SOME INTER-RELATIONS OF PLANTS AND ANIMALS
The most important and fundamental difference between the animal and
plant worlds is this: plants possess the power of manufacturing their
food out of the inorganic materials of which it is composed, while
animals cannot do this. Give an ordinary plant access to water with a
pinch of mineral salts in it, to the air, and to sunlight, and by the
agency of chlorophyll--the green colouring-matter of the leaves--the
miracle will be accomplished, and dead materials transformed into living
substance. Animals, on the other hand, are dependent for their
food-supply on organic material--that is, on either plant or animal
substances; and since they cannot live by taking in each other’s
washing--in other words, by eating each other--it follows that the
animal world is dependent on the plant world for its continued
existence. A porpoise may live on herrings, herrings on small fry, fry
in turn on minuter organisms, and so on down the scale; but their
ultimate source of food is the tiny Algæ which swarm in the water--the
_Plankton_ in Hensen’s original sense--which, alone in this chain, can
build up their bodies out of the sea and air. That these minute plants
can sustain the enormous drain upon them due to their use as a
food-supply by myriads of larger organisms is due to their vast numbers
and rapid increase. Sea-water favourable for plankton life may contain
several millions of individuals in every litre (about 1-3/4 pints);
while as a fair estimate for the seas which surround our own islands “at
least one” organism for every drop has been suggested.[6]
In the great abysses of the ocean, where vegetable life is absent, the
strange creatures which live there in utter darkness prey upon others,
and they again on others which belong to lesser depths, the ultimate
source of life being again the minute surface organisms which,
possessing chlorophyll, can make organic out of inorganic substances by
the energy obtained from sunlight. Thus only is life made possible in
the green hells of the sea
Where fallen skies and evil hues and eyeless creatures be.
On the land, the dependence of animals on plants is in large measure
direct, as the supply of vegetable food is abundant and widespread. The
largest land animals are all vegetable feeders; so are the majority of
our own native mammals, and in a great measure our birds; while most of
the creatures upon which the flesh-eating animals prey are themselves
vegetable feeders. The distribution of land animals over the globe is
thus dependent in large measure on the distribution of plants. On
account of the profusion and variety of plant life, and the fact that
most vegetable feeders can thrive on various sorts of plants, few
animals are restricted in their range by the presence or absence of any
particular species or genus, but complete dependence of this sort is by
no means unknown. The larvæ of some Butterflies, for instance, eat the
leaves of one plant only; the Peacock (_Vanessa io_) and the Small
Tortoiseshell (_V. urticæ_) are cases in point. The caterpillars of both
these species feed exclusively on the Common Nettle (_Urtica dioica_).
Should the efforts of farmers and gardeners succeed in exterminating
this unwelcome plant, these two butterflies would disappear from the
Earth. Sometimes absolute mutual dependence is found on both the animal
and vegetable sides. The American _Yucca filamentosa_, often grown in
our gardens, depends solely on the little moth _Pronuba yuccasella_ for
its pollination, just as the insect is absolutely dependent on the plant
(see p. 80), and other species of Yucca have each its particular
dependent moth, which feeds on no other plant, and whose flowers are
pollinated by no other.
Apart from such special cases, the general dependence of animals upon
plants is obvious, and is by no means confined to food-supply. Animals
of all grades, from human beings to Caddis Worms, construct houses of
vegetable materials; trees are the chosen home of large sections of our
fauna, and the herbs of the field are the world for millions of tiny
beings.
There’s never a leaf or a blade too mean
To be some happy creature’s palace.
