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Chapter XI
THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.
Insects visit the flowers of the same species as long as they can.
Cause of this habit.
Means by which bees recognise the flowers of the same species.
Sudden secretion of nectar.
Nectar of certain flowers unattractive to certain insects.
Industry of bees, and the number of flowers visited within a short time.
Perforation of the corolla by bees.
Skill shown in the operation.
Hive-bees profit by the holes made by humble-bees.
Effects of habit.
The motive for perforating flowers to save time.
Flowers growing in crowded masses chiefly perforated.
Bees and various other insects must be directed by instinct to search
flowers for nectar and pollen, as they act in this manner without
instruction as soon as they emerge from the pupa state. Their instincts,
however, are not of a specialised nature, for they visit many exotic
flowers as readily as the endemic kinds, and they often search for
nectar in flowers which do not secrete any; and they may be seen
attempting to suck it out of nectaries of such length that it cannot be
reached by them. (11/1. See, on this subject Hermann Muller
'Befruchtung' etc. page 427; and Sir J. Lubbock's 'British Wild Flowers'
etc. page 20. Muller 'Bienen Zeitung' June 1876 page 119, assigns good
reasons for his belief that bees and many other Hymenoptera have
inherited from some early nectar-sucking progenitor greater skill in
robbing flowers than that which is displayed by insects belonging to the
other Orders.) All kinds of bees and certain other insects usually visit
the flowers of the same species as long as they can, before going to
another species. This fact was observed by Aristotle with respect to the
hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper
published in 1736 in the Philosophical Transactions. It may be observed
by any one, both with hive and humble-bees, in every flower-garden; not
that the habit is invariably followed. Mr. Bennett watched for several
hours many plants of Lamium album, L. purpureum, and another Labiate
plant, Nepeta glechoma, all growing mingled together on a bank near some
hives; and he found that each bee confined its visits to the same
species. (11/2. 'Nature' 1874 June 4 page 92.) The pollen of these three
plants differs in colour, so that he was able to test his observations
by examining that which adhered to the bodies of the captured bees, and
he found one kind on each bee.
Humble and hive-bees are good botanists, for they know that varieties
may differ widely in the colour of their flowers and yet belong to the
same species. I have repeatedly seen humble-bees flying straight from a
plant of the ordinary red Dictamnus fraxinella to a white variety; from
one to another very differently coloured variety of Delphinium consolida
and of Primula veris; from a dark purple to a bright yellow variety of
Viola tricolor; and with two species of Papaver, from one variety to
another which differed much in colour; but in this latter case some of
the bees flew indifferently to either species, although passing by other
genera, and thus acted as if the two species were merely varieties.
Hermann Muller also has seen hive-bees flying from flower to flower of
Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and
repens; and even from blue hyacinths to blue violets. (11/3. 'Bienen
Zeitung' July 1876 page 183.)
Some species of Diptera or flies keep to the flowers of the same species
with almost as much regularity as do bees; and when captured they are
found covered with pollen. I have seen Rhingia rostrata acting in this
manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici
sepium. Volucella plumosa and Empis cheiroptera flew straight from
flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved
in the same manner with Potentilla tormentilla; and other Diptera with
Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica
hederaefolia and chamoedrys; but some flies visited indifferently the
flowers of these two latter species. I have seen more than once a minute
Thrips, with pollen adhering to its body, fly from one flower to another
of the same kind; and one was observed by me crawling about within a
convolvulus with four grains of pollen adhering to its head, which were
deposited on the stigma.
