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.
[Illustration: FIG. 61.—A single egg-tube or ovarian tube (usually
there are many) of an insect. The youngest and smallest eggs are at the
narrow end. _o_ _o_ are larger egg-cells with a striated shell or
envelope; _g_, nucleus of the egg-cell. The unshaded egg is one grown to
full size, and in the parthenogenetic aphis would develop where it is
without fertilisation into a young aphis.]
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.