Insect Migration

By C. B. Williams

Highlighting the significance of the widespread distribution of the migratory habit throughout the insect world. This edition is exclusive to newnaturalists.com

This is a pioneer book, a real milestone in the progress of biology. Only in recent years have the scientists begun to realise the significance of the widespread distribution of the migratory habit throughout the insect world.

Dr. Williams's own personal observations and adventures have played a fundamental part in the wakening of human consciousness to the extent to which insects migrate. His opportunities of studying the problem in remote corners of the world – such as British Guiana, Costa Rica, Egypt, Tanganyika and the Pyrenees – make the book as exciting as a world detective story. For Insect Migration deals with the subject on an international basis, with Britain – the home of the development of the present theories – as the natural peg on which a biological problem belonging to the world can properly be hung.

From 1932 to 1955 C. B. Williams was chief entomologist at the Rothamsted Experimental Station. This book is the distillation of a subject which has occupied him for nearly the whole of his life. His theories are marshalled and summarised with modesty, economy and skill. The New Naturalist is honoured to publish what will certainly prove to be, above all things, the stimulus for new search and fresh discoveries.

• Language: English
• Publisher: HarperCollins UK
• Release date: Apr 11, 2013
• ISBN: 9780007406326

Book preview

EDITORS’ PREFACE

WHEN the author of this book was a boy, the fact that large numbers of insects undertake migratory movements almost as remarkable as those of birds was unknown to all but a few observers. Even today the widespread distribution of the migratory habit throughout the insect world is fully recognised by a handful of scientists and skilful amateurs. Only in recent years has the significance of all sorts of interesting and absorbing facts been appreciated—yet the facts have been before naturalists’ eyes almost since naturalists began.

C. B. Williams has been interested in the migration of insects for nearly half a century. He was a great pioneer, a devoted worker in a subject that was once almost his alone.

This is a particularly happy book for the New Naturalist to publish since it makes the best of two worlds—Britain, and the world at large—only one of which is the normal province of the New Naturalist. We refer to the fact that the strict speciality of our series is the natural history of the British Isles. There are many fields of natural history which were pioneered in our islands, and many more that were pioneered by British naturalists. But not often can Britain be said to be the peg on which a biological problem belonging to the whole world can be hung properly.

The truth is that at present British material must be the mainstay of any book on insect migration as a general problem. British observations of the movements of insects are more abundant, more continuous and date back longer than those of any other country. Britain is the home of the development of the present theories of insect migration, and is also the headquarters of the main research organisation for the study of locusts. We have here a masterly summary of the extent of human knowledge on a world problem which is, at the same time, in essence, a book on British natural history.

In this distillation of his life’s work Dr. Williams has given us an up-to-date assessment of present knowledge of the subject. It will probably be found that there are members of every order of winged insects which migrate, though so far observation has largely been concentrated upon butterflies and moths, dragonflies, some beetles and two-winged flies, and the locusts.

As Dr. Williams recounts it the awakening of human consciousness of the extent to which insects migrate has been a slow and patchy story. In this story (we should underline, for he does not), his own personal observations and adventures in many parts of the world have played an important and fundamental part. Almost can this book be said to be a world detective story. Certainly some of the incidents and discoveries recounted in it must be as thrillng to the naturalist as any complicated piece of detection is to the reader of crime novels. The detective simile can usefully stay with us, for Dr. Williams has arranged his argument, his definitions and the steps of his discussion, with the inexorable tactics of the skilled, practical and philosophical investigator. He says, himself, that it is easy to make theories but difficult to find any that will fit the facts, all the facts and nothing but the facts. He marshals and summarises his own theories with modesty, economy and skill, and in a fine scientific spirit of dissatisfaction. This book is a pioneer book, a real milestone in the progress of biology. We are honoured to publish what will certainly prove to be, above all things, the stimulus for new search, exciting discoveries and interesting conclusions.

Dr. Williams’ career as a professional entomologist began in 1911, after he had graduated from Cambridge, at the John Innes Horticultural Institution in Surrey. After wide experience of economic entomology in Trinidad, Egypt and Tanganyika, during which he lost no opportunity to collect completely new information on the movements of insects, he joined the staff of Edinburgh University in 1929 as Lecturer in Agricultural and Forest Zoology.

After a visit to the University of Minnesota, U.S.A. as Guest Professor he became in 1932 chief entomologist to Rothamsted Experimental Station, where he remained till 1955. During the war, in 1943–44 he visited most of the countries in South America as a scientific liason officer for the British Council. In 1954 he was elected a Fellow of the Royal Society for pioneer work in two fields of biology, the migration of insects and the application of statistical methods to the study of insect populations. Since he retired from Rothamsted he has lived in the highlands of Inverness-shire working, on behalf of the Agricultural Research Council, on the relation of insect abundance to weather conditions.