Fabricius and Sprengel state that when flies have once entered the
flowers of Aristolochia they never escape,--a statement which I could
not believe, as in this case the insects would not aid in the
cross-fertilisation of the plant; and this statement has now been shown
by Hildebrand to be erroneous. As the spathes of Arum maculatum are
furnished with filaments apparently adapted to prevent the exit of
insects, they resemble in this respect the flowers of Aristolochia; and
on examining several spathes, from thirty to sixty minute Diptera
belonging to three species were found in some of them; and many of these
insects were lying dead at the bottom, as if they had been permanently
entrapped. In order to discover whether the living ones could escape and
carry pollen to another plant, I tied in the spring of 1842 a fine
muslin bag tightly round a spathe; and on returning in an hour's time
several little flies were crawling about on the inner surface of the
bag. I then gathered a spathe and breathed hard into it; several flies
soon crawled out, and all without exception were dusted with arum
pollen. These flies quickly flew away, and I distinctly saw three of
them fly to another plant about a yard off; they alighted on the inner
or concave surface of the spathe, and suddenly flew down into the
flower. I then opened this flower, and although not a single anther had
burst, several grains of pollen were lying at the bottom, which must
have been brought from another plant by one of these flies or by some
other insect. In another flower little flies were crawling about, and I
saw them leave pollen on the stigmas.
I do not know whether Lepidoptera generally keep to the flowers of the
same species; but I once observed many minute moths (I believe Lampronia
(Tinea) calthella) apparently eating the pollen of Mercurialis annua,
and they had the whole front of their bodies covered with pollen. I then
went to a female plant some yards off, and saw in the course of fifteen
minutes three of these moths alight on the stigmas. Lepidoptera are
probably often induced to frequent the flowers of the same species,
whenever these are provided with a long and narrow nectary, as in this
case other insects cannot suck the nectar, which will thus be preserved
for those having an elongated proboscis. No doubt the Yucca moth visits
only the flowers whence its name is derived, for a most wonderful
instinct guides this moth to place pollen on the stigma, so that the
ovules may be developed on which the larvae feed. (11/4. Described by
Mr. Riley in the 'American Naturalist' volume 7 October 1873.)With
respect to Coleoptera, I have seen Meligethes covered with pollen flying
from flower to flower of the same species; and this must often occur,
as, according to M. Brisout, 'many of the species affect only one kind
of plant." (11/5. As quoted in 'American Nat.' May 1873 page 270.)
It must not be supposed from these several statements that insects
strictly confine their visits to the same species. They often visit
other species when only a few plants of the same kind grow near
together. In a flower-garden containing some plants of Oenothera, the
pollen of which can easily be recognised, I found not only single grains
but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum,
and Linaria. Other kinds of pollen were likewise detected in these same
flowers. A large number of the stigmas of a plant of Thyme, in which the
anthers were completely aborted, were examined; and these stigmas,
though scarcely larger than a split needle, were covered not only with
pollen of Thyme brought from other plants by the bees, but with several
other kinds of pollen.
That insects should visit the flowers of the same species as long as
they can, is of great importance to the plant, as it favours the
cross-fertilisation of distinct individuals of the same species; but no
one will suppose that insects act in this manner for the good of the
plant. The cause probably lies in insects being thus enabled to work
quicker; they have just learnt how to stand in the best position on the
flower, and how far and in what direction to insert their proboscides.
(11/6. Since these remarks were written, I find that Hermann Muller has
come to almost exactly the same conclusion with respect to the cause of
insects frequenting as long as they can the flowers of the same species:
'Bienen Zeitung' July 1876 page 182.) They act on the same principle as
does an artificer who has to make half-a-dozen engines, and who saves
time by making consecutively each wheel and part for all of them.
Insects, or at least bees, seem much influenced by habit in all their
manifold operations; and we shall presently see that this holds good in
their felonious practice of biting holes through the corolla.
It is a curious question how bees recognise the flowers of the same
species. That the coloured corolla is the chief guide cannot be doubted.
On a fine day, when hive-bees were incessantly visiting the little blue
flowers of Lobelia erinus, I cut off all the petals of some, and only
the lower striped petals of others, and these flowers were not once
again sucked by the bees, although some actually crawled over them. The
removal of the two little upper petals alone made no difference in their
visits. Mr. J. Anderson likewise states that when he removed the
corollas of the Calceolaria, bees never visited the flowers. (11/7.