Turning to the other side of the picture, no such general dependence of
the plant world upon the animal world is found, but the inter-relations
of the two are many and varied, and in the absence of animals of one
kind or another whole groups of plants would become extinct. The cases
where plants derive their food-supply wholly from animals are indeed
rare, save near the bottom of the vegetable scale, and most of such
parasites are minute; one of the most noticeable in our own country is
the fungus _Cordyceps militaris_, which may be found growing on the dead
bodies of larvæ or pupæ which it has killed--a little scarlet,
club-shaped plant, about an inch in height. But some of the most highly
organized plants obtain _portions_ of their food-supply from animal
sources. Mention has already been made of the Sundews (_Drosera_),
Butterworts (_Pinguicula_), and Bladderworts (_Utricularia_), which
capture live insects, etc., by means of sensitive organs (as in the
first two cases) or ingenious traps (as in the last), and subsequently
digest them, and they will be dealt with later on (p. 186). Then there
is the Venus’ Fly-trap (_Dionæa_) and the well-known Pitcher Plants
(_Nepenthes_), which actively, as in the former case, or passively, as
in the latter, catch insects and digest them, by means of leaves
modified in very extraordinary ways. In all these instances the
advantage lies entirely on the side of the plant, just as in the case of
most of the plant-eating animals the advantage is wholly with the
animal. But in a large number of instances--many of them of a most
interesting nature--the inter-relations are such as to benefit both the
actors, each obtaining from the other what is useful to it. One of the
most conspicuous and widespread relationships of this kind is that
prevailing between flowers and insects, the insect receiving food in the
form of nectar, and at the same time carrying pollen from flower to
flower, without which transfer no fertile seed would be formed. To this
interchange of favours we shall return later (p. 81); meanwhile, it
will be well to consider a few of the cases in which the relationship
between plant and animal is continuous and more intimate, the two living
in very close relations to each other: to such cases the term
_symbiosis_ or “living together” is applied by naturalists. The
relations existing between certain trees and some species of ant are of
high interest, and illustrate well this phase of life. The Candelabra
Tree (_Cecropia peltata_) of the South American forests is liable to
attack by leaf-cutting ants (_Œcodoma_), which climb trees and bite off
thousands of leaves; these they cut up on the ground and carry to their
nests, where they form a basis for the growth of certain small fungi
which are a favourite food of the ants (compare the cultivation of
mushrooms as practised by gardeners). The Candelabra Tree protects
itself from these ravages by forming an alliance with another kind of
ant (_Azteca_). Along the hollow stems are little pits through which the
ants easily bore, and reach the convenient houses within, where they
live and bring up their young. At the base of the leaf-stalks, where the
greatest danger lies from the leaf-cutting ants, little tufts of hairs
are situated, among which are small white masses of nutritious material
much liked by the ants, and collected by them and stored within their
houses. So that these desirable trees are swarming with Aztec ants,
fierce little creatures--“it is one of the most bellicose ants that I
know, and its sting is most irritating,” writes Kerner--which congregate
especially at the leaf-stalks, the point of attack of the leaf-cutters.
The advantages of these arrangements to both the trees and the Aztec
ants are obvious.
A very remarkable instance of a different kind is supplied by the
relations existing between the American species of _Yucca_ and the small
white-winged moths of the genus _Pronuba_. The following succinct
account is given by Professor G. H. Carpenter:[7] “The female of these
moths has not only the palps of the first maxillæ developed, but the
region of the maxillæ (palpiger) whence they spring produced into a pair
of long, flexible, hairy processes. By means of these she collects from
the anthers pollen, which she deliberately carries to the stigma to
ensure fertilization. With her piercing ovipositor--a most abnormal
development among moths--she bores through the tissue of the pistil, and
by means of the flexible egg-tube, protrusible beyond the ovipositor,
lays her eggs close to the ovules of the _Yucca_. The caterpillar when
hatched feeds on the growing seed of the plant, which would never
develop were it not for the action of the _Pronuba_ moth. This action is
most wonderful, in that the moth herself gets no benefit from it. Her
food canal is degenerate, and her jaws, useless for sucking, are devoted
altogether to the gathering of the pollen; she does not feed in the
perfect state. Doubtless her ancestors did so, and were first attracted
to the _Yucca_ in search of honey, though the act of pollination is now
performed only for the sake of the offspring.”
Among certain lower animals and plants symbiotic connection is often
most intimate. For instance, in the body-wall of certain Sea Anemones
and Holothurians there are small green cells which were long believed
to be part of the animal, and which puzzled naturalists because they
contained chlorophyll, that remarkable green substance characteristic of
plants, which gives to them the power of forming food out of its raw
inorganic materials. These cells are now known to be minute seaweeds
(Algæ), which spend their lives in the animal tissues to the benefit of
both organisms. The plant, by virtue of its chlorophyll, absorbs carbon
dioxide, decomposes it, and gives out oxygen, which is eagerly seized on
by the animal. The animal in its turn liberates carbon dioxide, which is
required by the plant. Similar relations exist between Algæ and some of
the lowly Radiolarians and Foraminifera; in these cases, the animals
being very minute, the plant partner plays a more conspicuous rôle. It
is noteworthy that these Algæ are quite capable of living and
multiplying separately, free from the body of the animals, and the
animals also are capable of pursuing an independent existence.