THE EDITORS

AUTHOR’S PREFACE

I STARTED to collect insects when I was about twelve years old and, with encouragement at home, kept up my interest during school and university days. Throughout this period I had heard rumours of butterflies occasionally crossing the Channel from France to England, and also knew of the controversy about distinguishing, by the colour of the border, real British Camberwell Beauties from the worthless Continental specimens. There was, however, very definitely fixed in my mind the impression that insects were sedentary; that they kept very close to the spot where they had fed as caterpillars; and that individuals which wandered more than a few miles were quite exceptional and belonged to very few species. After all, the collector’s world was full of localities, some only a few acres in extent, to which one went to find rarities. Entomologists came all the way to the coast sandhills of Cheshire, which was my home, to collect Belted-Beauties; and one of my chief reasons for wishing to go to the University of Cambridge was that our only British Swallow-tail butterfly was confined to Wicken and some other relatively small areas in the neighbouring fenlands of East Anglia.

In 1916 my horizon was suddenly widened by my appointment in the Colonial Service as an agricultural entomologist in the West Indies. In August of this year I was searching for beneficial insects on an abandoned rubber estate in the backwoods of British Guiana. One day I noticed numbers of yellow butterflies crossing the garden of the house where I was staying, and after a few minutes I realised that all were flying steadily in one direction—towards the south-east.

As will be seen later, such flights had been known for a hundred years or more, but this was the first that had ever come my way.

The flight continued steadily for several days and I was able to trace the butterflies across low-lying rivers and forest land over a front of about ten miles. I then questioned people who had lived in the country and found that many had observed similar flights; some even said that they were of regular occurrence. I still, however, mentally filed such movements as rare events, and considered myself lucky to have seen one. Nevertheless, in the course of my next few years’ wanderings in this part of the world, I saw in Trinidad a much larger flight which lasted for over two weeks, and several smaller flights here and also in Costa Rica.

On leaving the West Indies in 1921 to take up an appointment in Egypt, I was still under the impression that these movements were a local curiosity and that, by leaving tropical America, I was also leaving behind the chance of further study. To my surprise, on my first voyage from England to Egypt, in July of this year, our ship ran through a thin but quite definite migration of Painted-Lady butterflies moving to the north across the Mediterranean on a front of at least a hundred miles, and over thirty miles from the African coast. It was only then that I began to realise that migration might be, not an extremely rare occurrence in the lives of a few species of butterflies, but perhaps a regular event in many.

During the six years that I lived in Egypt I saw three more directional flights of butterflies, two of Painted Ladies and one of a small blue butterfly; and I was called upon in the course of my work to investigate a moth (Agrotis ipsilon Rott.) damaging wheat and cotton, which turned out to be a winter resident in Egypt and a migrant to Europe in the summer. In 1927 I moved from Egypt to East Africa, only to find that my previous experience had been comparatively small. In north-eastern Tanganyika, between July 1927 and March 1929, I saw nine definite migrations of butterflies; and during the sixteen months from March 1928 to June 1929 flights were in progress (unless temporarily stopped by rain) on at least 290 days, which is about 60% of the days in the period. One of these flights lasted sixteen weeks, and was still continuing when I left the country! During this same period I also had the opportunity of studying in the field a serious outbreak of migratory locusts.

Since my return to Britain in 1929, the first three years in Edinburgh and from 1932–1955 in Hertfordshire, the flights that have come my way have been few and inconspicuous; but by careful watching, no less than five definite movements of Cabbage-White Butterflies have been seen passing through Harpenden, twenty-five miles north of London.

In September 1943 I was fortunate enough to return to the tropics for a visit to South America on behalf of the British Council. On the journey out our ship passed for nearly a week through a southerly migration of Painted Ladies off the west coast of Africa; and in my six months ashore I saw four migratory flights of butterflies, and collected over seventy other records by questioning local naturalists. Since then I have seen other flights, which will be discussed later, both in West Africa and in the Pyrenees.

Thus time and time again there has been forced upon my notice this problem of insect migration, not as a rare occurrence, but as something quite normal in the lives of millions of individuals belonging to hundreds of species, and recurring with regularity at certain seasons of the year in the same direction.

In 1930 I collected all the information that I had been able to find into a book The Migration of Butterflies (Edinburgh, Oliver & Boyd). The book that is here presented is an attempt to bring together both old and new information in a more easily readable form and to discuss many of the problems so raised. It will I think convince most naturalists that we must revise our earlier ideas about the distances that insects can travel by their own efforts, and I can only hope that the subject will prove to be as interesting to the reader as it has been to me.

