It is a constant visitor to my garden, though I have failed to
discover its nidus.
Alaptus minimus and fusculus have long been known to me. The
first-named is not more than one-seventy-second of an inch long,
with a sessile abdomen. Its wings are somewhat hatchet shaped, with
peculiar enlargements at the base of the inner margin. The lower
wings have a crimped appearance, arising from the mackerel marking.
This, and in fact all the family, are to be found running up the
glass in a greenhouse, especially on the window facing east. Alaptus
fusculus is one of the first to appear in spring--its peculiar jerky
gait will at once reveal its identity. Of this species I have bred
hundreds from the eggs of a psocid (Stenopscocus cruciatus), an
insect much like a common aphis, but very active and shy, having
a decided objection to being watched. But “all good things come
to those who know how to wait,” and wait he must for hours, days,
months, and years before the life history of a single species is made
out. Haliday mentions that Polynema (Cosmocoma) destroys the eggs
of the cabbage butterfly. I read this twenty-five years ago, but I
have never yet found one egg “struck.” Stenopscocus lays its eggs on
the leaves of various shrubs and trees. I have found them on lime,
oak, sycamore, hawthorn, and ivy. They are laid in patches of ten to
twenty, the female psocid carefully weaving a silken covering in an
endeavor to protect them from the attacks of enemies; but, alas! who
would imagine a fairy fly to be an enemy? Yet its microscopic size
enables it to pass unnoticed, and also permits its passing beneath
the silken screen, and, once there, woe betide the psocid’s eggs!
for the busy fairy taps one with her clubbed antennæ, mounts to
the summit, and then lets down the ovipositor until the barbed tip
touches the surface of the fresh-laid egg (it must be fresh). Now by
carefully focusing a good magnifier, we can observe the fairy taking
a firm hold of the surface of the egg with the two curved tips of
each of her exquisitely formed toes! Next we note that pressure is
being put upon the barbed ovipositor. The antennæ are pressed firmly
to the surface, and impress the observer with the fact that some very
serious business is in hand. Eleven minutes have passed without any
sign of a move, when just after eleven and a quarter the ovipositor
positively bumps through the shell. There is another serious pause,
and then up go the clubbed antennæ, and very carefully the ovipositor
is withdrawn until it slips back between the sheaths. The fairy
turns round, and with saliva from her mouth seals up the incision.
Another egg is ascended, bored, and stored with an egg--and so on
until the whole batch of twenty has been struck, and all chance of
any psocid emerging utterly ruined. I kept the first batch of eggs
which I had seen struck in October until the following year, when,
as the warm days of April arrived, I carefully examined them, until
one eventful day I observed one of the eggs had a tiny hole in it.
On placing it under the microscope I saw a pair of mandibles busily
at work nibbling away the eggshell, until at last the hole was large
enough to admit of the head being thrust through. After many efforts
the antennæ were freed, followed by the first pair of legs; then,
with this additional leverage at command, the thorax was lifted out,
the second pair of legs and part of the wings following, and after
much apparently painful effort the third and last pair of legs was
withdrawn, enabling the fairy to walk out, and to free those most
exquisitely delicate wings without a hitch. Now, taking a firm grip
on the empty eggshell, the fairy went through her toilet. Not a hair
or spine escaped attention--each and every part of this microscopic
marvel received the utmost attention; every hair forming the lovely
marginal fringe was brushed out and arranged in exact order. The
wings were raised several times to try them, and then away this atom
of perfection flew. Since my first seeing the ovipositor of a fairy
fly I have dissected many struck eggs, and in less than a minute had
the germ (laid by the fairy fly) under my microscope, and watched it
grow, and the cells divide again and again in such a marvelous manner
that I have been lost in wonderment or simply overcome. Flesh and
blood is not strong enough to carry on such watchings too long, but
the fascination is so great that time after time have I watched the
mysterious changes taking place--various organs being formed under
my eyes--the active larva gradually losing all its activity until
it appears in pupal form, and then new limbs and organs seem to be
evolved from nothing at all.
The next genus, Litus, is a peculiar one. Its one representative,
Litus cynipseus, much resembles a flea in color and form. It is the
only fairy fly that takes its time in walking. I have never seen it
in a hurry, neither have I ever seen the male--and the female is not
a plentiful species.
Eustochus is distinguished by having a deeply marked suture across
the club of the antennæ, and though Haliday mentions but one species,
I have found over a dozen; in fact, of this genus I can always find
specimens anywhere--on windows, in greenhouses, in railway carriages,
besides sweeping them up from grass and all kinds of herbage. I have
also seen and caught them flying, and have obtained many specimens
from spiders’ webs.