Let us turn now to the relations existing between flowers and insects,
which form one of the most picturesque and romantic features of field
life, and of which the materials for study and observation are ever at
our own doors. What is a flower? A flower is a group of modified leaves
set apart for the business of sexual reproduction. The essential parts
or _sporophylls_ are of two kinds, which may be borne on the same flower
or on separate flowers on one plant, or on separate plants. These are
the _stamens_, bearing _pollen grains_ (or _microspores_), from which
_male cells_ arise; and _carpels_, which contain _ovules_, each
enclosing an _embryo sac_ or _megaspore_, in which is an _ovum_ or
_female cell_.
Each stamen consists usually of a slender stalk, the _filament_, bearing
an oblong head, the _anther_, which contains four chambers, or pollen
sacs, filled with pollen grains; these, when mature, escape into the air
by the rupturing of the walls of the chambers.
Each carpel contains in its lower part an ovary, while its upper part
presents to the air a surface charged with nutrient substance, the
_stigma_, which is often raised on a slender stalk, the _style_.
To secure the production of seed, the first necessary step is
_pollination_, or the transfer of pollen from the stamen to the stigma.
When this is effected--the means will be considered immediately--and a
pollen grain alights on the surface of the stigma, which is usually
sticky or hairy to aid its retention there, the pollen grain commences
growth, and sends out a slender tube (the _pollen tube_), which pursues
its way through the substance of the stigma, down the style, into the
ovary, and from its tip a male cell passes out and fuses with the ovum.
In most flowers the pollen tube is not called on to make any great
effort of growth, the distance between stigma and ovary being very
small; but occasionally, as in Crocus and Lily, this may amount to half
a foot. The result of this act of fertilization is that the ovum and
ovule grow, the former forming eventually the _embryo_, or young plant,
the latter the _seed_ in which the embryo is enclosed. In order that
fertile seed may be produced it is often necessary, and usually
desirable, that the pollen which reaches the stigma should not belong to
the same flower, but to a different flower of the same species;
_cross-pollination_ being the rule among seed plants, _self-pollination_
the exception. To secure the former, and to avoid the latter, many
highly interesting devices are found, materially affecting the structure
and development of flowers.
The _essential_ parts of a flower, then, consist of stamens and carpels.
Flowers consisting of no other parts but either or both of these are not
common, but we may compare, for example, the rarely produced flowers of
the Duckweeds (_Lemna_), in which a tiny group of two stamens and a
carpel represents one flower, or, according to some views, a group of
three flowers. More commonly the flower is much more composite,
consisting mostly of four sets of organs, arranged in whorls or rings,
or more rarely in close spirals. In the centre is a group of carpels;
outside them--in other words, slightly lower on the stem--a ring, or two
rings, of stamens, few or many; then a ring of _petals_, forming the
_corolla_, usually coloured, leaf-like, and conspicuous; and outside of
them a ring of _sepals_, forming the _calyx_, generally green and
leaf-like. The main function of the calyx is protective; it encloses the
essential organs and guards them till they are mature, when the flower
opens and stamen and stigma play their parts. The calyx is usually
tough, and often covered with hairs, or with a sticky substance, to keep
the flower safe and ward off the attacks of insects or other small
devourers. If we turn to the corolla we find a singular variety of size,
form, and colour. To account for this, it is necessary to consider the
means by which pollen is distributed. There are two chief ways in which
pollen is conveyed from flower to flower--by means of the wind, and by
means of flying insects. If we examine wind-pollinated flowers, such as
Hazel (_Corylus_), Scotch Fir (_Pinus_), or Reed-mace (_Typha_), we
note the small size of the flowers and the great abundance of pollen.
Compare these with insect-fertilized flowers, such as Buttercup
(_Ranunculus_), Flax (_Linum_), Snapdragon (_Antirrhinum_), or one of
the Orchids. In these the flowers are much larger owing to the increased
size of the petals, which are of brilliant colour and of various shape.
