Chapter XI: Preface (11)

It does not follow because a plant is a native of a given country that it can be easily cultivated anywhere in that country, or that its finest cultivated varieties will be hardy. Only a few limited territories (owing to the nature of the soil, climate, and exposure) in Germany (chiefly in Bavaria), and in Kent, Sussex, Worcestershire, and Herefordshire, seem to be really favourable to hop-growing in Europe. Certain parts of the Pacific coast of the United States have of late years proved a very successful ground, although hops were introduced from Europe and first cultivated with considerable success in the State of New York. The same dangers and troubles attend the hop-crop in all these regions. These are blight, red-spider, mildew and mould, besides several less important insect pests. The hop-blight, or “black-blight,” is a plant-louse or aphis (Fig. 55) like the rose-aphis, and does great and increasing damage to the hop-crop in England, destroying the young and tender shoots in the months of June and July. In 1882 the hop-crop was reduced from 459,000 cwt. (of the preceding year) to 115,000 cwt. by this insect, and the wages paid for hop-picking from £350,000 to £150,000. These figures give an idea both of the damage done by blight and of the amount and value of the annual crop, for the mere picking of which so large a payment is made. Red-spider is a small mite or acarid which has done a good deal of damage in Kent. But mildew and mould are more serious. These are due to a delicate, thread-like kind of fungus, which spreads on the leaf. Many kinds are known in various parts of the world and on various plants. They may grow on one kind of plant without doing injury to it, but if they get on to another, cause deadly destruction of the foliage. It was an otherwise harmless mould, or leaf-fungus, which destroyed the coffee plantations of Ceylon. It had lived in the Ceylon forests on other plants without attracting notice; but when the coffee tree was introduced and cultivated in large areas, this little fungus seized on it, grew with terrible activity, and received the name “vastatrix” from the botanists who traced its history, and showed that it was the destroyer of the coffee plantations.

Hop-growers are constantly contending with these pests in the same way as other growers of crops have to contend with similar pests, but the hop-growers have the more difficult and delicate “patient” to steer through its diseases. The finest kinds of hops are not robust; it is a chance whether or no they will suffer from a wet and cold season, or other irregularity of climate, to such a degree as to fall ready victims to blight and mildew. Yet they pay better, provided the season is favourable, and so the grower risks planting the fine, delicate variety instead of being content with the more certain but smaller profits yielded by a more robust variety of hop. The hop-lice, or blight insects, are destroyed by washing with soft soap and quassia—a process requiring, even when a machine is used, a good deal of care and labour. Mildew and mould are destroyed and also prevented by dusting the hop-vines in hot summer weather with finely powdered sulphur. But both diseases can be combated by keeping the source of infection away from the hop-garden. The mould-fungus can be checked by burning all leaves and plants attacked by it within the hop-garden. If the infected leaves are left to rot they carry on the parasitic fungus to a new season.

An interesting fact has been discovered about the hop-blight aphis (called by zoologists _Phorodon humuli_). It appears that the winter brood of this little insect (when the hop-vine has died down) deposit their eggs on the bark of the sloe (the wild plum), and also that any cultivated plum trees serve them for the same purpose. When the hop is dead they must of necessity get nourishment and shelter from the plum tree. Clearly, then, if you can keep all plum trees at a distance of half a mile from your hop-garden you will render it very difficult, if not impossible, for the blight aphis to carry on from season to season. It will rarely, if ever, travel half a mile, and not in any number. But hop-growers have not always the control of the cultivation for half a mile around their hop-fields, though large growers should be able to acquire it. The skilful grower even finds it useful to leave one or two plum trees in the hop-field, so as to attract the winter brood of the blight aphis to them, and then he falls upon the devastating but minute rascals with quassia and other poisons, and ensures their destruction. The increase of plum orchards in the neighbourhood of hop-gardens is probably a chief cause of the increased loss by hop-blight of late years in Kent.

The hop-louse has other enemies besides the grower. These are the lady-birds (less prettily called “lady-bugs”), which feed greedily on the parasites, so that when the hop-grower sees plenty of them on a hop-vine he does not trouble to wash it. And there are other predaceous insects which tend to keep the hop-lice down. Cultivation and excessive production have resulted in putting, as it were, too heavy a task upon the natural enemies of the pest, whilst the more delicate but valuable varieties of hop cannot withstand the attacks of blight, which less valuable varieties would tolerate without fatal injury.

Another complicated and difficult problem for the hop-grower is the “curing” of the hops when gathered. He has to arrange to grow a number of varieties which will not be all ready for picking at the same moment, so that the hop-pickers may be employed for some six weeks, and gather each kind at the exact time of ripeness. Then the gathered hops have to be “dried” and “cured.” In Germany (where the highest-priced hops are produced) small cultivators dry them in the sun, and they are “cured” by the purchaser, but in England they are dried in kilns (called “oasts” in Kent) near the hop-grounds. They are cured with sulphur fumes on the spot as soon as dried. The object of the drying and curing is quickly to get rid of the water, which forms 75 per cent. of the weight of the green flower-heads, but is reduced by drying to 10 per cent., and to destroy the “mould” (fungus) which may be present, and to keep the hops free from new access of mould by the slight deposit of sulphur fumes on their surface. The drying and fumigating require a great deal of skill, and a fine crop may be injured or even rendered worthless by want of care, rapidity, and judgment in treating the freshly gathered flower-cones. It is said that it takes years to acquire the art, and that skilled hop-curers are more difficult to obtain than formerly.

