danger to sleeping babies. Body lice or fleas are even harder to eliminate.

After all you can't avoid lice and fleas by something as simple as mosquito netting. You must wash clothes and body regularly, but how can you ask people to do that who have no soap and no clean water?

It would be helpful if you could spray the bodies and clothes with insecticide, but you would have to find one that would kill the insects without killing the person. Certainly Paris green wouldn't do.

Instead of minerals, then, the search was on for some suitable organic substance. An organic substance is one that has a structure similar to the compounds contained in living tissue. There are many millions of different organic substances, and no two species of creatures act exactly alike in response to particular organic substances.

Might it not be possible to find an organic substance which would interfere with some of the chemical reactions that go on in insects, but not in other kinds of animals.

In 1935, a Swiss chemist, Paul Muller, began to search for such a compound. He wanted one that could be easily made and would therefore be cheap. It had to be without an unpleasant odour. It had to kill insects but be reasonably harmless to other kinds of life.

He narrowed down the search by studying different classes of organic compounds and then following up those classes that showed at least a little promise. He would study the chemical structure of those compounds that showed a little promise and would then try a slightly different compound to see if that had more promise. If it did, he would study the difference in structure and see how to make a still further difference that would be better still.

It took four years but in September of 1939 (the very month in which World War II started), Muffler came across a compound called "dichlorodiphenyltrichloroethane." That is a long name even for chemists and it is usually referred to by its initials, as DDT. This compound had first been prepared and described in 1874 but at that time there seemed nothing unusual about it. Now, however, Muller discovered that DDT was the very thing he was looking for. It was cheap, stable, and odourless, fairly harmless to most forms of life, but meant death to insects.

By 1942, preparations containing DDT were beginning to be manufactured for sale to the public, and in 1943, it had its first dramatic use. The city of Naples, in Italy, had been captured by Allied forces and, as winter came on, typhus began to spread.

It wasn't possible to make the population strip off their clothes, burn them, and put on new clothes, so something else was done. Soldiers and civilians were lined up and sprayed with a DDT solution. The lice died and typhus died with them. For the first time in human history, a winter epidemic of typhus had been stopped in its tracks.

To show that this was no accident the same thing was done in Japan in late 1945, after the American occupation. Since World War II, DDT and other organic insecticides have been used in large quantities. Tens of thousands of tons are produced each year. The United States alone spent over a billion dollars for such insecticides in the single year of 1966. Not only are our crops saved but the various insect-spread diseases are all but wiped out. Since DDT wipes out mosquitoes and flies, as well as lice, malaria is now almost unknown in the United States. Less than a hundred cases a year are reported and almost all are brought in from abroad.

Yet this does not represent a happy ending. The use of organic insecticides has brought troubles in its train. Sometimes such insecticides don't work because they upset the balance of nature.

For instance, DDT might be fairly deadly to an insect we want to kill, but even more deadly to another insect that lives on the first one. Only a few harmful insects survive but their insect enemies are now all dead. In a short time, the insects we don't want are more numerous than they were before the use of DDT.

Then, too, organic insecticides don't kill all species of insects. Some insects have a chemical machinery that isn't affected by these poisons; they are "resistant." It may happen that a resistant insect could do damage to our crops but usually doesn't because some other insect is more numerous and gets the lion's share of the food.

If DDT kills the damaging insect, but leaves the resistant insect behind, then that resistant insect can multiply enormously. It then becomes a great danger and DDT can't touch it.

In fact, even among those species of insects that are killed by DDT there are always a few individuals that differ chemically from the rest and are resistant. They survive when all other individuals are killed. They multiply and then a whole species of resistant insects comes into existence.

Thus, as the years pass, DDT has become less effective on the house fly, for instance. Some resistance was reported as early as 1947, and this has been growing more serious. By now almost every species of insect has developed resistance, including the body louse that spreads typhus.

Finally, even though organic insecticides are not very poisonous to creatures other than insects, they are not entirely harmless either. If too much insecticide is used, some birds can be poisoned. Fish are particularly easy to kill, and if insecticides are used on water to kill young insects, young fish may also go in great numbers.

