all our indigenous 'flowers,' are, as a rule, fertilized by means of
insects, and it is amazing to see in what diverse ways, often highly
specialized, they have adapted themselves to the visits of insects.
Thus there are flowers in which the nectar lies open to view, and
these can be feasted on by all manner of insects; there are others in
which the nectar is rather more concealed, but still easily found, and
reached by insects with short mouth-parts, e.g. large flowers blooming
by day and bearing much pollen, like the Magnolias. These have been
called beetle-flowers, because they are visited especially by the
honey-loving Longicorns.
Other flowers blooming by day are especially adapted to fertilization
by means of bees; they are always beautifully coloured, often blue;
they are fragrant, and contain nectar deep down in the flower, where
it can only be reached by the comparatively long proboscis of the
bee. Different arrangements in the different flowers secure that
the bee cannot enjoy the nectar without at the same time effecting
the cross-pollination. Thus the stamens of the meadow sage (_Salvia
pratensis_) are at first hidden within the helmet-shaped upper lip of
the flower (Fig. 41, _st´_), but bear lower down on their stalk a short
handle-like process, which turns the pollen-bearing anther downwards
(_st´´_) as soon as it is pressed back by an intruding insect. The
pollen-sacs then strike downwards on the back of the bee, and cover it
with pollen. When the bee visits another more mature flower, the long
style, which was at first hidden within the helmet, has bent downwards
(_gr´´_), and now stands just in front of the entrance to the flower,
so that the bee must rub off a part of the pollen covering its back on
to the stigma, and fertilization is thus effected.
There are other flowers which are specially disposed to suit
the visits of the humble-bees, as, for instance, _Pedicularis
asplenifolia_, the fern-leaved louse-wort, a plant of the high Alps
(Fig. 42). The first thing that strikes us about this plant is the
thickly tufted hair covering on the calyx (_k_), which serves to keep
off little wingless insects from the flower; then there is the strange
left-sided twisting of the individual flowers, whose under lip allows
only a strong insect like the humble-bee to gain access, towards the
left, to the corolla-tube (_kr_), in the depths of which the nectar is
concealed. While the humble-bee is sucking up the nectar it becomes
dusted over with pollen from the anthers, which falls to dust at a
touch, and when it insinuates itself into a second flower its powdered
back comes first into contact with the stigma of the pistil (_gr_)
which projects from the elongated bill-shaped under lip, dusting it
over with the pollen of the first visited flower. Butterflies and
smaller bees cannot rob this flower; it is strictly a humble-bee's
flower.
[Illustration: FIG. 42. Alpine Lousewort (_Pedicularis asplenifolia_).
_A_, flower seen from the left side, enlarged three times; the arrows
show the path by which the humble-bee enters. _B_, the same flower,
seen from the left, after removal of the calyx, the lower lip and the
left half of the upper lip. _C_, ovary (_ov_), nectary (_n_), and base
of style. _D_, tip of style, bearing the stigma. _E_, two anthers
turned towards one another. _o_, upper lip. _u_, lower lip. _gr_,
style. _st_, anthers. _kr_, corolla-tube. _k_, calyx.]
There are not a few of such flowers adapted to a very restricted
circle of visitors, and in all of them we find contrivances which
close the entrance to all except what we may call the welcome insects;
sometimes there are cushions of bristles which prevent little insects
from creeping up from below, or it is the oblique position of the
flower which prevents their getting in from the stem; sometimes it is
the length and narrowness of the corolla-tube, or the deep and hidden
situation of the nectar, which only allows intelligent insects to find
the treasure.
Very remarkable are those flowers which are adapted to the visits of
flies, for they correspond in several respects to the peculiarities
of these insects. In the first place, flies are fond of decaying
substances and the odours given off by these, and so the flowers which
depend for their cross-fertilization on flies have taken on the dull
and ugly colours of decay, and give out a disagreeable smell. But flies
are also shy and restless, turning now hither, now thither, and cannot
be reckoned among the 'constant' insect visitors, that is to say, they
do not persistently visit the same species; it is, therefore, evident
that they might easily carry away the pollen without any useful result
ensuing. Moreover, their intelligence is of a low order, and they do
not seek nectar with the perseverance shown by bees and humble-bees. It
is not surprising, therefore, to find that many of the flowers adapted
for the visits of flies are so constructed that they detain their
visitors until they have done their duty, that is to say, until they
have effected, or at least begun, the process of cross-pollination.