The natural difficulties and fluctuations with which the English hop-grower has to contend are made far more serious by the fact that he does not know what will be the yield of the American and German hop-plantations, and so cannot prepare beforehand for the demands of the market. It appears that ice-storage is now being made use of in some districts to hold over any excess of produce of particular kinds of hop beyond the special demand for those kinds. But a formidable source of trouble exists (and, it appears, must always exist) in the enormous changes and expansion of the brewing industry in all parts of the globe. It is actually the case that there has been a greatly increased and unforeseen demand for hops of less highly developed aroma, for the purpose of brewing light ales with little of the perfume given by the finest and hitherto most highly priced hops. So that, having expended skill and money to produce the finest hops, and having been favoured by the weather, a grower may find that his pains have been thrown away, and that there is a sudden falling-off in the demand for the beautiful high-priced crop which he has gathered in. There is no remedy for these world-wide fluctuations in the market, and the only way in which the grower can protect himself is by combining with others to procure information from every part of the world as to the probable production and the probable demand of the various qualities of hops a year or more in advance of his planting. More has been done in America and in Germany in this way than in England, and it is probable that the future success or failure of hop-growing in this country depends more on the possibility of obtaining correct information in regard to the tendencies of production in all hop-growing countries, and in regard to the demand in all the brewing industries of the world, than on anything else.

This brief sketch of the hop-growing industry is sufficient to show what a very difficult problem is before those who desire to take legislative measures for the preservation of the old industry of the hop-garden in this country. But it must not be at once assumed, because the case is a difficult and complicated one, that nothing can be done, and that the beautiful hop-vines and the finest hops are necessarily to be banished from the English soil.

XXXV

GREEN-FLIES, PLANT-LICE, AND PARTHENOGENESIS

The minute “green-flies” which attack all kinds of plants, and among which are ranked the hop-louse or hop-blight, the rose aphis or green-fly of rose trees, the woolly blight or aphis of apple trees and pear trees, and the terrible vine-killer—the _Phylloxera vastatrix_—form a special group of bug-like insects known as the Aphides. They have soft cylindrical bodies, six legs, sometimes two pairs of transparent wings, sometimes none, and a sharp beak (in some kinds this is one and a half times as long as the body), with which they prick the soft parts of plants, when they suck up the juices which issue from the wound (Fig. 59). There is in the temperate regions of the world a special kind of aphis or plant-louse peculiar to each of many kinds of flowering plants, including most trees. A very complete, illustrated account of the kinds or species of British aphides, amounting to some two hundred, was produced by the late Mr. Buckton, F.R.S., and published by the Ray Society.

There are many facts of extraordinary interest about these tiny swarming insects. In the first place, they are closely related to the minute scale-insects or _Coccidæ_, several species of which produce the celebrated lac of lacquer-work and the dyes known as lake, cochineal, and kermes, the latter a dye manufactured in South Europe and used to colour wool and cloth crimson before cochineal reached us from Mexico. The _Coccidæ_ include also the “mussel-scale” and other destructive diseases of fruit trees. A beautiful purple colour can be extracted from crushed masses of some kinds of aphides (as well as from _Coccidæ_), and has been used as a dye. The aphides have very generally a green colour, like many insects (caterpillars and leaf insects) which pass their lives upon green leaves and feed on them. It is often supposed that this green colour is merely the green colouring matter (so-called chlorophyll) of the leaf, taken up by the insects in feeding on the leaf. But this is not so; it is a peculiar substance derived in a crude state from the plant-juice, but digested in the stomach and completed in the insects’ blood and tissues. Then, again, the aphides produce curious secretions, often in great abundance, which surround them as the lac surrounds the lac-insect. The threads which are produced in such abundance, by the woolly aphis of apple trees, as to look like masses of cotton wool adhering to the twigs of the tree, are of this nature.

Another curious production of the aphides—common on the leaves of elms and other trees infested by them—is known as “honey-dew.” It is sticky and sweet, and was supposed by old writers to have distilled from the stars, or otherwise to have dropped from heaven. It is this sweet secretion which has led to the establishment of a most curious friendship between ants and aphides, or plant-lice. It has long been known that an ant will approach an aphis, and tickle it, when at once the aphis exudes from its cornicles (see Fig. 60) a drop of sweet honey-dew, which the ant swallows—just as a man may milk a cow and drink the milk. And the resemblance goes further, for the ants take possession of certain aphides, and keep them either underground or in specially constructed chambers, where they can gain ready access to them and “milk” them for honey-dew. There has been a certain amount of exaggeration in the description of these facts by some of the older writers; but it is undoubtedly true that some species of ants keep special flocks or herds of aphides, and feed on their sweet secretion.