Organic insecticides are also washed into the soil. Eventually, they are broken down by bacteria but not very quickly. Some accumulates in the soil, then in the plants that grow in the soil, then in the animals that eat the plants. All animals, including man, have a little bit of DDT inside ourselves. Not enough to hurt us so far, but it is there.

For that reason, attempts have been made to control insects by means that don't involve chemicals.

For one thing, there are certain strains of plants which are naturally resistant to particular insects. These strains might be cultivated.

Then, too, crops might be rotated; one crop might be grown one year, another crop the next. In this way, an insect which flourished one year might drop to very low levels the next when the wrong plants were grown, plants it could not eat. It would have to start from scratch again and its numbers would stay low. Or else one might break up the fields so that not too large an area would be devoted to a single crop. That would make it harder for an insect to spread.

Here's something else-insects have their enemies. The enemy might exist in one part of the world but not in another. It might be another insect or some kind of parasite. If it could be introduced in places where the insect we were after was flourishing, the numbers of that insect might be brought under control.

Modern science has worked up a number of additional devices for killing insects. Bats eat insects and locate them by emitting very shrill squeaks, squeaks too shrill for us to hear. The sound waves of these squeaks bounce off the insect, and the bat, by listening for the echo, knows where the insect is.

Naturally, insects have developed an instinctive avoidance of such a sound. If a device is set up to send out these shrill "ultrasonic" squeaks, insects stay away from a wide area near it.

Another device is just the opposite-to attract rather than to repel. Insects can find each other over large distances because they can smell each other. Female moths give off an odour that a male moth of the same species can detect many hundreds of yards away. Female moths can tell by smell a good spot on which to lay eggs.

Chemists have worked to isolate the chemicals that give off this attractive odour. Once they isolate it, they can place it on certain spots to attract insects. If those spots are sprayed with insecticide, too, insects could die in great numbers. Only a little insecticide would have to be used; it wouldn't have to be spread everywhere; and it would be less likely to affect other forms of life.

Or else a female could be induced to lay eggs in an unsuitable place by means of a sprayed odour, so that the eggs would not develop.

Then, too, male insects can be subjected to radioactivity that destroys some of their internal organs so they cannot fertilize the female's eggs. If such sterilized males are released, the females end up laying eggs that cannot develop. An insect called the "screwworm," which infests cattle in south-eastern United States, was almost wiped out by this method.

But all that mankind is doing today is not yet enough. The insecticides are too poisonous and the other methods are a little too fancy for undeveloped countries where the insect menace is greatest. Is there something better we can do to help feed the doubled population of 2000?

Actually, the 1960s are seeing the development of an exciting new way of battling insects, a way that makes the insects fight themselves, so to speak. To understand how this should be, let's consider how insects grow.

An insect has two chief stages to its life. In its young form, it is a "larva"; later on, it is an "adult." The two forms are very often completely different in appearance.

Consider the caterpillar, for instance. It is a larva, a wingless, wormlike creature with stumpy little leg-like structures. Eventually, though, it becomes a moth or butterfly, with the usual six legs of the insect, and often with gorgeous wings. Similarly, the housefly develops out of its egg as a tiny, wormlike "maggot."

The reason for two such different forms is that the two have widely different specialities. The larva spends almost all its time eating and growing. It is almost what we might call an eating machine with all its makeup concentrated on that. The adult, on the other hand, is an egg-laying machine. Sometimes adult insects do nothing but lay eggs. Mayflies live less than a day after they reach the adult stage and don't even have proper eating apparatus. In their short adult life they just lay eggs; they don't have to eat.

The change from larva to adult is called "metamorphosis." Sometimes the metamorphosis is not a very startling one. A young grasshopper looks quite grasshopperish, for instance.

Where the metamorphosis is a thoroughgoing one, as in the case of the caterpillar, the insect must pause in its life cycle to make the enormous change within its body. It is almost as though it must go back into an original egg stage and start again. It becomes a motionless, apparently dead object, slowly changing within and making itself over until it is ready to step forth as an adult. In this motionless intermediate stage it is called a "pupa."

There are insect species which act in such a way as to protect this defenceless pupa stage. In its final period as a larva, it will produce thin jets of liquid from special openings in its abdomen. These jets harden into a tough fibre which the larva weaves round about itself until it is completely enclosed. This is the "cocoon" within which the pupa remains hidden till metamorphosis is done. It is the fibre from the cocoon of the silkworm moth that provides mankind with silk.