[Illustration: FIG. 43. Flower of Birthwort (_Aristolochia clematitis_)
cut in half. _A_, before pollination by small flies. _b_, the bristles.
_B_, after pollination. _P_, pollen mass. _N_, stigma, _b_, the
bristles. _b´_, their remains. After H. Müller.]
[Illustration: FIG. 44. Alpine Butterwort (_Pinguicula alpina_). _A_,
section through the flower. _K_, calyx. _bh_, bristly prominences.
_sp_, spur. _st_, stamen. _n_, stigma. _B_, stigma and stamen more
magnified. After H. Müller.]
Our birthwort (_Aristolochia clematitis_) and the Cuckoo-pint
(_Arum maculatum_) are pit-fall flowers, whose long corolla-tubes
have an enlargement at the base, in which both pistil and stamens
are contained. In the birthwort (Fig. 43) the narrow entrance-tube
is thickly beset with stiff hairs (_A_, _b_), whose points are
all directed towards the base. Little flies can creep down quite
comfortably into the basal expansion, but once there they are kept
imprisoned until the flower, in consequence of the pollination of
the stigma, begins to wither, the first parts to go being these very
bristles (_B_, _b´_), whose points, like a fish-weir, prevented the
flies from creeping out. Other 'fly-flowers,' as for instance the
Alpine butterwort (_Pinguicula alpina_) (Fig. 44), securely imprison
the plump fly as soon as it has succeeded in forcing itself in far
enough to reach, with its short proboscis, the nectar contained in the
spur (_sp_) of the corolla. The backward-directed bristles hold it fast
for some time, and it is only by hard pressing with the back against
the anthers (_st_) lying above it, and against the stigma (_n_), that
it ultimately succeeds in getting free, but it never does so without
having either loaded itself with pollen, or rubbed off on the stigma
the pollen it brought with it from another similar flower. The Alpine
butterwort is protogynous, that is to say, the pistil ripens first,
the pollen later, so that the possibility of self-fertilization is
altogether excluded.
It would be impossible to give even an approximate idea of the
diversity of the contrivances for securing fertilization in flowers
without spending many hours over them, for they are different in
almost every flower, often widely so, and even in species of the same
genus they are by no means always alike; for not infrequently one
species is adapted to one circle of visitors, and its near relative
to another. Thus the flower of the common Daphne (_Daphne mezereum_)
(Fig. 45, _A_ and _C_) is adapted to the visits of butterflies, bees,
and hover-flies, while its nearest relative (_Daphne striata_) (Fig.
45, _B_ and _D_) has a somewhat narrower and longer corolla-tube, so
that only butterflies can feast upon it. This example shows that there
are exclusively 'butterfly flowers,' but specialization goes further,
for there are flowers adapted to diurnal and others to nocturnal
Lepidoptera. The former have usually bright, often red colours, and
a pleasant aromatic fragrance, and in all of them the nectar lies at
the bottom of a very narrow corolla-tube. To this class belong, for
instance, the species of pink, many orchids, such as _Orchis ustulata_,
and _Nigritella angustifolia_ of the Alps, which smells strongly of
vanilla; also the beautiful campion (_Lychnis diurna_) and the Alpine
primrose (_Primula farinosa_). The flowers adapted to nocturnal
Lepidoptera are characterized by pale, often white colour, and a strong
and pleasant smell, which only begins to stream out after sunset, and
indeed many of these flowers are quite closed by day. This is the case
with the large, white, scentless bindweed (_Convolvulus sepium_), which
is chiefly visited and fertilized by the largest of our hawk-moths
(_Sphinx convolvuli_). The pale soapwort (_Saponaria officinalis_)
exhales a delicate fragrance which attracts the Sphingidæ from afar,
and the sweet smell of the honeysuckle (_Lonicera periclymenum_) is
well known, and has the same effect; an arbour of honeysuckle often
attracts whole companies of our most beautiful Sphingidæ and Noctuidæ
on warm June nights, to the great delight of the moth-collecting youth.
[Illustration: FIG. 45. _Daphne mezereum_ (_A_ and _C_) and Daphne
striata (_B_ and _D_). The former visited by butterflies, bees, and
flies, the latter by butterflies only. _A_ and _B_, vertical sections
of the flowers. _St_, stamens. _Gr_, style. _n_, nectary. _C_ and _D_,
flowers seen from above. After H. Müller.]