Mymar pulchellus is, without doubt, the most extraordinary of the
whole family. My first specimen I caught in a spider’s web when
living near Finsbury Park. It was very much held by viscid globules,
but after many hours’ work appeared in Canada balsam a splendid
mount. No insect has exercised my power of thought more than this
one; but its life history remains a mystery still, though I imagine
I am within measurable distance of discovering its nidus. The
posterior wings are abnormal--mere bristles--and yet they are of
immense service when hooked into the anterior ones, the black specks
just before the tips of these bristle-like wings being the three
hooklets.
The next representative, Cosmocoma, is Haliday’s Polynema, which, he
states, affects the eggs of the cabbage butterfly. C. fumipennis is
of the most strikingly beautiful character. I once saw the male of
this, but it was on the glass outside the greenhouse, while I was
inside and the door was locked. My feelings at that moment can be
better imagined than described.
Caraphractus cinctus has received a good deal of attention and
notoriety from the fact that in 1862 Sir John Lubbock discovered that
it was aquatic in its habits, using its wings for swimming under
water. It was christened Polynema natans by the worthy discoverer of
its natatorial habits; but in 1896 it fell to my lot to prove it to
be identical with Haliday’s Caraphractus.
Anaphes is found in almost any garden, and is frequently confounded
with Eustochus, but the solid club is an unfailing character. It is
also somewhat larger than Eustochus.
Anagrus contains many species of very delicate yellow fairy
flies, always present in gardens and about ponds--in fact, almost
everywhere. I have been again fortunate in discovering the life
history of several species belonging to this genus. One is parasitic
in eggs of dragon-flies, and three others in different kinds of eggs
of frog-hoppers, which have for over thirty years eluded my search.
One more genus remains to be noticed, and it is the most fairy-like
of all the Mymaridæ, viz., Camptoptera papaveris. It is the smallest
of the family, the female being but one-eighty-fifth of an inch long.
Mystery surrounds this gem, and yet it is most plentiful in certain
localities. The male, of which I have only taken two specimens,
measures just one-ninety-second of an inch from head to tail, and yet
is absolutely perfect in every part.
INSECT TRANSFORMATIONS
--ANDREW WILSON
Most people are aware, as a piece of commonplace knowledge, that many
animals, before arriving at their mature or adult state, undergo a
series of changes in form, of a more or less complete character.
To such a series of changes the naturalist applies the term
“metamorphosis”; and the study of the disguises which an animal may
in this way successively assume forms one of the most interesting and
fascinating subjects that can attract the notice of the observer.
The great insect-class presents us with the most familiar examples of
these changes, and the butterflies and moths exemplify metamorphosis
in its most typical aspect. Thus we know that from the egg of the
butterfly, deposited by the short-lived parent upon the leaves of
plants, a crawling grub-like creature is first developed. This form
we name the “larva” or “caterpillar”; and if we might fail to
recognize its relationship to the bright denizen of the air as far
as outward appearance is concerned, we might also be at a loss to
reconcile its internal structure with that of the perfect butterfly.
Thus the latter is winged; possesses a mouth and digestive system,
adapted for the reception and assimilation of flower-juices; and
wholly differs in structure and habits from its worm-like progeny.
The caterpillar is provided with a mouth furnished with jaws, and
adapted for biting or mastication; its digestive system presents
a type differing widely from that of the perfect form; and its
crawling, terrestrial habits appear in strong contrast to the light
and ethereal movements of its parent.
The life of this larva may be accurately described as one devoted
solely to its nourishment. Its entire existence, while in the
caterpillar state, is one long process of continuous eating and
devouring. By means of its jaws it nips and destroys the young leaves
of plants, much to the gardener’s annoyance; and so rapidly does
its body increase in size, that the first skin with which its body
is provided soon cracks and bursts like a tight-fitting coat, and a
process of moulting ensues. As the result of this process the larva
emerges, clad in a new skin, adapted to the increased size of its
body. This second skin may similarly become inadequate to accommodate
its ever-increasing growth, and a second process of moulting produces
in turn a new investment. In this way the caterpillar may change
its coat many times--twenty-one moultings have been counted in the
development of the Mayflies--and on arriving at the close of its
larval stage of existence, may present a very great increase in
size, as compared with the dimensions it presented at the beginning
of its life.
But, sooner or later, the caterpillar appears to sicken, and to
become quiescent. Its former state of activity is exchanged for one
of lethargy, from which it awakes to begin an operation of a novel
and different nature from that in which it has been previously
engaged. It begins to spin a delicate silky thread by means of a
special apparatus, situated in the head, and which consists of
silk-glands and of an organ named the “spinneret.” Within the silken
case or “cocoon” which it thus constructs with the thread of the
spinneret, the caterpillar-body is soon inclosed; the first stage of
its existence comes to an end; and the second or cocoon stage, marked
by outward quiescence and apparent rest, becomes known to us as that
of the “pupa,” “chrysalis,” or “nymph.”