All this requires careful organization. For instance, it is a problem for a larva just to grow. The larva is surrounded by a thin, but tough, cuticle made of a substance called "chitin." This protects it and gives it an anchor for its muscles, but chitin doesn't expand with the body.

As a larva grows, its cuticle becomes tighter and tighter about it. Finally, the cuticle splits and is shed. The larva is said to "moult." From the split cuticle, the larva wriggles. It is expanded now and is distinctly bigger now that the cuticle which had been holding it in like a tight girdle is gone. A new, but roomier, cuticle quickly forms and within it the larva grows again.

But what makes the cuticle split at just the right time? The answer is that there is an automatic chemical control involved. Any living creature is a complex setup of automatic self-regulating chemical machinery. This is true even of the human being and it was only at the very opening of-the twentieth century that biologists began to have an inkling as to how some of this machinery worked.

In the human being there is a large gland called the pancreas. It manufactures a digestive juice which enters the small intestine and mixes with food emerging from the stomach. The interesting thing is that the pancreas doesn't bother wasting its juice when the small intestine is empty. Nothing happens until food enters the small intestine and then, instantly, the pancreatic juice starts flowing.

Something automatic must be involved and in 1902, two English biologists, William Maddock Bayliss and Ernest Henry Starling, discovered what it was.

The food in the stomach is mixed with a strongly acid juice. When the food emerges from the stomach and enters the small intestine, the touch of its acidity has a chemical effect on the intestinal lining and causes it to produce a substance which Bayliss and Starling called "Secretin."

Secretin is discharged into the bloodstream and is carried to all the body. When it reached the pancreas, it brings about a chemical effect that causes the pancreas to begin to manufacture and discharge its juice.

Secretin is a substance which rouses the pancreas to activity. In 1905, Bayliss suggested that secretin, and all other substances like it, be called "hormones," from a Greek word meaning "to arouse."

The process of moulting seems to be an automatic process controlled by a hormone. As the larva grows, there is growing pressure from the cuticle. When the pressure reaches a certain point, a hormone is triggered. It pours into the larva's bloodstream and when it reaches the cuticle that cuticle is made to split.

The hormone that does this has been given the name "ecdysone," from a Greek word meaning "to moult."

But moulting doesn't go on forever. After a certain number of moults, there is a sudden change. Instead of continuing to grow in order to prepare the way for still another moult, the larva begins to undergo metamorphosis instead.

Can this be because a second hormone is involved? Is there a second hormone that suddenly appears after a certain number of moults and brings about the metamorphosis?

Not quite. In 1936, an English biologist, Vincent Brian Wigglesworth, was working with a certain disease-spreading, blood-sucking bug called Rhodnius. In the course of his experiments, he thought it would be useful to see what would happen if he cut off the head of the larva of these bugs.

Naturally, if you cut off the head of a mammal or a bird, the creature would die and that would be all. An insect, however, is far less dependent on its head, and life could continue in some ways.

But different parts of the body produce different hormones and some can be produced in the head. By cutting off the head of a larva, Wigglesworth could tell what hormones the insect head might be producing. After all, the headless larva would grow differently than one with a head would and the differences might be at least partly due to the missing head-hormones.

Wigglesworth did indeed notice a change. As soon as he had cut off the head, the larva went into a moult and emerged as an adult. (Rhodnius was not one of the bugs that went through a pupa stage.)

It did this even when it was nowhere near ready for such a change. It hadn't moulted enough times; it was far too small. Yet it did change and a miniature adult would appear.

But if metamorphosis was caused by the production of a hormone, how could cutting off the head produce it? Cutting off the head should cause the loss of a hormone, not its production.

Wigglesworth argued that the head produced a hormone that prevented metamorphosis. As long as it was produced ecdysone, the moulting hormone, did its work; the larva moulted and grew, moulted and grew. At a certain point, though, in the course of the life of the normal insect, something happened which cut off the supply of this head hormone. Without that hormone, ecdysone couldn't work even though it was present, and metamorphosis began.

If the head were cut off, the supply of the hormone was destroyed at once and metamorphosis began even though the insect body wasn't really ready for it.