'Gardeners' Chronicle' 1853 page 534. Kurr cut off the nectaries from a
large number of flowers of several species, and found that the greater
number yielded seeds; but insects probably would not perceive the loss
of the nectary until they had inserted their proboscides into the holes
thus formed, and in doing so would fertilise the flowers. He also
removed the whole corolla from a considerable number of flowers, and
these likewise yielded seeds. Flowers which are self-fertile would
naturally produce seeds under these circumstances; but I am greatly
surprised that Delphinium consolida, as well as another species of
Delphinium, and Viola tricolor, should have produced a fair supply of
seeds when thus treated; but it does not appear that he compared the
number of the seeds thus produced with those yielded by unmutilated
flowers left to the free access of insects: 'Bedeutung der Nektarien'
1833 pages 123-135.) On the other hand, in some large masses of Geranium
phaeum which had escaped out of a garden, I observed the unusual fact of
the flowers continuing to secrete an abundance of nectar after all the
petals had fallen off; and the flowers in this state were still visited
by humble-bees. But the bees might have learnt that these flowers with
all their petals lost were still worth visiting, by finding nectar in
those with only one or two lost. The colour alone of the corolla serves
as an approximate guide: thus I watched for some time humble-bees which
were visiting exclusively plants of the white-flowered Spiranthes
autumnalis, growing on short turf at a considerable distance apart; and
these bees often flew within a few inches of several other plants with
white flowers, and then without further examination passed onwards in
search of the Spiranthes. Again, many hive-bees which confined their
visits to the common ling (Calluna vulgaris), repeatedly flew towards
Erica tetralix, evidently attracted by the nearly similar tint of their
flowers, and then instantly passed on in search of the Calluna.
That the colour of the flower is not the sole guide, is clearly shown by
the six cases above given of bees which repeatedly passed in a direct
line from one variety to another of the same species, although they bore
very differently coloured flowers. I observed also bees flying in a
straight line from one clump of a yellow-flowered Oenothera to every
other clump of the same plant in the garden, without turning an inch
from their course to plants of Eschscholtzia and others with yellow
flowers which lay only a foot or two on either side. In these cases the
bees knew the position of each plant in the garden perfectly well, as we
may infer by the directness of their flight; so that they were guided by
experience and memory. But how did they discover at first that the above
varieties with differently coloured flowers belonged to the same
species? Improbable as it may appear, they seem, at least sometimes, to
recognise plants even from a distance by their general aspect, in the
same manner as we should do. On three occasions I observed humble-bees
flying in a perfectly straight line from a tall larkspur (Delphinium)
which was in full flower to another plant of the same species at the
distance of fifteen yards which had not as yet a single flower open, and
on which the buds showed only a faint tinge of blue. Here neither odour
nor the memory of former visits could have come into play, and the tinge
of blue was so faint that it could hardly have served as a guide. (11/8.
A fact mentioned by Hermann Muller 'Die Befruchtung' etc. page 347,
shows that bees possess acute powers of vision and discrimination; for
those engaged in collecting pollen from Primula elatior invariably
passed by the flowers of the long-styled form, in which the anthers are
seated low down in the tubular corolla. Yet the difference in aspect
between the long-styled and short-styled forms is extremely slight.)