Pollen is mostly much reduced in quantity, since insects flying direct
from flower to flower afford a far more economical mode of distribution
than is offered by the wind. The pollen grains, moreover, are sticky and
covered with tiny spines or knobs, to render them more liable to adhere
to the body or head of an insect; the pollen grains of wind-fertilized
flowers being, on the other hand, smooth, dry, and dust-like. Again,
these insect-pollinated flowers usually possess little glands which
secrete nectar, the sugary syrup which by digestion in a bee’s body
becomes honey. Here, then, is the inter-relation established: the insect
helps the plant by carrying its pollen from flower to flower, and in its
turn is helped by the provision of delicious food. And what about the
showy petals, and the fragrance that so often marks these entomophilous
flowers? They are advertisements, designed to catch the attention of the
necessary insects as they fly about. Not only does the corolla by its
bright colour attract insects, but markings of various shapes and tints
upon the petals are generally held to be honey-guides--sign-posts
directing the insects to the nectar and to the pollen. These are
especially conspicuous in many of the irregular flowers to which
reference will be made shortly, in which the insects are encouraged to
approach the flowers in a particular way. An example
[Illustration: FIG. 14.--FLOWER OF ERODIUM PETRÆUM. 2/1.]
of such markings, as seen in the genus Erodium, is shown in Fig. 14. It
is interesting to note the various ways in which flowers render
themselves conspicuous in order to attract insects. In the majority of
Seed Plants, such as the Buttercup, Pea, Rose, Foxglove, it is the
corolla, formed either of separate petals, as in the first three, or of
petals fused together, as in the last, which by its bright colour or
colours renders the flower noticeable. In other species the calyx takes
on the function of advertisement, the corolla being in comparison
insignificant--we may study examples of this in the Anemones,
Hellebores, and Marsh Marigold (_Caltha palustris_). It is worth
examining this last, to see how its coloured _sepals_ resemble and
fulfil the same function as the _petals_ of its cousins the Buttercups.
Or, again, sepals and petals may combine in showiness, both sets being
brightly coloured in one or more tints--compare the Columbine
(_Aquilegia_), Larkspur (_Delphinium_), Milkwort (_Polygala_), and the
marvellous flowers of Orchids. In the great group of the Monocotyledons,
indeed, to which
[Illustration: FIG. 15.--UMBEL OF ASTRANTIA CARNIOLICA, SHOWING
PETAL-LIKE RING OF COLOURED LEAVES (BRACTS). 1/1.]
the Orchids belong, sepals and petals usually combine in form and colour
to form one corolla-like envelope (then called a _perianth_). In many
other plant groups--for instance, the _Dipsacaceæ_ (such as the
Scabious), _Umbelliferæ_, and _Compositæ_--conspicuousness is obtained
by a grouping together of a large number of small flowers. In the Cow
Parsnep (_Heracleum Sphondylium_) the outer petal of the marginal
flowers of the large umbel is much enlarged, which enhances this effect.
In _Astrantia_, an interesting genus of _Umbelliferæ_, the bracts take
on the appearance of a ring of large petals, and surround the group of
small flowers (Fig. 15). The same thing may be noticed in the outer
blossoms of the close flower-head of the Field Scabious (_Knautia
arvensis_). In many _Compositæ_ the process is carried still farther; in
the Common Daisy (_Bellis perennis_) the outer flowers have each a long
strap-shaped expansion of the corolla, which is of a different colour
(white) from that of the corollas of the inner flowers, which are
yellow. In the Dandelion (_Taraxacum officinale_) all the flowers have a
yellow strap-shaped corolla. In the Guelder Rose (_Viburnum Opulus_) the
outer flowers are entirely devoted to advertisement, consisting each of
a big white corolla, while only the small inner flowers possess stamens
and pistil and are capable of producing the brilliant scarlet berries.