Wigglesworth called this hormone from the insect head "juvenile hormone" because it kept the insect in its juvenile, or youthful, form. He also located tiny glands, barely visible without a microscope, behind the brain of the larva and these, Wigglesworth was certain, produced the hormone.

What Wigglesworth found to be true of Rhodnius was true of other insects, too; of the silkworm caterpillar, for instance. It seems that all insects that undergo metamorphosis do so because the supply of juvenile hormone stops at a certain time.

Wigglesworth's suggestion about the glands in the head was quickly shown to be correct. In 1938, a French biologist, Jean Bounhiel, worked out a delicate technique for removing the tiny hormone-producing glands from a small silkworm caterpillar and placing them in a large one.

The large silkworm caterpillar was about ready to enter its pupal stage, which meant that its glands had stopped producing juvenile hormone. The glands from the small caterpillar, however, were still capable of producing the hormone. When the glands from the small caterpillar were grafted into the large one, the large caterpillar suddenly found itself with a new supply of juvenile hormone. Instead of entering the pupal stage, it continued to moult one or two extra times.

Naturally, it continued to grow, too, and when it finally did switch to the pupa, it was a considerably larger-than-normal one, and out of it emerged a considerably larger-than-normal adult moth.

At this point, Carroll Williams of Harvard University stepped onto the scene. He transferred hormone-producing glands, not to another larva, but to the pupa of a silkworm. The pupa was well along in metamorphosis. It wasn't supposed to be exposed to any juvenile hormone at all; it was past that stage. But what if juvenile hormone were forced upon it?

Williams had his answer at once. The presence of juvenile hormone seemed to stop the metamorphosis, or at least slow it down. When the adult moth finally appeared it was incomplete. Some of it had not changed over.

Williams found that the more gland material he inserted into the pupa, the more incomplete the metamorphosis. He could use the amount of incompleteness of metamorphosis to judge how much juvenile hormone were present in the glands at different stages in the life of the larva.

He could also determine if there were juvenile hormone anywhere else in an insect body, and here he stumbled over something that was a complete surprise.

In 1956, Williams found that an insect called the "Cecropia moth" produced a large quantity of juvenile hormone just before entering the adult stage, after having passed through the pupa stage entirely without it. Why they do this nobody knows.

This juvenile hormone is stored in the abdomen of the moth for some reason. Only the male moth does it, not the female. Only one other kind of moth, as far as is known, stores juvenile hormone in this fashion. All other insects do not.

Even if biologists don't know the reason for any of this, it still turned out to be a useful fact. The tiny glands that produce juvenile hormone in larvas contain so little that it is just about impossible to extract a useful amount. The reserve supply in the abdomen of the male Cecropia moth is so large, on the other hand, that the hormone can be produced in visible quantities.

Williams produced an extract from the abdomens of many such moths; a few drops of golden oil that contained huge quantities of juvenile hormone. Now he had plenty of material with which he could experiment further.

One Cecropia abdomen supplied enough hormone to block completely the metamorphosis of ten pupas of almost any kind of moth or butterfly. The extract did not even have to be injected into the pupa. If some were just applied to the skin of the pupa, enough hormone leaked into the inner tissues to upset the metamorphosis.

The metamorphosis could be so badly upset, if enough juvenile hormone were used, that the pupa could not develop at all. It simply died.

The thought at once occurred to Williams that here might be a potential insecticide that would have great advantages over every other kind known. After all, it turned the insect's own chemistry against itself.

An insect couldn't grow resistant to juvenile hormone, as it could to any other sort of insecticide. It had to respond to its own hormones. If it didn't, it would die.

In other words, an insect had to respond to juvenile hormone at the right time or it would die. And if it did respond at the right time, then it would also respond at the wrong time and still die. Either way, the insect was dead.

Even more important, the juvenile hormone would be no danger to forms of life other than insects. It affected only insects and has no effect whatever (as far as has been found so far) on any form of life other than insects.

Of course, it is one thing to kill a few pupas in a laboratory and quite another to kill vast quantities out in the fields. Thousands of tons of insecticides are needed for the work that must be done and it would be impossible to get thousands of tons out of Cecropia moths.