The conspicuousness of the corolla does not suffice to induce repeated
visits from insects, unless nectar is at the same time secreted,
together perhaps with some odour emitted. I watched for a fortnight many
times daily a wall covered with Linaria cymbalaria in full flower, and
never saw a bee even looking at one. There was then a very hot day, and
suddenly many bees were industriously at work on the flowers. It appears
that a certain degree of heat is necessary for the secretion of nectar;
for I observed with Lobelia erinus that if the sun ceased to shine for
only half an hour, the visits of the bees slackened and soon ceased. An
analogous fact with respect to the sweet excretion from the stipules of
Vicia sativa has been already given. As in the case of the Linaria, so
with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some
species of Trifolium, I have watched the flowers day after day without
seeing a bee at work, and then suddenly all the flowers were visited by
many bees. Now how did so many bees discover at once that the flowers
were secreting nectar? I presume that it must have been by their odour;
and that as soon as a few bees began to suck the flowers, others of the
same and of different kinds observed the fact and profited by it. We
shall presently see, when we treat of the perforation of the corolla,
that bees are fully capable of profiting by the labour of other species.
Memory also comes into play, for, as already remarked, bees know the
position of each clump of flowers in a garden. I have repeatedly seen
them passing round a corner, but otherwise in as straight a line as
possible, from one plant of Fraxinella and of Linaria to another and
distant one of the same species; although, owing to the intervention of
other plants, the two were not in sight of each other.
It would appear that either the taste or the odour of the nectar of
certain flowers is unattractive to hive or to humble-bees, or to both;
for there seems no other reason why certain open flowers which secrete
nectar are not visited by them. The small quantity of nectar secreted by
some of these flowers can hardly be the cause of their neglect, as
hive-bees search eagerly for the minute drops on the glands on the
leaves of the Prunus laurocerasus. Even the bees from different hives
sometimes visit different kinds of flowers, as is said to be the case by
Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9.
'Gardeners' Chronicle' 1844 page 374.) I have known humble-bees to visit
the flowers of Lobelia fulgens in one garden and not in another at the
distance of only a few miles. The cupful of nectar in the labellum of
Epipactis latifolia is never touched by hive- or humble-bees, although I
have seen them flying close by; and yet the nectar has a pleasant taste
to us, and is habitually consumed by the common wasp. As far as I have
seen, wasps seek for nectar in this country only from the flowers of
this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10.
The same fact apparently holds good in Italy, for Delpino says that the
flowers of these three plants are alone visited by wasps: 'Nettarii
Estranuziali, Bulletino Entomologico' anno 6.), and Tritoma; the two
former plants being endemic, and the two latter exotic. As wasps are so
fond of sugar and of any sweet fluid, and as they do not disdain the
minute drops on the glands of Prunus laurocerasus, it is a strange fact
that they do not suck the nectar of many open flowers, which they could
do without the aid of a proboscis. Hive-bees visit the flowers of the
Symphoricarpus and Tritoma, and this makes it all the stranger that they
do not visit the flowers of the Epipactis, or, as far as I have seen,
those of the Scrophularia aquatica; although they do visit the flowers
of Scrophularia nodosa, at least in North America. (11/11. 'Silliman's
American Journal of Science' August 1871.)
The extraordinary industry of bees and the number of flowers which they
visit within a short time, so that each flower is visited repeatedly,
must greatly increase the chance of each receiving pollen from a
distinct plant. When the nectar is in any way hidden, bees cannot tell
without inserting their proboscides whether it has lately been exhausted
by other bees, and this, as remarked in a former chapter, forces them to
visit many more flowers than they otherwise would. But they endeavour to
lose as little time as they can; thus in flowers having several
nectaries, if they find one dry they do not try the others, but as I
have often observed, pass on to another flower. They work so
industriously and effectually, that even in the case of social plants,
of which hundreds of thousands grow together, as with the several kinds
of heath, every single flower is visited, of which evidence will
presently be given. They lose no time and fly very quickly from plant to
plant, but I do not know the rate at which hive-bees fly. Humble-bees
fly at the rate of ten miles an hour, as I was able to ascertain in the
case of the males from their curious habit of calling at certain fixed
points, which made it easy to measure the time taken in passing from one
place to another.