Although outwardly still, and although all the former activity
appears to have been exchanged for a state of dull repose, changes
of active kind, and of marvelous extent, are meanwhile proceeding
within the cocoon or pupa-case. The elements of the caterpillar’s
form are being gradually disintegrated or broken down, and built
up anew in the form and image of the adult butterfly. Old textures
and garments are being exchanged for new ones; particle by particle
the outward and inward structures of the larva are being replaced
by others proper to the mature being; and in due course, and after
a longer or shorter period, the cocoon is ruptured, and the perfect
form emerges--a bright and beautiful creature, furnished with wings
and active senses, and rejoicing in the exercise of its new-born
functions amid the sunlight and the flowers.
Such is an outline of the familiar process by which the larva or
caterpillar of the butterfly becomes transformed or developed, to
form the “imago” or perfect and adult form. And if we review the
stages exemplified in the process, we shall be able to detect in
each an obvious harmony and correspondence both with the preceding
and with the succeeding stage. Thus we find that the life of the
perfect and mature insect is at the best of a comparatively short
and transient nature, and its energies are directed chiefly to
reproduction--to the deposition of eggs, from which new individuals
will, in due course, be reproduced. The larval stage, on the
contrary, is devoted to nutrition; to the laying up, as it were, of
a store of nourishment, sufficient to last throughout the lifetime
of the being, and to sustain it while its adult functions are being
performed.
Indeed, the entire lifetime of the higher insect may be divided into,
or comprised within, two distinct periods. The first of these latter
is the nutritive period, represented by the caterpillar-state, when
the nutrition of the body is mainly provided for; and the second
period, no less defined than the first, is included in the life of
the perfect form, which is devoted to reproducing the species. This
last we might therefore term the reproductive period of insect-life.
All insects, however, do not exemplify “metamorphosis” in so perfect
a manner as does the butterfly. The beetles, flies, bees, etc., and
many other insects, undergo a process of metamorphosis essentially
resembling that of the butterfly; the characteristic feature of
this form of development being that while the caterpillar stage
is passed in activity, the pupa or chrysalis is quiescent; and
from this resting-pupa the active, winged insect comes forth. The
dragon-flies, crickets, grasshoppers, and their allies, undergo, on
the other hand, a less perfect series of changes than the foregoing
insects. The young grasshopper, on leaving the egg, bears first
a close resemblance to the perfect insect. It is, further, not
of worm-like conformation, and in these two points differs from
the larva of the other forms. Then, thirdly, it does not inclose
itself in a cocoon-case, but passes its chrysalis stage in a free
and active condition. In this respect it again differs from the
butterfly chrysalis; and its perfect form is attained simply by the
development of the wings. So that, in reality, the chief difference
between the young and the perfect form of the grasshopper consists
in the non-development in the former of the wings, which are thus
characteristic of the adult form.
The dragon-flies illustrate an essentially similar kind of
metamorphosis, but also exemplify differences in the details of
their development. The young of the dragon-fly are active creatures,
inhabiting the water of pools; the eggs from which they are produced
having been deposited by the parent in bunches on the leaves of
water-plants. The larvæ are of brownish color, and possess six legs,
and a peculiar apparatus of jaws, consisting of a pair of nippers
attached to a movable, rod-like stem. This apparatus can be folded
upon the head, when it gives to the larva the appearance of being
masked, and hence the name of “mask” which has been applied to this
structure. But on the approach of some unwary insect, the jaws can
be rapidly extended to seize the unfortunate victim, and convey it
to the mouth of its captor. The dragon-fly’s young are thus purely
aquatic in habits, and propel themselves along by ejecting water,
which has been used in breathing, from the posterior extremity of the
body.
Having arrived at the close of its chrysalis-stage of
development--the chrysalis differing from the larva simply in its
greater size, and in the development of the wings and perfect body
within the pupa-skin--the insect at length fixes its body to some
water-plant. The pupa-skin next splits along the back, and the
mature, winged insect slowly emerges therefrom. The crumpled wings
soon dry, and acquire their normal consistence; and the dragon-fly,
freed from the trammels of a mundane existence, mounts into the air,
and “revels in the freedom of luxury and light.” Tennyson has aptly
described this change in his lines:
To-day I saw the dragon-fly
Come from the wells where he did lie.
An inner impulse rent the veil
Of his old husk: from head to tail
Came out clear plates of sapphire mail.