I cannot conclude this account of flower-adaptations without
considering the orchids somewhat more in detail, for it is among
them that we find the most far-reaching adaptations to the visits of
insects. Among them, too, great diversity prevails, as is evident
from the fact that Darwin devoted a whole book to the arrangements
for fertilization in orchids, but the main features are very much the
same in the majority. Figure 46 gives a representation of one of our
commonest species (_Orchis mascula_), A shows the flower in side view,
_B_ as it appears from in front. The flower seems as it were to float
on the end of the stalk (_st_), stretching out horizontally the spur
(_sp_) which contains the nectar. Between the large, broad under lip
(_U_), marked with a honey-guide (_sm_), and offering a convenient
alighting surface, and the broad, cushion-like stigma (_n_) lies the
entrance to the spur. Fertilization occurs in the following way:--The
fly or bee, when it is in the act of pushing its proboscis into the
nectar-containing spur, knocks with its head against the so-called
rostellum (_r_), a little beak-like process at the base of the stamens
(_p_). The pollen masses are of very peculiar construction, not falling
to dust, but forming little stalked clubs, with the pollen grains glued
together, and so arranged that they spring off when the rostellum is
touched and attach themselves to the head of the insect, as at _D_ on
the pencil (Fig. 46). When the bee has sucked up the nectar out of the
spur, and then proceeds to penetrate into another flower of the same
species, the pollinia have bent downwards on its forehead (_E_), and
must unfailingly come in contact with the stigma of the second flower,
to which they now remain attached, and effect its fertilization. What
a long chain of purposeful arrangements in a single flower, and no
interpretation of them is available except through natural selection!
[Illustration: FIG. 46. Common Orchis (_Orchis mascula_). _A_, flower
in side view. _st_, stalk. _sp_, spur with the nectary (_n_). _ei_,
entrance to the spur. _U_, lower lip. _B_, flower from in front. _p_,
pollinia. _Sm_, honey-guide. _ei_, entrance to the nectar. _na_,
stigma. _r_, rostellum. _U_, lower lip. _C_, vertical section through
the rostellum (_r_), pollinium (_p_). _ei_, entrance. _D_, the pollinia
removed and standing erect on the tip of a lead-pencil. _E_, the same,
somewhat later, curved downwards.]
And how diversely are these again modified in the different genera
and species of orchids, of which one is adapted to the visits of
butterflies exclusively, as _Orchis ustulata_, another to those of
bees, as _Orchis morio_, and a third to those of flies, as _Ophrys
muscifera_. These flowers are adapted to insect visits in the minutest
details of the form of the petals, which are smooth, as if polished
with wax, where insects are not intended to creep, but velvety or hairy
where the path leads to the nectar, and at the same time to the pollen
and the stigma. And then there is the diversity in the form and colour
of the 'honey-guides' on the 'alighting surface,' that is, the under
lip of the flower, upon which the insect sits and holds fast, while it
pushes its head as far as possible into the spur, so that its proboscis
may reach the nectar lying deep within it! Even though we cannot
pretend to guess at the significance of every curve and colour-spot in
one of the great tropical orchids, such as _Stanhopea tigrina_, yet we
may believe, with Sprengel, that all this has its significance, or has
had it for the ancestors of the plant in question, and in fact that
the flower is made up of nothing but adaptations, either actual or
inherited from its ancestors, although sometimes perhaps no longer of
functional importance.
So far, then, we have illustrated the fact that there are hundreds and
thousands of contrivances in flowers adapted solely to the visits of
insects and to securing cross-fertilization, and these adaptations go
so far that we might almost believe them to be the outcome of the most
exact calculation and the most ingenious reflection. But they all admit
of interpretation through natural selection, for all these details,
which used to be looked upon as merely ornamental, are directly or
indirectly of use to the species; directly, when, for instance, they
concern the dusting of the insect with the pollen; indirectly, when
they are a means of attracting visits.
Moreover, the evidence of the operation of the processes of selection
becomes absolutely convincing when we consider that, as in symbiosis,
there are always two sets of adaptations taking place independently
of one another--those of the flowers to the visits of the insects,
and those of the insects to the habit of visiting the flowers. To
understand this clearly we must turn our attention to the insects, and
try to see in what way they have been changed by adapting themselves to
the diet which the flowers afford.