Other small insects nourish themselves on the enormous swarms of plant-lice in a less gentle way, but a way which man is very glad to see in active operation, namely, by biting them and sucking out their soft entrails—thus destroying them in great numbers. The lady-bird beetle is especially active in this matter, both when it is a grub and on attaining its adult form. A trustworthy observer saw as many as forty aphides consumed by a lady-bird in an hour. Where the plant-lice or aphides abound, there come also in countless swarms the beetles known as lady-birds. In the year 1869, such a cloud of these beetles passed over and settled on the fields and gardens of Kent, Sussex, and Surrey, as to cause something like terror; it was impossible to walk in the lanes without crushing hundreds under foot. But the little lady-birds are not like the terrible locust, which appears in millions and devours all vegetation before it; on the contrary, they are what are called “beneficials,” and come solely to feed on and destroy the plant-lice of the hops, plum trees, and apple trees. A first-rate hop crop in the year 1870 was the consequence of the abundance of lady-birds in 1869. It is this beneficent activity of the lady-birds which has given them their name. In Italy they are called _Bestioline del Signore_, also _Madonnine_, and _Marioline_, and in France _Bête à Dieu_. In English they are “our lady’s blessed bugs,” which save the crops from destruction.

The exertions of the aphides in pricking the plants they infest so as to get at their juices lead to the growth of galls on the leaves, and also on the rootlets of many plants, and often the leaves become rolled up into bag-like bodies filled with aphides. Many trees and smaller plants are killed by these attacks, but it is probable that where the plants have not been rendered delicate by nursing and cultivation, and where the aphides are not a strange foreign kind, introduced by man’s carelessness or by some rarely (if ever) occurring wind or flood, the aphides do not actually destroy any plants by their visitation, excepting the weaklings, and that their numbers are kept within bounds by their natural enemies the lady-birds and other such carnivorous insects.

We must now notice the most interesting of all the wonderful things which have been discovered about these tiny insects, which are even smaller than fleas. Any one who has a rose-garden and chooses to spend some hours a day in studying the “green-fly” can follow out the facts. They reproduce themselves—that is to say, propagate—with astounding rapidity. The great Linnæus, a hundred and fifty years ago, came to the conclusion, from his observation of one kind or species, that in one year a single aphis would produce a quintillion of descendants! Without insisting upon the exact numbers in different kinds of aphides, we may say that that is a fair indication of the rate at which they produce young. No sooner does a mother aphis produce some thirty or forty young, than in a few hours or days, according to the warmth of the season and the abundance of food, these young have grown to full size and themselves each produce the same number of young, and so on through the summer, and even into the autumn. Nineteen generations in sixteen weeks have been counted in some kinds of the plant-lice. Hence it is no wonder that these little creatures increase exceedingly and cover the leaves and shoots on which they feed; no wonder that they furnish a plentiful nourishment for the lady-birds which prey on them. But the most curious thing is this, that these abundant and rapidly reproducing broods of aphis are _all females_, and that they do not lay eggs, but extrude their young in a more or less complete state of development, that is to say, they are viviparous. They are all females! It is only late in the season that males are produced!

In fact, the summer broods of the “green-fly” and other aphides which do so much damage to rose bushes, hops, and other cultivated plants, are produced by females alone, without the intervention of a male. These minute insects present true instances of that very remarkable and interesting occurrence which is called “parthenogenesis,” or virginal propagation. It is further a noteworthy thing that the virginal aphis mothers do not lay or deposit eggs, but that the young grow from the eggs inside their mothers (Fig. 61), and are only extruded when they are complete little six-legged insects, capable of walking, and ready to feed themselves by stabbing the soft leaves of the plant on which they find themselves, and sucking up its juices. The summer aphides are spoken of as being both “viviparous” and “parthenogenetic.” The words are really useful, and we cannot get on without them.

No case is known to medical men or to naturalists of the birth of young from an unimpregnated or virgin mother among what are called the higher animals—those which are classed as vertebrates, and include mankind, mammals, birds, reptiles, batrachians, and fishes. But though uncommon, this virginal reproduction (or “parthenogenesis”) does occur constantly in a very few kinds of small insects and in some small shrimp-like creatures. It has excited the greatest interest amongst naturalists from the early days when it was first observed until the present, and it has been very carefully studied in the past thirty years.