With respect to the number of flowers which bees visit in a given time,
I observed that in exactly one minute a humble-bee visited twenty-four
of the closed flowers of the Linaria cymbalaria; another bee visited in
the same time twenty-two flowers of the Symphoricarpus racemosa; and
another seventeen flowers on two plants of a Delphinium. In the course
of fifteen minutes a single flower on the summit of a plant of Oenothera
was visited eight times by several humble-bees, and I followed the last
of these bees, whilst it visited in the course of a few additional
minutes every plant of the same species in a large flower-garden. In
nineteen minutes every flower on a small plant of Nemophila insignis was
visited twice. In one minute six flowers of a Campanula were entered by
a pollen-collecting hive-bee; and bees when thus employed work slower
than when sucking nectar. Lastly, seven flower-stalks on a plant of
Dictamnus fraxinella were observed on the 15th of June 1841 during ten
minutes; they were visited by thirteen humble-bees each of which entered
many flowers. On the 22nd the same flower-stalks were visited within the
same time by eleven humble-bees. This plant bore altogether 280 flowers,
and from the above data, taking into consideration how late in the
evening humble-bees work, each flower must have been visited at least
thirty times daily, and the same flower keeps open during several days.
The frequency of the visits of bees is also sometimes shown by the
manner in which the petals are scratched by their hooked tarsi; I have
seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of
their flowers thus sadly defaced.
PERFORATION OF THE COROLLA BY BEES.
I have already alluded to bees biting holes in flowers for the sake of
obtaining the nectar. They often act in this manner, both with endemic
and exotic species, in many parts of Europe, in the United States, and
in the Himalaya; and therefore probably in all parts of the world. The
plants, the fertilisation of which actually depends on insects entering
the flowers, will fail to produce seed when their nectar is stolen from
the outside; and even with those species which are capable of
fertilising themselves without any aid, there can be no
cross-fertilisation, and this, as we know, is a serious evil in most
cases. The extent to which humble-bees carry on the practice of biting
holes is surprising: a remarkable case was observed by me near
Bournemouth, where there were formerly extensive heaths. I took a long
walk, and every now and then gathered a twig of Erica tetralix, and when
I had got a handful all the flowers were examined through a lens. This
process was repeated many times; but though many hundreds were examined,
I did not succeed in finding a single flower which had not been
perforated. Humble-bees were at the time sucking the flowers through
these perforations. On the following day a large number of flowers were
examined on another heath with the same result, but here hive-bees were
sucking through the holes. This case is all the more remarkable, as the
innumerable holes had been made within a fortnight, for before that time
I saw the bees everywhere sucking in the proper manner at the mouths of
the corolla. In an extensive flower-garden some large beds of Salvia
grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower
perforated, and many scores were examined. I have seen whole fields of
red clover (Trifolium pratense) in the same state. Dr. Ogle found that
90 per cent of the flowers of Salvia glutinosa had been bitten. In the
United States Mr. Bailey says it is difficult to find a blossom of the
native Gerardia pedicularia without a hole in it; and Mr. Gentry, in
speaking of the introduced Wistaria sinensis, says "that nearly every
flower had been perforated." (11/12. Dr. Ogle 'Pop. Science Review' July
1869 page 267. Bailey 'American Naturalist' November 1873 page 690.
Gentry ibid May 1875 page 264.)
As far as I have seen, it is always humble-bees which first bite the
holes, and they are well fitted for the work by possessing powerful
mandibles; but hive-bees afterwards profit by the holes thus made. Dr.
Hermann Muller, however, writes to me that hive-bees sometimes bite
holes through the flowers of Erica tetralix. No insects except bees,
with the single exception of wasps in the case of Tritoma, have sense
enough, as far as I have observed, to profit by the holes already made.