I should like to acknowledge here that scarcely any of this work would have been possible without the interest of the hundreds of naturalists who have put on record their observations on insect movements. Especially I wish to express my thanks to Captain T. Dannreuther for his enthusiasm for over twenty years in organising the collection of records from the British Isles, to Mr. R. A. French for many years help, and, in another field, to Dr. B. Uvarov for his inspiring leadership in the study of locusts.

Much of the preparation of this book was done while I held the position of Head of the Department of Entomology at Rothamsted Experimental Station, Harpenden, England.

C.B.W.

PART ONE

Introductory

CHAPTER 1

GENERAL INTRODUCTION TO INSECT MIGRATION

IT HAS BEEN briefly explained in the preface that the problem of insect migration has been brought to the notice of many field naturalists—including myself—chiefly by observations of large numbers of insects flying steadily for hours or days in one definite direction. At first it was found difficult to believe that these movements were anything but quite local affairs covering at the most a few miles; but the number of records increased so rapidly, and the distances obviously traversed by the insects proved to be so large, that the narrowness of the original explanations was brought into question.

Our minds had been partly prepared for the idea of long distance movements of insects by discoveries in the field of bird migration, but to offset this there was a widespread misconception, still found in many places today, that the life of an adult insect is so short—a day or two at the most—that no long distance movement would be possible.

It must therefore be made clear to begin with that, while there are insects whose adult winged life is measured only in days, the majority live several weeks, and there are many (out of the hundreds of thousands of known species) that live for several months. Among these long-lived species are included the locusts and many of the migrant butterflies and moths that will be discussed below.

Evidence of migration, however, does not come only from these directional flights, which although not often seen in temperate countries, may be so conspicuous in the tropics as to become objects of folklore and superstition. In countries like the British Isles, where insects have been studied for a century or more, we know that certain species are present at one time of the year, but at other seasons cannot be found in any stage of development; they must therefore move in and out of the area. The evidence is of course negative, but it plays a definite part in the piecing together of the jig-saw puzzle of migration.

There are also many observations of the sudden appearance of large numbers of insects, often of one or very few species, without any evidence of breeding in the district—for example the Silver-Y moth on the south coast of England; or, rather less conclusive, the sudden disappearance of large numbers without any evidence of local death or destruction. Of importance also are the records of winged insects on ships and light-ships far out at sea; but we must examine carefully all such cases to see if they are likely to be due to deliberate migration, to drift on the wind, or to human interference.

I have tried in this book to fit together thousands of these small and often quite casual observations into a connected whole, so that we can discover what kinds of insects migrate; where they start, what route they take and where they stop; and at what seasons they fly. It is only after such comparatively simple facts have been clarified that we can face more difficult questions such as what makes them start, how they keep their direction, and how they survive a long journey. Finally we may ask the most difficult question of all—why do insects migrate? Although this question is so difficult to answer satisfactorily, it is nearly always the first to be asked.

It should be pointed out here that all questions of How and Why have a dual nature. We can ask for example why Painted Lady butterflies migrate: this is a problem of the origin and evolution of the habit. But we have also to consider the much more detailed problem of why or how a particular individual butterfly is migrating in a particular direction at a particular time! In my opinion it is by finding answers to the second that we may be able to solve the first of these questions.

The ways in which insects can travel over long distances may be roughly divided into active and passive. Among the passive means are distribution by air or water currents (for we know that every day millions of insects are carried distances up to hundreds of miles by these means), and by human agencies such as ships, trains, motor cars and aeroplanes. The transmission of injurious insects from one country to another by modern transport has already cost mankind many millions of pounds in loss of crops and by the spread of disease. The spread of insects is active when the movement is determined by the insect itself; and if such a flight, instead of being haphazard in direction, is straightened out so as to result in a change of habitat, I use the word migration to describe it.

There have been many definitions of this word and perhaps the most disputed point is whether or not a flight in two directions—a to-and-fro movement of the population at different seasons—is an essential part of a migration. This double movement occurs in birds and fishes, but until recently it was believed not to occur in insects. However, as will be seen later, more and more species of insects, previously known to move only in one direction, have been found on closer study to have a return flight at a different season. In the cases where no such evidence exists the available information is often so small that we are not in a position to say that the return flight does not occur. Absence of evidence is not evidence of absence. In this connection we must remember that the life cycle of an insect is usually a year or less, and that their high birth rate and high death rate result in great and rapid fluctuations in numbers. Thus the insect population available to move in one direction at one season of the year is often very different from that available for a reverse flight at another season.