In a cultivated form of this, commonly called the Snowball Tree, the
advertisement flowers only are present, forming a globe of white
blossom, and no fruit is produced in consequence. The Dwarf Cornel
(_Cornus suecica_), a little Dogwood growing on many Scottish moors,
bears what looks like a white flower with a purple centre. On
examination it is seen that the four white petal-like structures are
really foliage-leaves, which have taken on the duty of advertising the
group of small purple blossoms which they enclose (Fig. 16). A similar
and very gorgeous effect is produced in several Spurges often seen in
greenhouses, such as _Euphorbia fulgens_, _E. splendens_, and _E.
punicea_; in these the upper foliage-leaves are large and coloured
brilliant scarlet, the flowers which accompany them being quite small.
These aggregations of flowers with their flaunting flags are in general
an invitation to all comers; the nectar in the blossoms lies open to
every hungry insect, and pollination is effected in a rather promiscuous
and messy way; not only flying insects--bees, butterflies, beetles, and
flies of many sorts--but also ants and other creatures which creep up
the stems from the ground, assemble for the feast, and incidentally
transfer from flower to flower pollen which may adhere to their bodies.
[Illustration: FIG. 16.--DWARF CORNEL (CORNUS SUECICA), 2/3, AND SINGLE
FLOWER ENLARGED.]
In a large number of flowers such general feasting is discountenanced,
insect traffic is regulated, the visits of insects of little or no
service to the plants is discouraged, and special arrangements are made
to attract and minister to the needs of those insects whose visits are
of most benefit. Except where flowers are borne in clusters, creeping
creatures like ants are of no service; for in the course of the journey
“by land” from one flower to another, there is a strong probability of
any pollen which the insect may be carrying being rubbed off before the
next blossom is reached; small flying insects are likewise frequently
useless. In many plants the visits of such pedestrians and small fry is
very distinctly discouraged. Of different devices which serve this end,
the most conspicuous and effective include barriers to the passage of
stem-climbers, and devices in the flower preventive of the visits of
unwelcome guests. We may take a few instances from among British plants,
which the reader may with a little diligence find and study for himself.
Several members of the Pink family (_Caryophyllaceæ_) produce a sticky
secretion which is a very effectual bar to the passage of small walking
animals. In the English Catchfly (_Silene anglica_), Night-flowering
Catchfly (_S. noctiflora_) and the Nottingham Catchfly (_S. nutans_),
hairs are present all over the leaves and stems, from the tips of which
a gummy substance exudes, which is a fatal trap for small insects.
Kerner, in his interesting book, “Flowers and their Unbidden Guests,”
states that on the sticky stems of the last, in the Tyrol, he identified
the remains of sixty different kinds of insects--ants, ichneumons,
beetles, bugs, flies, and so on. The Red German Campion (_Lychnis
Viscaria_) has an extremely sticky ring below each joint of the stem and
inflorescence, which is most fatal to any creature which attempts to
climb to the flowers. Other instances, such as the Petunia or Moss Rose,
will occur to the reader. Another familiar kind of barrier is the
presence on the calyx or involucre of a palisade of stiff hairs or
prickles, such as may be studied in the Thistles; in some plants a
downward-pointing ring of stiff hairs at each joint serves the same
purpose. In the Japanese Wineberry (_Rubus phœnicolasius_), often grown
in gardens, the calyx, like the stem, is densely clothed with bright red
slender spines (Fig. 17). It opens to allow the inconspicuous petals to
expand, and then closes again and resumes its protective rôle till the
scarlet fruit approaches maturity.
[Illustration: FIG. 17.--JAPANESE WINEBERRY (RUBUS PHŒNICOLASIUS).
Leaf and panicle, 2/3; flower after pollination; ripe fruit, both
slightly enlarged.]