Even humble-bees do not always discover that it would be advantageous to
them to perforate certain flowers. There is an abundant supply of nectar
in the nectary of Tropaeolum tricolor, yet I have found this plant
untouched in more than one garden, while the flowers of other plants had
been extensively perforated; but a few years ago Sir J. Lubbock's
gardener assured me that he had seen humble-bees boring through the
nectary of this Tropaeolum. Muller has observed humble-bees trying to
suck at the mouths of the flowers of Primula elatior and of an
Aquilegia, and, failing in their attempts, they made holes through the
corolla; but they often bite holes, although they could with very little
more trouble obtain the nectar in a legitimate manner by the mouth of
the corolla.
Dr. W. Ogle has communicated to me a curious case. He gathered in
Switzerland 100 flower-stems of the common blue variety of the monkshood
(Aconitum napellus), and not a single flower was perforated; he then
gathered 100 stems of a white variety growing close by, and every one of
the open flowers had been perforated. (11/13. Dr. Ogle 'Popular Science
Review' July 1869 page 267. Bailey 'American Naturalist' November 1873
page 690. Gentry ibid May 1875 page 264.) This surprising difference in
the state of the flowers may be attributed with much probability to the
blue variety being distasteful to bees, from the presence of the acrid
matter which is so general in the Ranunculaceae, and to its absence in
the white variety in correlation with the loss of the blue tint.
According to Sprengel, this plant is strongly proterandrous (11/14. 'Das
Entdeckte' etc. page 278.); it would therefore be more or less sterile
unless bees carried pollen from the younger to the older flowers.
Consequently the white variety, the flowers of which were always bitten
instead of being properly entered by the bees, would fail to yield the
full number of seeds and would be a comparatively rare plant, as Dr.
Ogle informs me was the case.
Bees show much skill in their manner of working, for they always make
their holes from the outside close to the spot where the nectar lies
hidden within the corolla. All the flowers in a large bed of Stachys
coccinea had either one or two slits made on the upper side of the
corolla near the base. The flowers of a Mirabilis and of Salvia coccinea
were perforated in the same manner; whilst those of Salvia grahami, in
which the calyx is much elongated, had both the calyx and the corolla
invariably perforated. The flowers of Pentstemon argutus are broader
than those of the plants just named, and two holes alongside each other
had here always been made just above the calyx. In these several cases
the perforations were on the upper side, but in Antirrhinum majus one or
two holes had been made on the lower side, close to the little
protuberance which represents the nectary, and therefore directly in
front of and close to the spot where the nectar is secreted.
But the most remarkable case of skill and judgment known to me, is that
of the perforation of the flowers of Lathyrus sylvestris, as described
by my son Francis. (11/15. 'Nature' January 8, 1874 page 189.) The
nectar in this plant is enclosed within a tube, formed by the united
stamens, which surround the pistil so closely that a bee is forced to
insert its proboscis outside the tube; but two natural rounded passages
or orifices are left in the tube near the base, in order that the nectar
may be reached by the bees. Now my son found in sixteen out of
twenty-four flowers on this plant, and in eleven out of sixteen of those
on the cultivated everlasting pea, which is either a variety of the same
species or a closely allied one, that the left passage was larger than
the right one. And here comes the remarkable point,--the humble-bees
bite holes through the standard-petal, and they always operated on the
left side over the passage, which is generally the larger of the two. My
son remarks: "It is difficult to say how the bees could have acquired
this habit. Whether they discovered the inequality in the size of the
nectar-holes in sucking the flowers in the proper way, and then utilised
this knowledge in determining where to gnaw the hole; or whether they
found out the best situation by biting through the standard at various
points, and afterwards remembered its situation in visiting other
flowers. But in either case they show a remarkable power of making use
of what they have learnt by experience." It seems probable that bees owe
their skill in biting holes through flowers of all kinds to their having
long practised the instinct of moulding cells and pots of wax, or of
enlarging their old cocoons with tubes of wax; for they are thus
compelled to work on the inside and outside of the same object.