In order to appreciate this matter it is necessary to know the chief facts about the ordinary process of reproduction in animals and plants. All animals and plants are built up of minute particles of living matter called “cells” (see p. 170). Really, these are not cells, or hollow boxes, or cases. We use the word “cell” for the contents of a cell. Each is a droplet of protoplasm or living matter lying in a small or large envelope or case of dead matter which it has produced around itself (Fig. 61). Observers using their microscopes saw at first only the case, and called it a “cell,” and the word “cell” is now used almost universally for the soft stuff within the cell (see p. 173). Each soft cell of “plasm” or “protoplasm” has a very special structure. The existence in it of a central kernel, or “nucleus” of peculiarly active substance, is the most obvious feature. These “cells” are so small (for instance, those which build up the human body) that from one to two thousand could be placed side by side on a line an inch long. They are the “units” which make up the body of an animal or plant, just as bricks and planks and rods make up a building constructed by human contrivance. Two most important things about them are—first, that each is always the seat of chemical activity, absorbing liquid material, changing it and either fixing it or throwing it out in a new chemical condition; and, second, that as a result each cell grows, and after a very little growth divides into two. This “dividing into two” is immensely important, for in this way the number of cells forming a very young or small animal or plant is increased from a few thousands to many millions whilst the organism grows. And not only that, but we find on tracing the young animal or plant back to its beginning as an individual that it actually started as a single cell. The germ of every living thing, then, is a single nucleated particle of protoplasm—a cell which we call the “egg-cell,” because “eggs” are merely shells and packing to hold and protect this all-important egg-cell.

Every individual flower, tree, insect, snail, fish, and man started as a single egg-cell, which became detached from the mother’s body. Take the case of a common marine animal, the star-fish. At the breeding season, early in the year, the female star-fish discharges thousands of these egg-cells into the sea-water. Each floats separately in a delicate case of its own. Before any one of those floating egg-cells can commence to divide so as to build up a new mass of cells—a new young star-fish—it must undergo the process of “fertilisation.” That is to say, its substance must fuse with that of a “sperm-cell.” These “sperm-cells” are discharged into the sea-water in countless thousands by the male star-fishes. They are excessively minute, actively wriggling threads, swollen out at one end to form a little knob, the “nucleus” of the sperm-cell (see p. 134 for figures of the spermatozoa, and eggs of the oyster). The water is rendered cloudy by the abundance of these microscopic filaments, which are called “spermatozoa.” One sperm-cell, or spermatozoon comes into contact, in the sea-water, with each of the discharged floating egg-cells. It burrows into it and fuses or melts and mixes with the substance of the egg-cell. The whole process is easy to watch with a microscope, and I am writing of what I, in common with many others, have actually seen.

The egg-cell after this process consists really of the substance of two equal cells—the egg-cell and the sperm-cell—completely fused so as to form a single cell, having a single “nucleus,” which has resulted from the fusion of the nucleus of the egg-cell with that of the sperm-cell. Now, and not before, the egg-cell can divide, take up nourishment, and continue to divide and grow, so as to form a constantly increasing mass of young cells, a young animal which gradually assumes the form of a star-fish. All animals, and plants, too, reproduce themselves in this way. When the animal or plant is not aquatic in its habits the sperm-cell and the egg-cell cannot be discharged and take their chance of coming into contact with one another outside the parent’s body; the sperm-cells are, in such cases, received into a chamber of the egg-producing parent’s body, and there the fusion of the egg-cells with them, one sperm-cell to one egg-cell, takes place. Parthenogenesis then consists in the omission of the fusion of a sperm-cell with the egg-cell. The egg-cell develops, divides again and again, and produces the young animal without the addition to it of a sperm-cell—without, in fact, being “fertilised,” as it is called. That is what happens in the summer broods of the little plant-lice or aphides (Fig. 57). When, however, the cold weather comes the virgin mothers suddenly produce two kinds of young—males as well as females—and then the solitary winter egg, which the late autumn females lay to last through winter until spring, is fertilised by a sperm-cell derived from the late produced autumn male (Fig. 56) in the ordinary way.

Another parthenogenetic animal is the rare little fresh-water shrimp called _Apus_, which goes on multiplying in this manner in wayside ponds for years, thousands of female individuals being produced in successive seasons, laying their eggs and carrying on the race for an indefinite time until at last—one fine day—we do not know why then and not before, that rare creature a male _Apus_ is hatched. Why these and one or two other such small shrimps and insects are able to set aside the almost universal law as to the necessity for fertilisation of the egg-cell by a sperm-cell, naturalists have not yet found out. It is quite certain that these exceptional creatures have been derived from ancestors which had their eggs fertilised in the regular way, and that this elimination of the male is a special device, an innovation.

There are incomplete attempts at it in other insects. Thus it has been discovered that the queen bee produces only females from the eggs which are fertilised before she lays them. When the stock of sperm-cells which she received from a drone in her nuptial flight is exhausted, or if we carefully remove by a painless operation the internal sac in which they are stored, the eggs are no longer fertilised, but they are not rendered sterile or abortive. They develop into drones! And drones or male bees are produced in no other way, and only drones are so produced, never worker-females (so-called neuters) nor queens.