As is well known, several orders of insects possess mouth-parts which
are suited for sucking up fluids, and these have evolved, through
adaptation to a fluid diet, from the biting mouth-parts of the
primitive insects which we see still surviving in several orders.
Thus the Diptera may have gradually acquired the sucking proboscis
which occurs in many of them by licking up decaying vegetable and
animal matter, and by piercing into and sucking living animals. But
even among the Diptera several families have more recently adapted
themselves quite specially to a flower diet, to honey-sucking, like the
hover-flies, the Syrphidæ, and the Bombyliidæ, whose long thin proboscis
penetrates deep into narrow corolla-tubes, and is able to suck up the
nectar from the very bottom. The transformation was not so important
in this case, since the already existing sucking apparatus only
required to be a little altered.
Again, in the order Hemiptera (Bugs) the suctorial proboscis does not
owe its origin to a diet of flowers, for no member of the group is now
adapted to that mode of obtaining food.
[Illustration: FIG. 47. Head of a Butterfly. _A_, seen from in front.
_au_, eyes. _la_, upper lip. _md_, rudiments of the mandibles. _pm_,
rudimentary maxillary palps. _mx´_, the first maxillæ modified into the
suctorial proboscis. _pl_, palps of labium or second maxillæ, cut off
at the root, remaining in _B_--which is a side view. _at_, antennæ.
Adapted from Savigny.]
The proboscis of the Lepidoptera, on the other hand, depends entirely
on adaptation to honey-sucking, and we may go the length of saying that
the order of Lepidoptera would not exist if there were no flowers.
This large and diverse insect-group is probably descended from the
ancestors of the modern caddis-flies or Phryganidæ, whose weakly
developed jaws were chiefly used for licking up the sugary juices of
plants. But as flowering plants evolved the licking apparatus of the
primitive butterflies developed more and more into a sucking organ, and
was ultimately transformed into the long, spirally coiled suctorial
proboscis as we see it in the modern butterflies (Fig. 47). It has
taken some pains to trace this organ back to the biting mouth-parts of
the primitive insects, for nearly everything about it has degenerated
and become stunted except the maxillæ (_mx´_). Even the palps (_pm_)
of these have become so small and inconspicuous in most of the
Lepidoptera that it is only quite recently that remains of them have
been recognized in a minute protuberance among the hairs. The mandibles
(_md_) have quite degenerated, and even the under lip has disappeared,
and only its palps are well developed (_B_, _pl_). But the first
maxillæ (_mx´_), although very strong and long, are so extraordinarily
altered in shape and structure that they diverge from the maxillæ of
all other insects. They have become hollow, probe-like half-tubes,
which fit together exactly, and thus form a closed sucking-tube of most
complex construction, composed of many very small joints, after the
fashion of a chain-saw, which are all moved by little muscles, and are
subject to the will through nerves, and are also furnished with tactile
and taste papillæ. Except this remarkable sucking proboscis there are
no peculiarities in the body of the butterfly which might be regarded
as adaptations to flower-visiting, with a few isolated exceptions, of
which one will be mentioned later. This is intelligible enough, for
the butterfly has nothing more to seek from the flower beyond food for
itself; it does not carry stores for offspring.
The bees, however, do this, and accordingly we find that in them the
adaptations to flower-visiting are not confined to the mouth-parts.
As far as we can judge now, the flower-visiting bees are descended
from insects which resembled the modern burrowing-wasps. Among these
the females themselves live on nectar and pollen, and build cells in
holes in the ground, and feed their brood. They do not feed them on
honey, however, but on animals--on caterpillars, grasshoppers, and
other insects, which they kill by a sting in the abdomen, or often only
paralyse, so that the victim is brought into the cells of the nest
alive but defenceless, and remains alive until the young larva of the
wasp, which emerges from the egg, sets to work to devour it.
[Illustration: FIG. 48. Mouth-parts of the Cockroach (_Periplaneta
orientalis_), after R. Hertwig. _la_, upper lip or labrum. _md_,
mandibles. _mx_^1, first maxillæ, with _c_, cardo, _st_, stipes, _li_,
internal lobe or lacinia, _le_, external lobe or galea, and _pm_, the
maxillary palp. _mx_^2, the labium or second maxillæ, with similar
detailed parts.]