But it is in the flower itself that we find the most ingenious
arrangements to encourage useful and discourage useless visitors, to
assist the former to pollinate the flower, and while offering nectar to
the welcome guest to deny it to the unwelcome. The first stage in this
specialization is that the flower, instead of having its axis vertical,
and facing the sky, is turned on its side by the curving of its stalk,
and looks out horizontally. The effect of this is to cause a flying
insect on approaching the flower to alight in a particular
position--namely, on the lowest petal. Following on the adoption of this
attitude the next stage in development is seen in the parts of the
flower beginning to alter their shape and position relative to each
other and often also their colour. Thus, beginning with a quite regular
flower, we can arrange a series showing more and more asymmetry. The
tendency is generally for the lowest petal to become enlarged and often
conspicuously marked, providing a broad, convenient platform on which
insects may alight, while the remainder form walls and roof, protecting
the important parts within and by their shape, which is often narrowed
and tubular behind, barring access to all but chosen visitors. To find a
full series illustrating these transformations we do not need to go to
plants widely separated in their affinities. In the Buttercup order
(_Ranunculaceæ_) alone every gradation may be found. The flowers of the
Buttercups themselves are upright and quite regular. In the Larkspur
(_Delphinium_) the flower is turned on its side, and a puzzling
combination of coloured sepals and petals--five bright blue unequal
sepals and a single large purplish petal of peculiar shape with a long
hollow spur behind--produces a quite irregular blossom. The process is
carried farther again in the Monkshood (_Aconitum_), in whose well-known
blue flower the sepals and petals combine to produce a strikingly
irregular blossom, with the upper sepal arching over into a great hood
protecting the rest of the flower. In such irregular flowers the
essential parts--the pollen-producing and pollen-receiving portions, or
stamens and stigma--also alter their position and form, and are so
placed that an insect, visiting the flower to obtain nectar (which is
generally stored at the back, well out of the way), must of necessity
receive pollen on its body, and probably deposit pollen on the stigma.
To describe the variety and ingenuity of these devices as found in
different flowers might well occupy several chapters, and only one or
two examples can be quoted here; familiar wild flowers are chosen, and
the reader should examine them for himself to understand their
structure. In the well-known Pea type, one great petal arches over the
flower; two narrow ones stand one on either side; the remaining two
stand on edge below, with their margins in contact, enclosing the
stamens and pistil. An insect visiting the flower alights naturally on
the _keel_ or pair of lower petals. Pressed down by its weight, these
open, often with a sudden movement like bursting, and dust the insect
with pollen. Compare also the flowers of the Snapdragons (_Antirrhinum_)
and Toadflaxes (_Linaria_), in which the upper and lower lips of the
corolla meet like a closed mouth, which can be forced open only by a
strong insect like a bee, and is safe from predatory visits of smaller
fry (Fig. 18). In the Sages (_Salvia_) the corolla is tubular at the
base; there is a large lobed lip on which visiting insects alight, and a
hooded roof above arching over the stamens and pistil, which are placed
close against it, overhanging the entrance to the corolla-tube, at the
base of which the nectar is stored. The stamens, only two of which are
developed, have each a hinge near the top, the part above the hinge
being like a curved rod supported near its middle. These two curved rods
stand normally in a vertical position, so that their lower ends partly
block the entrance to the tube; the pollen is borne at their upper ends.
Should a bee insert its head down the tube in search of nectar, it
pushes the lower ends of the hinged rods upwards, with the result that
their upper ends swing downward against the bee’s back, dusting it with
pollen just at that part of its body which, if the bee should visit a
rather older flower, would come in contact with the stigma, the slender
stalk of which (the _style_) increases in length during the period of
flowering, and is in consequence the more liable to be encountered.
[Illustration: FIG. 18.--FLOWER AND FRUIT OF LINARIA PURPUREA. 2/1.]
Only one more instance can be referred to, which can be tested by the
reader any summer day wherever any of our native Orchids grow. In
these, the most highly specialized of all plant groups as regards
pollination by insects, the general arrangement of the flower is often
somewhat similar in a general sense to the last case; but here the
sepals and petals which between them form the platform, tube, sides, and
roof of the flower, are all separate and often differently and
elaborately coloured. The essential organs are greatly modified and
hardly recognizable at first. There is only one stamen, producing two
clusters of pollen, which are embedded in the roof of the flower. Each
possesses a slender stalk which terminates in a little sticky disc which
projects from the general surface. The pollen grains are held together
in a mass by fine threads, and the whole with its stalk--the
_pollinium_--resembles a lemonade bottle in shape. The stigma is also
embedded, forming a sticky surface in the roof of the flower behind the
stamen. When an insect inserts its head into the flower, its forehead
comes in contact with the sticky ends of the pollinia, which adhere, so
that on leaving the flower the insect flies away with the pollen
sticking to its forehead like two little horns. And now a remarkable
thing happens. The stalks of the pollinia, drying rapidly in the air,
contract unequally, and become curved, so that the pollinia bend forward
into a horizontal position. When the insect visits another flower and
thrusts in its head, the pollen consequently comes in contact with the
sticky stigmatic surface farther down the tube, and cross-pollination is
effected.