Another curious fact is that in rearing moths in captivity some naturalists have quite unexpectedly found that when they have hatched out female moths from the chrysalids and kept them from the moment of hatching quite apart from the male moths (which are of another size and colour, and easily distinguished), these females will sometimes lay eggs—unfertilised eggs—which give birth to caterpillars, which feed and complete all their changes. The second generation of moths so produced are male and female, but the females, being kept apart again, produce a parthenogenetic brood, and the process has been repeated to a third generation. These instances are very rare. The remarkable thing about them is that, apparently, the parthenogenesis is only due to the experimental interference of an entomologist, and that unless some such accident had befallen the moths, the eggs would have been fertilised in the usual way, since there was no deficiency of male moths. These facts have led to many interesting speculations, and are particularly curious in regard to the inquiry as to what determines the sex of offspring, about which sensational announcements are sometimes made in the foreign correspondence columns of our newspapers. Here we find the parthenogenetic eggs of the moths producing both males and females, those of the aphides and the pond-shrimp producing predominantly females, and those of the queen bee producing exclusively males (drones). Biologists have not yet arrived at a solution of the problem raised by these divergent results.

It is necessary, in regard to this subject, to remember that many lower animals and plants can reproduce or propagate by separating “buds,” or large bits of their bodies, built up of thousands of cells, and, therefore, not to be confused with the single egg-cell. The egg-cell is a cell specially prepared for fusion with a sperm-cell, necessitating—except in very rare instances—the union in the new individual or young of living material from two separate parental organisms, and, therefore, in many cases, from two widely separate lines of ancestry. A snippet, or bit cut from a begonia leaf, will produce a new individual plant; a bit cut or torn from a polyp will similarly give rise to a new individual: but the parthenogenetic egg is not to be confused with these masses of cells. It is a true egg-cell which might have been fertilised, and it is found in animals such as insects and crustaceans, which are more highly elaborated in structure than any which, like the polyps and zoophytes, multiply by buds and cuttings.

XXXVI

THE DEADLY PHYLLOXERA

It was only after long and patient investigation that the various broods of the terrible Phylloxera which, between 1868 and 1888, destroyed half the vineyards of France, became known, their relations to one another determined, and the final cure for the devastation caused by them decided upon and put into practice.

In all ordinary plant-lice or green-fly (aphides) at the end of the summer, the last parthenogenetic brood produces a generation of distinct males and females, which differ a good deal in appearance from the virginal broods of the spring and summer. Each female, after receiving sperm-cells from a male, lays a single egg, which consists of a fertilised egg-cell enclosed in an egg-shell. It is deposited in a safe place in a crack of the bark of a tree, or on the rootlets of some plant, and remains unchanged through the winter. In the spring from every such egg hatches a single female aphis, which feeds and increases in size. In a very short time (a week or so) this solitary female (Fig. 58) proceeds to produce, without male intervention, young which grow from true egg-cells which are not laid but remain inside her. The young are born or pass out of her as small six-legged insects. They feed and grow up, and in turn produce “parthenogenetically” and viviparously broods of young like themselves. The first female thus hatched from the winter egg is called a “foundress,” or “stock-mother,” because she starts a whole colony of young which, by virginal propagation of successive broods, may number many millions in a season. These are known as “virgin-mothers” (Fig. 57), and eventually their later generations always produce males and females, so that we distinguish, in the course of a year, four sets of aphides, starting from the egg, namely (1) the foundresses, (2) the numerous generations of virgin-mothers, (3) the males, and (4) the egg-laying females.

In different kinds of plant-lice any of these “sets” may be either winged or wingless (Figs. 55, 56, 59); many generations of the virgin-mothers are wingless, but not all, in all species. According to the species or kind of aphis and its requirements in regard to the plants on which it feeds, wings are developed so as to enable the aphis to fly from one tree or locality to another, or are not developed if the aphis has to remain where it was born. The whole series of successive broods of some kinds of aphis remain on one plant and about the same part of it, and then there is little need for wings. Others have their summer broods on the twigs or leaves, but the later broods descend in winter to the roots of the same plant. The woolly aphis of the apple trees and pear trees behaves in this way; other species again produce a late-winged brood, which leaves the plant on which its parents were feeding, and travels some distance to the twigs or to the roots of a quite distinct kind of plant to produce an autumn brood, and from these the final males and females are born, and the winter eggs are then deposited. The hop-louse leaves the hop when the hop-vine dies down in autumn. The abundant wingless form (Fig. 58) of which there have usually been ten generations, produces at last a winged “migrant” brood (Fig. 59) which flies away to plum trees and sloe bushes, perhaps a quarter of a mile distant. There the migrants produce wingless females on the plum tree. They are followed to the plum trees by a final migrant brood from the dying hops which are males—the first yet seen (Fig. 56). The males fertilise the wingless females born on the plum tree—and the latter lay each one fertilised egg in the crevices of the bark of the plum tree near the young buds. Winter now sets in: all are dead except the eggs. In the following late spring a foundress hatches out from each egg so deposited. The “foundress” (Fig. 58) in this species, the hop aphis, is wingless. She produces parthenogenetically and viviparously a brood of wingless females. They similarly produce on the plum tree a third generation of virgin females, but these have wings! (Fig. 55). They fly back to the hop-vines, which are now well risen from the ground and offer abundant juice to the wingless virgin brood which escapes from the winged migrants as soon as they have settled on the hop, and feed and grow and produce new wingless broods (Fig. 57) in rapid succession.