In the early part of the summer of 1857 I was led to observe during some
weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus),
whilst attending to the fertilisation of this plant, and daily saw
humble- and hive-bees sucking at the mouths of the flowers. But one day
I found several humble-bees employed in cutting holes in flower after
flower; and on the next day every single hive-bee, without exception,
instead of alighting on the left wing-petal and sucking the flower in
the proper manner, flew straight without the least hesitation to the
calyx, and sucked through the holes which had been made only the day
before by the humble-bees; and they continued this habit for many
following days. (11/16. 'Gardeners' Chronicle' 1857 page 725.) Mr. Belt
has communicated to me (July 28th, 1874) a similar case, with the sole
difference that less than half of the flowers had been perforated by the
humble-bees; nevertheless, all the hive-bees gave up sucking at the
mouths of the flowers and visited exclusively the bitten ones. Now how
did the hive-bees find out so quickly that holes had been made? Instinct
seems to be out of the question, as the plant is an exotic. The holes
cannot be seen by bees whilst standing on the wing-petals, where they
had always previously alighted. From the ease with which bees were
deceived when the petals of Lobelia erinus were cut off, it was clear
that in this case they were not guided to the nectar by its smell; and
it may be doubted whether they were attracted to the holes in the
flowers of the Phaseolus by the odour emitted from them. Did they
perceive the holes by the sense of touch in their proboscides, whilst
sucking the flowers in the proper manner, and then reason that it would
save them time to alight on the outside of the flowers and use the
holes? This seems almost too abstruse an act of reason for bees; and it
is more probable that they saw the humble-bees at work, and
understanding what they were about, imitated them and took advantage of
the shorter path to the nectar. Even with animals high in the scale,
such as monkeys, we should be surprised at hearing that all the
individuals of one species within the space of twenty-four hours
understood an act performed by a distinct species, and profited by it.
I have repeatedly observed with various kinds of flowers that all the
hive and humble-bees which were sucking through the perforations, flew
to them, whether on the upper or under side of the corolla, without the
least hesitation; and this shows how quickly all the individuals within
the district had acquired the same knowledge. Yet habit comes into play
to a certain extent, as in so many of the other operations of bees. Dr.
Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus
multiflorus that certain individuals went exclusively to the
perforations, while others of the same species visited only the mouths
of the flowers. (11/17. Dr. Ogle 'Pop. Science Review' April 1870 page
167. Mr. Farrer 'Annals and Magazine of Natural History' 4th series
volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861
exactly the same fact with Trifolium pratense. So persistent is the
force of habit, that when a bee which is visiting perforated flowers
comes to one which has not been bitten, it does not go to the mouth, but
instantly flies away in search of another bitten flower. Nevertheless, I
once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it
entered the mouths of some flowers and cut holes into the others. Dr.
Hermann Muller informs me that in the same district he has seen some
individuals of Bombus mastrucatus boring through the calyx and corolla
of Rhinanthus alecterolophus, and others through the corolla alone.
Different species of bees may, however, sometimes be observed acting
differently at the same time on the same plant. I have seen hive-bees
sucking at the mouths of the flowers of the common bean; humble-bees of
one kind sucking through holes bitten in the calyx, and humble-bees of
another kind sucking the little drops of fluid excreted by the stipules.
Mr. Beal of Michigan informs me that the flowers of the Missouri currant
(Ribes aureum) abound with nectar, so that children often suck them; and
he saw hive-bees sucking through holes made by a bird, the oriole, and
at the same time humble-bees sucking in the proper manner at the mouths
of the flowers. (11/18. The flowers of the Ribes are however sometimes
perforated by humble-bees, and Mr. Bundy says that they were able to
bite through and rob seven flowers of their honey in a minute: 'American
Naturalist' 1876 page 238.) This statement about the oriole calls to
mind what I have before said of certain species of humming-birds boring
holes through the flowers of the Brugmansia, whilst other species
entered by the mouth.