If only the chemical structure of the juvenile hormone were known. It would then be possible to manufacture it from other chemicals; or else manufacture something that was close enough to do the job. Unfortunately, the structure was not known.

Williams and a colleague, John Law, sat in their Harvard Laboratories one summer day in 1962, wondering if they could reason out what the structure might be. A lab assistant, listening to them, suggested a particular type of compound as a joke.

John Law thought he would go along with the gag. It wouldn't be too difficult to make the compound, or something with a name very like the lab assistant's joke. With scarcely any trouble at all, he produced an oily solution of a mixture of substances which he intended to show the young assistant and say, "Well, here is the compound you joked about."

Still as long as he had it, he tried it first on insect pupas. To John Law's everlasting surprise, it worked! It worked unbelievably well. It was over a thousand times as powerful as the extract from Cecropia abdomens. An ounce of Law's solution would kill all the insects over an area of two and one-half acres-at least all the insects that were metamorphosing.

This substance is "synthetic juvenile hormone." It contains at least four different chemicals, and none of them seems to have a structure like that of the natural hormone.

Synthetic juvenile hormone works on all insects tested, including the mosquito that spreads yellow fever and the louse that spreads typhus. Yet it doesn't affect any creature other than insects. It would be no danger to birds, fish, mammals, or man.

Still, killing all insects is a little too much. That would upset the balance of nature.

We want to kill only certain insects, only one species out of a thousand. This could be done perhaps with the natural juvenile hormone. Each different group of species of insects manufactures its own kind of juvenile hormone which works for itself but not for others. Perhaps then, you can use a particular juvenile hormone and get just the insect you're after and no other kind.

For instance, a biologist in Prague, Czechoslovakia, named Karel Sláma, was trying to make natural juvenile hormone work on a harmless insect called the "red linden bug." He used the technique developed by Carroll Williams, but the extract from Cecropia moths didn't affect the red linden bugs. It might kill moths and butterflies but it had no effect at all on the red linden bugs. The red linden bugs must have a juvenile hormone so different from those of moths and butterflies that the effects didn't cross.

Williams heard of these experiments and was most curious. In the summer of 1965, Williams asked Sláma to bring his red linden bugs to Harvard and to come with them. Sláma came, and together the two men began to grow the bugs. In Prague, Sláma had grown them by the tens of thousands and their way of growing was always the same. The larvas went through exactly five moults and then moved into the adult stage. (The red linden bug does not go through a pupa stage.)

Yet at Harvard this did not happen. Bug after bug went through the fifth moult. Then, instead of changing into an adult, they stayed larvas and tried to moult a sixth time. Usually, they didn't make it, but died. In the few cases where a bug survived the sixth moult, they died when they attempted a seventh moult. About 1,500 insects died in the Harvard laboratories, where none had died in Prague.

Why? It was as though the bugs had received a dose of juvenile hormone and couldn't stop being larvas-but no juvenile hormone had been given them.

Williams and Sláma tried to think of all possible differences between the work at Harvard and the work in Prague. In Harvard, the red linden bugs were surrounded by all sorts of other insects which were involved in juvenile hormone experiments. Perhaps some of the hormone got across somehow. The other insects were therefore removed but the red linden bugs still died.

Could the glassware have been contaminated during cleaning? Maybe. So Williams ordered new glassware that had never been used. The bugs still died.

Could there be something wrong with the city water? Williams got spring water, but the bugs still died.

Altogether fourteen different possibilities were thought of and thirteen were cancelled out. One thing remained, and one only -

Strips of paper were placed into the jars in which the red linden bugs were grown. They were slanted against the sides, as a kind of path for the bugs to scurry along. (That seemed to keep them more active and in better health.) Of course, the paper used at Harvard was not the same as the paper used in Prague. Williams was, in fact, using strips of ordinary paper towels produced by a local manufacturer.

Williams proceeded to check that. He used strips of chemically pure filter paper instead. At once, the bugs stopped dying.

There was something in the paper towels that acted like juvenile hormone and upset the chemical machinery of the larvas. It kept them moulting after they should have stopped doing so and that killed them. Williams called the mysterious substance that did this the "paper factor." Later, it received the more chemical sounding name of "juvabione."