In the cases of many of these highly specialized flowers, one is no less
struck with the perfection of the arrangements made for preventing
self-pollination, than those adapted to securing cross-pollination. But
in a few, on the contrary, self-pollination is specially arranged for.
It must be pointed out that the insects which pollinate these
specialized flowers have in many cases acquired modifications in their
structure corresponding to the modifications in the flowers which they
frequent. In the more specialized forms, indeed, plant and animal have
become entirely dependent on each other; the plants would become extinct
in the absence of the special insects through whose agency they are able
by pollination to produce fertile seed; and the insects would likewise
die out if the flowers to whose nectar and pollen they look for food
were not available.
As regards the kinds of insects which visit flowers for food, these are
very numerous and belong to almost every section of that large class. In
many, such as Neuroptera, Orthoptera, Hemiptera, Coleoptera, there is
very little special adaptation for their flower-feeding habits, and
these insects visit flowers, such as the _Umbelliferæ_, in which the
nectar and pollen are freely exposed, and lie open to all. Many of the
Diptera, or Flies, are in the same case; but in some families, such as
the _Bombyliidæ_, high specialization for securing food from flowers is
found: the creatures are provided with elongated probosces for sucking
nectar even when it is deeply hidden, and no other food is used by the
insects in their adult stage. But it is among the long-tongued Bees and
the Lepidoptera (Butterflies and Moths) that the highest degree of
adaptation in this direction is found; and the modifications are
associated with those flowers which have become most highly specialized
for insect pollination, and most completely dependent on it. In the Bees
the legs have become much modified for the gathering of pollen, and the
mouth is a long flexible sucking-tube which when not in use is carried
rolled up in a spiral. The pollen, on which food alone the young bees
are fed, is gathered and stored among rows of hairs on the legs, and in
the more highly specialized forms it is wetted with honey so as to form
a compact mass, easily carried and easily removed when the nest is
reached. The balls of pollen thus formed are sometimes nearly the size
of the body of the bee, and may contain one to two hundred thousand
grains of pollen. The formation of the mouth is beautiful and
complicated, adapted to the rapid sucking up of nectar even if deeply
placed in the flower. The nectar is stored in the body of the bee, and
subsequently transferred to the waxen honey-cells in the hive. In the
Butterflies and Moths the mouth parts are also modified for sucking, and
as these insects do not build nests or take care of their offspring as
Bees do the mouth is formed solely for the purpose of securing the
nectar which is their only food. The proboscis varies greatly in length
in different groups, according to the kind of flower which they visit.
In the Owl Moths (_Noctuidæ_) it is sometimes only eight millimetres
(1/3 inch) long; in many of the Butterflies it is about half an inch. In
the Hawk-moths it attains a remarkable development, necessitated no
doubt by the habit of these insects of not alighting on or entering a
flower, but hovering in front of it as a Humming Bird does, and sucking
up the nectar while thus poised. The proboscis of the Convolvulus
Hawk-moth measures 65 to 80 millimetres (2-1/2 to 3-1/4 inches), and
some of the Tropical allies of this moth have probosces twice or even
three times that length. These species feed on the nectar of flowers
with tubular corolla of corresponding dimensions. Most of the Hawk-moths
feed only at dusk, and as the time is short they take advantage of their
powers of rapid flight to visit (and incidentally to pollinate) a very
large number of flowers in a short period. Moreover, in common with most
of the more specialized flower-feeding insects, they do not visit the
flowers of different species indiscriminately, but dash to blossom after
blossom of whatever single species they have selected. Hermann Müller
records watching Humming-bird Hawk-moths (_Macroglossa stellatarum_) at
work at the summit of the Albula Pass; one visited 106 flowers of _Viola
calcarata_ in under 4 minutes; another 194 blossoms of the same plant in
6-3/4 minutes.
The day-flying Butterflies display none of this restless energy. The
sunshine is pleasant and the day long. They wander aimlessly in their
beauty from flower to flower, sun themselves on the warm ground, or
“whirl through the air with the first good comrade that by chance
appears.” They are the flowers of the air, and our country rambles are
made more joyous by their careless companionship.