Migration for the purpose of this book is considered to be a continued movement in a more or less definite direction, in which both movement and direction are under the control of the animal concerned. Often there is known to be a return flight to the original habitat, but I do not consider this as an essential part of the definition.

Every migration consists of three parts; first an emigration from the original home; second a trans-migration or movement through an area where the migrants are only in passage; and thirdly an immigration into the new area where the animals settle down. When one says that a particular insect is an immigrant, it is only in reference to the particular country at the end of its flight; but it is impossible to be an immigrant without having previously been an emigrant. Each observer is apt to consider more particularly the status of an insect in his own country, but the complete association of emigration and immigration must always be kept in mind.

In any particular area the different kinds of insects will fall into one of the following classes with respect to their migrations—and of course the same species will often fall into different classes in different countries. Where examples are known for the British Isles they are given below:—

Permanent resident, no migration known (Small Copper Butterfly).

Permanent resident, but migrating within the limits of the area (no known British example, but the Bogong Moth of Australia would fit in here).

Residents, reinforced by immigration:

Immigrants of same race as residents (Large White Butterfly).

Immigrants of a different race (Swallow-tail Butterfly).

Resident and regular migrants (Red Admiral Butterfly).

Immigrants which breed only at one time of the year (in Britain, the summer) but do not become permanently established (Clouded Yellow and Painted Lady Butterflies).

Immigrants which seldom or never breed (Camberwell Beauty).

The facts about the migration of birds are so widely known that in a book on the subject it is possible to start directly on the discussion of problems raised by their known behaviour; and this is of course the most interesting part of the study. But so little is known of the facts about the migration of insects, even by the entomologist (and still less so by the general public), that the next few chapters must be devoted to an outline of what we know about the movements of a number of our butterflies (Chapter 3), of butterflies abroad (Chapter 4), of moths at home and abroad (Chapter 5) and of the locusts, dragonflies, ladybird beetles, hoverflies, and many other insects both in this country and elsewhere (Chapter 6). After this readers should be in a position to be critical of the discussion in Part 3.

The object of most scientific work is to find some good or better working explanation of observed phenomena. There are however, two dangers to be avoided. The first lies in trying to explain the behaviour of an animal in terms of its value to man. This anthropocentric attitude is less widespread now than formerly, but one is still asked, for example, What good are Painted Lady butterflies to us? as though that was the test of the justification of their existence!

The second danger is more widespread, and is to a certain extent the result of the over-enthusiastic Darwinism of a few decades ago. It consists in thinking that when one has discovered a possible advantage to the species resulting from any particular habit or structure, the habit is thereby explained. This is far from being the case, and I believe that even if it can be shown that migration is of survival value to some particular species, that alone does not explain why or how the species began to develop the habit; nor does it throw any light on the mechanism by which a particular individual comes to be migrating in a particular district at a particular moment; nor does it explain why many species that are closely related to migrants seem to be equally abundant and equally successful in spite of the fact that they have never developed this habit.

All these questions and problems must be kept in mind in looking over the next few chapters which survey the facts, or rather the observations, on which any theories must be based. After these have been digested the problems must be re-examined to see what we can make of them.

It is so easy to make theories, but so difficult to find any which will fit the facts, all the facts and nothing but the facts.

I should just like to add a word on the subject of scientific and popular names. Where possible in this book I have used popular English names, as so many amateur naturalists seem to prefer them. It should, however, be remembered that the classical names are international, while popular names are local. The Painted Lady of England is La Belle Dame in France, and Der Distelfalter in Germany, but is Pyrameis or Vanessa cardui throughout the world. Many rare or foreign butterflies have no popular names. It is quite easy, if one tries, to get used to the classical names, which are not always longer or more difficult than the popular ones. May I quote the case of the amateur gardener who described a botanist as a man who couldn’t call a chrysanthemum a chrysanthemum, but had to use a long Latin name for it!

CHAPTER 2

A BRIEF HISTORY OF THE STUDY OF INSECT MIGRATION

BY FAR the earliest mention of any migrant insect is to be found in the Book of Exodus (written about 1,500 B.C.) and refers to the plague of locusts in Egypt, saying the Lord brought an east wind upon the land all that day and all the night, and when it was morning the east wind brought the locusts. And the locusts went up all over the land of Egypt…they covered the face of the whole earth…and there remained not any green thing, either tree or herb of the field, through all the land of Egypt. And later The Lord turned an exceeding strong west wind which took up the locusts and drove them into the Red Sea.

It is interesting to note that the relation between the flight of locusts and wind direction has been investigated in the last ten years or so, after having been neglected for nearly three thousand!

Two records of migration of butterflies have come down to us from the