He dried his wings: like gauze they grew:
Thro’ crofts and pastures wet with dew
A living flash of light he flew.
In these latter instances, as in the case of the butterfly, the
nutrition of the insects has been proceeding during the earlier
stages of life, and has been fitting them for entering upon the
final part of their existence, which may extend for a longer or
shorter period, but which is mainly devoted to the continuation of
the species. The time occupied in the development of insects varies
greatly in different groups. Cold and damp appear to delay this
process. The chrysalis of a butterfly has been kept for two years
in an icehouse, without undergoing development; while on removal
to a warm place it became transformed into the winged insect. The
cockchafer occupies three years in its development, the duration of
life in its perfect state being probably only a single year.
STRUGGLE FOR EXISTENCE
--CHARLES DARWIN
Among organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be
called species or sub-species or varieties; what rank, for instance,
the two or three hundred doubtful forms of British plants are
entitled to hold, if the existence of any well-marked varieties be
admitted. But the mere existence of individual variability and of
some few well-marked varieties, though necessary as the foundation
for the work, helps us but little in understanding how species arise
in nature. How have all those exquisite adaptations of one part of
the organization to another part, and to the conditions of life,
and of one organic being to another being, been perfected? We see
these beautiful co-adaptations most plainly in the woodpecker and the
mistletoe; and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of a bird; in
the structure of the beetle which dives through the water; in the
plumed seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic
world.
Again, it may be asked, how is it that varieties, which I have
called incipient species, become ultimately converted into good and
distinct species, which in most cases obviously differ from each
other far more than do the varieties of the same species? How do
those groups of species which constitute what are called distinct
genera, and which differ from each other more than do the species of
the same genus, arise? All these results follow from the struggle for
life. Owing to this struggle, variations, however slight and from
whatever cause proceeding, if they be in any degree profitable to
the individuals of a species, in their infinitely complex relations
to other organic beings and to their physical conditions of life,
will tend to the preservation of such individuals, and will generally
be inherited by the offspring. The offspring, also, will thus have
a better chance of surviving, for, of the many individuals of any
species which are periodically born, but a small number can survive.
I have called this principle, by which each slight variation, if
useful, is preserved, by the term Natural Selection, in order to mark
its relation to man’s power of selection. But the expression often
used by Mr. Herbert Spencer of the Survival of the Fittest is more
accurate, and is sometimes equally convenient.
The elder De Candolle and Lyell have largely and philosophically
shown that all organic beings are exposed to severe competition. In
regard to plants, no one has treated this subject with more spirit
and ability than W. Herbert, Dean of Manchester, evidently the result
of his great horticultural knowledge. Nothing is easier than to
admit in words the truth of the universal struggle for life, or more
difficult--at least I have found it so--than constantly to bear this
conclusion in mind. Yet unless it be thoroughly ingrained in the
mind, the whole economy of nature, with every fact on distribution,
rarity, abundance, extinction, and variation, will be dimly seen
or quite misunderstood. We behold the face of nature bright with
gladness, we often see superabundance of food; we do not see or
we forget that the birds which are idly singing round us mostly
live on insects or seeds, and are thus constantly destroying life;
or we forget how largely these songsters, or their eggs, or their
nestlings, are destroyed by birds and beasts of prey; we do not
always bear in mind that, though food may be now superabundant, it is
not so at all seasons of each recurring year.
I should premise that I use this term Struggle for Existence in a
large and metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the life of
the individual, but success in leaving progeny. Two canine animals,
in a time of dearth, may be truly said to struggle with each other
which shall get food and live. But a plant on the edge of a desert is
said to struggle for life against the drought, though more properly
it should be said to be dependent on the moisture. A plant which
annually produces a thousand seeds, of which only one on an average
comes to maturity, may be more truly said to struggle with the
plants of the same and other kinds which already clothe the ground.
The mistletoe is dependent on the apple and a few other trees, but
can only in a far-fetched sense be said to struggle with these
trees, for, if too many of these parasites grow on the same tree, it
languishes and dies. But several seedling mistletoes, growing close
together on the same branch, may more truly be said to struggle with
each other. As the mistletoe is disseminated by birds, its existence
depends on them; and it may metaphorically be said to struggle with
other fruit-bearing plants, in tempting the birds to devour and thus
disseminate its seeds. In these several senses, which pass into each
other, I use for convenience’ sake the general term of Struggle for
Existence.
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season
or occasional year, otherwise, on the principle of geometrical
increase, its numbers would quickly become so inordinately great that
no country could support the product. Hence, as more individuals
are produced than can possibly survive, there must in every case be
a struggle for existence, either one individual with another of the
same species, or with the individuals of distinct species.