Williams and Sláma went on to try all kinds of paper. They found that almost any American newspaper and magazine contained the factor. Red linden bug larvas that crawled over them never made it to the adult stage. On the other hand, paper from Great Britain, the European continent, or Japan, did not have it and the bugs lived well on such paper. (That's why they lived in Prague.)

Could it be that American manufacturers put something in paper that other manufacturers did not? A check with the manufacturers showed they didn't. Well, then, what about the trees from which the paper was made.

They began to test extracts from the trees and found one called the "balsam fir" which was much used for American paper but which did not grow in Europe. It was particularly rich in paper factor, and this paper factor could be obtained from the tree in large quantities.

Here is an interesting point. The paper factor works on only one group of insect species, the one to which the red linden bug happens to belong. If Sláma had brought with him some insect from another group of species, the paper factor might have gone undiscovered.

The paper factor is an example of an insecticide that will kill only one small group of insects and won't touch anything else. Not only are fish, birds, and mammals safe, but so are all insects outside that one group of species.

To be sure, the red linden bug is harmless and there is no purpose in killing it, but the red cotton bug, which eats up half of India's cotton crop, is closely related to it. The red cotton bug can also be hit by the paper factor and experiments are underway to see how well it will work in India's cotton fields.

Paper factor catches bugs at the time of their metamorphosis. This is better than nothing but it still isn't quite as good as it might be. By the time the metamorphosis is reached, the insect has spent a lot of time as a larva-eating, eating, eating.

Then any insects that happen to survive the paper factor for some reason can lay a great many eggs. They will develop into larvas that will eat and eat and eat and will only be caught at the metamorphosis.

It would be better if insects were caught at the beginning of the larval stage rather than at the end.

And they can be! It turns out that the eggs, like the period of metamorphosis, must be free of juvenile hormone. In 1966, Sláma placed eggs of the red linden bug on paper containing the factor and-if the eggs were fresh enough and weren't already on the point of hatching-they didn't hatch.

Then he tried it on adult females that were ready to lay eggs but hadn't laid them yet. He placed a drop of the factor on the adult's body and found that it worked its way inside and, apparently, even into the eggs. At least such a female laid eggs that didn't hatch.

The paper factor was more valuable than ever now, for it could be used to catch the insects at the very beginning of their life.

But why should the balsam fir possess a compound that acts like juvenile hormone? The answer seems clear. Insects eat plants and plants must have developed methods of self-protection over the millions of years of evolution.

A good method of self-protection is for the plants to develop substances that taste bad to insects or that kill them. Plants which happen to develop such substances live and flourish better than those that don't.

Naturally, a plant would develop a substance that would affect the particular insects that are dangerous to it. It seemed that if biologists were to make extracts from a large variety of plants, they might find a variety of substances that would kill this type of insect or that. In the end, they would have, perhaps, a whole collection of insecticides to use on particular insect pests. We would be able to attack only the insects we want to attack and leave the rest of nature alone. By 1968, indeed, some fifteen such plant self-defence chemicals were located.

Then, too, in 1967, Williams took another step in this direction, while with an expedition exploring the interior of the South American country Brazil. There the large river Rio Negro flows into the Amazon. The name means "Black River" because its waters are so dark.

Williams noticed there were surprisingly few insects about the banks of the river and wondered if the trees might not contain considerable paper factor of different kinds. Then he wondered if the darkness of the river water might not come from its soaking substances out of the trees that lined its bank. If so, the river water might contain all kinds of paper factors.

Tests have shown that the Rio Negro does have insecticide properties. Perhaps many different paper factors will be extracted from it in endless supply. Perhaps other rivers may be found to be as useful.

In 1968, Sláma synthesized a hormone like compound which was the most powerful yet. An insect treated with such a hormone would pass a bit of it on to any other insect with which it mated. One treated insect could sterilize hundreds of others.

So things look quite hopeful. Between the supplies found in nature and between the chemicals that can be formed in the laboratory, we may get our insect enemies at last.

This will mean that man's supply of food and fibre will increase. It will mean that a number of diseases will no longer threaten him, and he will be able to work harder to produce goods.

In that case, we may well be able to feed, clothe, and take care of all the billions who will swell Earth's population in the next forty years or so.... And by that time we may have learned to control our own numbers and we will then be safe.

From: Twentieth Century Discovery by Isaac Azimov