The motive which impels bees to gnaw holes through the corolla seems to
be the saving of time, for they lose much time in climbing into and out
of large flowers, and in forcing their heads into closed ones. They were
able to visit nearly twice as many flowers, as far as I could judge, of
a Stachys and Pentstemon by alighting on the upper surface of the
corolla and sucking through the cut holes, than by entering in the
proper way. Nevertheless each bee before it has had much practice, must
lose some time in making each new perforation, especially when the
perforation has to be made through both calyx and corolla. This action
therefore implies foresight, of which faculty we have abundant evidence
in their building operations; and may we not further believe that some
trace of their social instinct, that is, of working for the good of
other members of the community, may here likewise play a part?
Many years ago I was struck with the fact that humble-bees as a general
rule perforate flowers only when these grow in large numbers near
together. In a garden where there were some very large beds of Stachys
coccinea and of Pentstemon argutus, every single flower was perforated,
but I found two plants of the former species growing quite separate with
their petals much scratched, showing that they had been frequently
visited by bees, and yet not a single flower was perforated. I found
also a separate plant of the Pentstemon, and saw bees entering the mouth
of the corolla, and not a single flower had been perforated. In the
following year (1842) I visited the same garden several times: on the
19th of July humble-bees were sucking the flowers of Stachys coccinea
and Salvia grahami in the proper manner, and none of the corollas were
perforated. On the 7th of August all the flowers were perforated, even
those on some few plants of the Salvia which grew at a little distance
from the great bed. On the 21st of August only a few flowers on the
summits of the spikes of both species remained fresh, and not one of
these was now bored. Again, in my own garden every plant in several rows
of the common bean had many flowers perforated; but I found three plants
in separate parts of the garden which had sprung up accidentally, and
these had not a single flower perforated. General Strachey formerly saw
many perforated flowers in a garden in the Himalaya, and he wrote to the
owner to inquire whether this relation between the plants growing
crowded and their perforation by the bees there held good, and was
answered in the affirmative. Hence it follows that the red clover
(Trifolium pratense) and the common bean when cultivated in great masses
in fields,--that Erica tetralix growing in large numbers on
heaths,--rows of the scarlet kidney-bean in the kitchen-garden,--and
masses of any species in the flower-garden,--are all eminently liable to
be perforated.
The explanation of this fact is not difficult. Flowers growing in large
numbers afford a rich booty to the bees, and are conspicuous from a
distance. They are consequently visited by crowds of these insects, and
I once counted between twenty and thirty bees flying about a bed of
Pentstemon. They are thus stimulated to work quickly by rivalry, and,
what is much more important, they find a large proportion of the
flowers, as suggested by my son, with their nectaries sucked dry.
(11/19. 'Nature' January 8, 1874 page 189.) They thus waste much time in
searching many empty flowers, and are led to bite the holes, so as to
find out as quickly as possible whether there is any nectar present, and
if so, to obtain it.
Flowers which are partially or wholly sterile unless visited by insects
in the proper manner, such as those of most species of Salvia, of
Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less
completely to produce seeds if the bees confine their visits to the
perforations. The perforated flowers of those species, which are capable
of fertilising themselves, will yield only self-fertilised seeds, and
the seedlings will in consequence be less vigorous. Therefore all plants
must suffer in some degree when bees obtain their nectar in a felonious
manner by biting holes through the corolla; and many species, it might
be thought, would thus be exterminated. But here, as is so general
throughout nature, there is a tendency towards a restored equilibrium.
If a plant suffers from being perforated, fewer individuals will be
reared, and if its nectar is highly important to the bees, these in
their turn will suffer and decrease in number; but, what is much more
effective, as soon as the plant becomes somewhat rare so as not to grow
in crowded masses, the bees will no longer be stimulated to gnaw holes
in the flowers, but will enter them in a legitimate manner. More seed
will then be produced, and the seedlings being the product of
cross-fertilisation will be vigorous, so that the species will tend to
increase in number, to be again checked, as soon as the plant again
grows in crowded masses.
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