The phylloxera of the vine is a plant-louse or aphis, which exhibits an interesting adaptation of winged and wingless broods to the requirements of the insect’s nutrition and multiplication. A “foundress” hatches from an egg on the bark of the vine where it has passed the winter. It proceeds to attack the young leaves and to produce a brood of young. The leaves of the vine when thus attacked swell up and produce galls, in which the young phylloxera are enclosed, and there the phylloxeras continue to multiply, producing more galls and thus destroying the leaves. Some of the young broods now crawl down the vine to its roots; others stay on the leaves and continue their destructive work there. There are several varieties of form and size amongst these broods. Those which go to the roots attack the rootlets and produce knobs and swellings on them, leading to their destruction as feeding organs. Meanwhile the root-phylloxeras multiply exceedingly, and those on the leaves are still feeding and multiplying. From one foundress mother as many as twenty-five millions are produced in six months. At last in the autumn the root-parasites produce a winged generation of virgin-mothers, which come up from the ground and fly away to other vines, upon which they produce males and females. These females each lay a fertilised egg on the bark of the previously healthy vine, and so the infection is spread. The root-infesting forms continue to multiply, and in warm climates there is no cessation of this process even in winter.

This parasite—the _Phylloxera vastatrix_—was introduced with some American species of grape-vine—brought over as experimental samples from Colorado—about 1864. In its native country it does comparatively little harm, for the roots of the American species of vine are, though attacked by it, not seriously injured. They have the property of throwing out new rootlets when those already existing are punctured and injured by the phylloxera, and so are not killed by the attack, as is the European grape-vine.

The introduction of this deadly parasite to Europe was a mere chance, due to ignorance and stupid want of supervision of importations on the part of the Government, such as is common in this country, though less so in France and Germany—part of the blind mixing-up of the nicely adjusted products of all parts of the earth which civilised man is always bringing about with disastrous and terrifying results. In twenty years France lost 400 million pounds in consequence; three million acres of vineyards were destroyed. Other countries—Germany, Italy, and the Cape—also suffered. All sorts of remedies were suggested and tried, such as the application of poisons to the roots and the sinking of the vineyards under water. Gradually the only effective method of dealing with the case has been established. The old European vine-stocks or standards have been grubbed up in all but the very choicest vineyards, and American vines have been planted in their place. On to these have been grafted cuttings of the local French vines, and they have taken kindly to their new conditions. The produce of the French vineyards is now greater than it has ever been. It had fallen from an annual yield of 1,300,000,000 gallons to 650,000,000—but in 1900 it had risen again to a yield of more than 1,400,000,000 gallons.

This history is a striking instance of the vast importance to civilised communities of a knowledge and control of even such minute living things as the plant-lice, and of the extraordinarily large results which obscure living things may produce. It must tend to convince reasonable men of the importance of accurate knowledge as to living things and of the necessity of expending public money in constantly improving and extending that knowledge.

An ingenious illustration of the enormous fecundity of the plant-lice occurs to me as worth giving in conclusion. The late Professor Huxley—a careful and trustworthy authority—calculated that the produce of a single aphis would, in the course of ten generations, supposing all the individuals to survive, “contain more ponderable substance than five hundred millions of stout men; that is, more than the whole population of China.” And this calculation is held by some authorities to be below rather than above the mark!

XXXVII

CLOTHES MOTHS

The way in which the lives of all animals and plants are interwoven with that of other animals and plants, often in obscure and unsuspected ways, comes home to man when he contemplates the numbers and variety of living things which exist with him and upon him—that is to say, at his expense and to the detriment of the stores which he accumulates, the clothing with which he covers himself, and the buildings which he constructs. Man not only has carefully taken a number of animals and plants in hand and cultivated them as food-givers, as sources of clothing, and other useful material, but, much to his annoyance, he finds, per contra, that other animals (and plants, too), with similar self-seeking habit, make use of him in his turn, and of his belongings, with a complete disregard of his convenience, treating him and his arrangements as so much available “food-stuff,” and showing no atom of respect to him as the lord of creation. Just as in dealing with the more deadly attacks of disease-producing parasites, so in meeting the destructive invasions made by his fellow-creatures of all sizes and kinds in search of food and shelter—man has to be continually on the alert, and to wage a constant warfare, unless he will consent to see himself and his possessions moth-eaten, fly-blown, worm-burrowed, reduced to fragments and powder. And this warfare he has incessantly carried on with increasing skill and knowledge from the earliest times of which we have any record.

The sparrow and the rat, of which there has lately been much talk, are examples of fairly large, easily detected enemies of this kind. The almost ultra-microscopic bacteria—similar to those which produce disease by multiplying in the living body—are examples of the most minute living pests which injure man by causing sourness, putrefaction, and destructive rot in his food and stores. Every year civilised man is gaining greater knowledge of these “ferment organisms,” and vastly increased skill in preserving his possessions, such as food and drink, from the attacks of their ubiquitous swarms. Between the larger depredators, such as birds and rats, and the smallest, such as the microscopic bacteria and moulds (to whom alone putrefaction is due, and without whom it would never occur), there are a host of small troublesome creatures, which belong chiefly to the group of animals called “insects”—beetles, moths, flies, and bugs—which give man incessant occupation in warding off their attacks upon his food, his clothes, his furniture, his buildings, his crops and fruit trees, and his domesticated animals. The study of these things and of the means of grappling with them is the fascinating occupation of those who are called “economic” zoologists and botanists. Of course, in order to carry on their inquiries successfully they have to bring to bear on the questions they investigate as complete and thorough a knowledge as possible of all the kinds of animals and plants, and of their ways of feeding, reproducing, and protecting themselves in natural conditions.