Before I go on to explain the origin of the sucking proboscis of the
bee from the biting mouth-parts of the primitive insects I must first
briefly consider the latter.
The biting mouth-parts of beetles, Neuroptera, and Orthoptera (Fig.
48), consist of three pairs of jaws, of which the first, the mandibles
(_md_), are simply powerful pincers for seizing and tearing or chewing
the food. They have no part in the development of the suctorial
apparatus either in bees or in butterflies, so they may be left out
of account. The two other pairs of jaws, the first and second maxillæ
(_mx_^1 and _mx_^2), are constructed exactly on the same type, having
a jointed basal portion (_st_) bearing two lobes, an external (_le_)
and an internal (_li_), and a feeler or palp, usually with several
joints, directed outwards from the lobes (_pm_ and _pl_). The second
pair of maxillæ (_mx_^2) differs from the first chiefly in this, that
the components of the pair meet in the median line of the body, and
fuse more or less to form the so-called 'under lip' or labium. In the
example given, the cockroach (_Periplaneta orientalis_), this fusion
is only partial, the lobes having remained separate (_le_ and _li_);
and the same is true of the bee, but in this case the inner lobes have
grown into a long worm-like process which is thrust into the nectar in
the act of sucking.
[Illustration: FIG. 49. Head of the Bee. _Au_, compound eyes. _au_,
ocelli. _at_, antennæ. _la_, upper lip. _md_, mandibles. _mx_^1, first
maxillæ, with _pm_, the rudimentary maxillary palp. _mx_^2, second
maxillæ with the internal lobes (_li_) fused to form the 'tongue.'
_le_, the external lobes of the second maxillæ, known as 'paraglossæ.'
_pl_, labial palp.]
Even the burrowing-wasps exhibit the beginnings of variation in this
direction, for the under lip is somewhat lengthened and modified into
a licking organ. The adaptation has not gone much further than this,
even in one of the true flower-bees, _Prosopis_, which feeds its larvæ
with pollen and honey, and it is only in the true honey-bee that the
adaptation is complete (Fig. 49). Here the so-called 'inner lobe' of
the under lip (_li_) has elongated into the worm-shaped process already
mentioned; it is thickly covered with short bristles, and is called
the 'tongue' of the bee (_li_). The outer lobes of the under lip have
degenerated into little leaf-like organs, the so-called accessory
tongue or paraglossa (_le_), while the palps of the under lip (_pl_)
have elongated to correspond with the tongue, and serve as a sensitive
and probably also as a smelling organ, in contrast to the palps of the
first maxillæ, which have shrunk to minute stumps (_pm_). The whole
of the under lip, which has elongated even in its basal portions,
forms, with the equally long first maxillæ, the proboscis of the bee.
The first maxillæ are sheath-like half-tubes, closely apposed around
the tongue, and form along with it the suctorial tube, through which
the nectar is sucked up. Thus, of the three pairs of jaws in insects,
only the first pair, the mandibles, have remained unaltered, obviously
because the bee requires a biting-organ for eating pollen, for kneading
wax, and for building cells.
But bees do not only feast on nectar and pollen themselves, they carry
these home as food for their larvæ. The form already mentioned,
_Prosopis_, takes up pollen and nectar in its mouth, and afterwards
disgorges the pulp as food for its larvæ, but the rest of the true
bees have special and much more effective collecting-organs, either a
thick covering of hair on the abdomen, or along the whole length of the
posterior legs, or finally, a highly developed collecting apparatus,
such as that possessed by the honey-bee--the basket and brush on the
hind leg. The former is a hollow on the outer surface of the tibia, the
latter a considerable enlargement of the basal tarsal joint, which, at
the same time, is covered on the inner surface with short bristles,
arranged in transverse rows like a brush. The bee kneads the pollen
into the basket, and one can often see bees flying back to the hive
with a thick yellow ball of pollen on the hind leg. In those bees which
collect on the abdomen, like _Osmia_ and _Megachile_, the pollen mass
forms a thick clump on the belly, and in the case of _Andrena_ Sprengel
observed long ago that it sometimes flew with a packet of pollen bigger
than its own body on the hind leg.
All these are contrivances which have gradually originated through the
habit of carrying home pollen for the helpless larvæ shut up in the
cells. They have developed differently in the various groups of bees,
probably because the primary variations with which the process of
selection began were different in the various ancestral forms.