One of the most widely celebrated and anciently detested of insect pests is the clothes moth. It is the caterpillar of this moth which is objectionable—biting off, eating, and using to weave a case the hair of furs and the fine filaments of woollen fabrics. Not every one is able to recognise the clothes moth, which is a very small creature of a greyish-yellow colour. The wings when set for flying measure only half an inch in expanse, and when the moth is walking or at rest, shut closely to the body so as to give it an almost cylindrical shape, with an attenuated snout. Much bigger moths occasionally get into our rooms, but do no harm. These little clothes moths lay their eggs on fur or wool, and the caterpillars which hatch from them do the damage. The moths themselves have no jaws and take no food. But the caterpillar or grub, though soft and readily crushed, has a pair of very hard, minute, dark-coloured jaws, with which it works away, cropping the fur and wool on which it lives. The moths are seen in houses commonly between January and October, and it is, of course, the object of the victimised householder to destroy them before they can lay eggs, or, what is more practical, to keep woollen and fur clothes away from their reach. Things which are in daily use are not very liable to receive a deposit of eggs from the clothes moth, and as a rule the enemy may be kept at bay by daily shaking and beating the things in question, and hanging them up in the air. But coats, flannels, etc., which are hidden away, left quietly in drawers or cupboards, offer the undisturbed conditions which the clothes moth seeks. There is no safety for them unless they are wrapped up or shut in with a quantity of naphtol or of camphor, or, as is nowadays more usual, placed in a refrigerating chamber.

The little caterpillar which does all the damage is of a dull white colour, with a reddish head. It is remarkable for the fact that it makes a sort of movable tunic or case for itself out of the hair or wool which it crops, and it crawls about protected by this case. There are not many insects which thus construct portable cases for themselves when in the grub or caterpillar state of life. Such “cases” must not be confused with the very similar “cocoons” by which some moth-grubs surround themselves (as, for instance, the silkworm moth) when their growth is completed, and they become quiescent and hard, and are known as chrysalids. Such “cocoons” are constructed in the same way as the lining of the clothes moth’s case, by threads of silk secreted by the caterpillar, but they are made once for all when the grub has ceased activity. The little clothes moth caterpillar, on the other hand, has continually to enlarge its tunic or case as it itself increases in size. There is a hole at the end, from which the head and three legs of the caterpillar emerge, so that it can crawl and feed freely. The outer surface of the case consists of cut lengths of the fibre on which the grub is living, and so is protective in resembling the surrounding material and hiding the minute ravager. It is easy enough for the little grub to add a bit to the case at the end from which its head protrudes, and, being very flexible, it can turn right round in the tube and put its head out at the other end and secrete a bit more there, cementing cut hairs to the outer surface. But in order to increase the breadth of the tube or case, the caterpillar has, from time to time, to undertake a formidable operation. It actually slits up the case lengthwise for about half its extent, and fills in the gaping space with new material; then it cuts up the opposite face of the same half of the tube, and puts in a new patch there. And after that, it has to treat the remaining half of the tube in the same way, making two more cuts, one opposite the other, and filling in the gap in each case as before. Students of these little creatures have amused themselves by changing the position of the caterpillar and its case, from fur or wool of one colour to fur or wool of another colour, and in this way the industrious caterpillar is made to work in different coloured fibre in successive enlargements of his case, so that it becomes a Joseph’s coat of many colours.

An interesting fact about the movable case made by the clothes moth caterpillar is that the nearest thing in nature to it is the case made by the aquatic grubs or caterpillars of another kind of insects—the caddis-worms (“case-worms”) which are common in ponds and streams. They show extraordinary powers in making their cases so that they balance nicely in the water, as the animal crawls along on the bottom of a pool, with his head and six legs emerging from one end of the case. Caddis-worms are of various kinds or species, and some attach to their cases little broken sticks, others minute empty snail-shells, others the fine green threads of water-plants. The caddis-worm becomes changed into a delicate fly, with transparent wings, just as the clothes-grub becomes changed into a moth—and it is an interesting fact that the caddis-flies, though they are classed with the May-flies and such net-winged insects, and not with the moths and butterflies (the _Lepidoptera_, or insects with wings covered with dust-like scales, which give the colour and patterns to the wings), yet agree with moths in having some scales on the wings and with one kind of minute moth, namely, the clothes moth, in having grubs which make movable cases.

The clothes moth caterpillar was known to the Romans by the name _Tinea_, and is described with correct detail by the Roman naturalist Pliny. Modern naturalists have accepted this name _Tinea_ as that of the genus to which the clothes moth belongs. There are thirty different British species of _Tinea_, of which four are guilty of attacking animal fabric, and so causing trouble to man. The one which builds a case and is the titular chief of the clan of clothes moths—“the” clothes moth, just as one may say “the” Macintosh—is scientifically indicated by the name _Tinea pellionella_. The other three do not form movable cases when in the caterpillar stage, and attack coarser stuff than fur and fine wool. One of them is known as the “tapestry moth,” because its caterpillar establishes itself in old tapestry and carpets, and burrowing into these thickish materials is concealed without the aid of any self-provided tunic or case. The name _Tinea_ is often used by entomologists in an expanded form as _Tineina_, to indicate the whole series of minute moths of which the genus _Tinea_ is only one little group. Many of these moths are much smaller even than the clothes moth, and they are found in all parts of the world and in all sorts and conditions of life—in relation to trees, shrubs, and plants of all kinds. It has been estimated that there are as many as 200,000 distinctly marked different kinds of these minute creatures. The insect collectors and students who occupy themselves with the magnificent butterflies and larger moths (of which there are an enormous variety of kinds) refuse to deal with the somewhat dull-looking and almost innumerable minute moths which are classed as _Micro-lepidoptera_, in contrast to the _Macro-lepidoptera_ (or big moths and butterflies). Consequently they have become the favourite study of a few enthusiasts, who are known as Micro-lepidopterists, and have a wide but not uninteresting field of exploration all to themselves. The _Micro-lepidoptera_ include, besides the _Tineina_, a group of less minute though small moths, with narrow, fringed wings, amongst which are the window moth, the milk moth, the tabby moth, the meal moth, and the grease moth. Though the clothes moths may well be described as “tiny” moths, yet the word _Tinea_, as applied to them, has no such origin, but is the name given to the destructive grub by the Romans. The same word has unfortunately been applied by medical men and botanists to a vegetable parasite which causes a skin disease (ringworm) resulting in baldness. The _Tinea calvans_ of the doctors has only this in common with the moth _Tinea pellionella_—that it causes hair to disappear and baldness to ensue; but the vegetable parasite attacks the hair on a living man’s head, the caterpillar that on his fur coat.

XXXVIII

STONE AND WOOD BORERS

Boring into wood is a favourite proceeding on the part of many small creatures, insects, shrimps, and ship-worms, by which they not only acquire nourishment, but at the same time penetrate more and more deeply into safe quarters and concealment. It is not surprising that it has become the necessary and regular mode of life of a host of small animals, and consequently that man who wants wood in good sound blocks and planks for his various constructions is a good deal put out by the voracity of the wood-boring community. To some extent he has given up the task of checking their proceedings, and now uses metal where he formerly used wood, but that only applies to a limited field. Wood is still the great material of rough construction, and the main substance used in fittings and furniture.

In our own country and in most parts of the world there are large grubs or caterpillars, such as those of the goat moth, three inches long and as thick as one’s finger, which eat into the stems of trees and spoil the timber. The grub of the handsome moth known as the wood leopard is another of these. It attacks poplar trees, and we used to take it in numbers in the London parks and squares when I was a collector. The goat moth is specially destructive to willow trees. But there are a very large series of smaller grubs and adult insects which injure trees or bore or devour wood already cut and dried. Among these are the saw-flies and a number of beetles, and in Sicily and the tropics there are the wonderful white ants which are not ants at all, but more like May-flies. The destruction caused by these borers and eaters of wood is increased by the fact that when they have riddled a piece of wood, moisture penetrates it, and vegetable “moulds” flourish within it and complete the break-up. Among the most destructive borers of wood are those which attack the ships and piers of wood placed by man in the sea. These are certain shell-fish, called ship-worms (_Teredo_), which are really peculiarly modified mussels. There is also a tiny shrimp-like creature, the _Limnoria terebrans_, which does enormous damage by its borings to piers of wood erected in the sea. True insects do not flourish in the sea. There are marine bivalve shell-fish which bore into clay, sandstone, chalk, and even into hard granite-like rock. They do not use jaws or teeth for this purpose, but the surface of their shells, which are sharp and spiny, and also the sand which adheres to their soft muscular bodies like emery powder to the pewter-plate of a lapidary’s wheel. You may see the large and small holes made by _Pholas_ (called also “the piddock”) and other bivalve shell-fish in the clay and chalk rocks of the seashore on most parts of the English coast.

Most boring animals swallow the material which they excavate in the act of boring, just as the earth-worm swallows the soil into which it bores, and as many sand-worms do, throwing out from the hind end of the body, in the form of a little coiled-up heap, a vast quantity of undigested matter which has passed through them. But many insects which swallow some of the material disengaged by their jaws remove, in addition, a large quantity which is ejected from the boring as powder, like sawdust, and others do not swallow any of the material into which they bore. So, too, the _Pholas_ and marine-boring mussels do not swallow the material which they loosen. It is a very slow process, the boring in rock, and the fine particles rubbed away by incessant movement are carried off in the sea-water.