Charles Darwin.

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Gallinacese, and throws some light on the singular instinct of the ostrich.
In this family several hen birds unite and lay first a few eggs in one nest
and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs, but, as with the cuckoo, at intervals of two or three days. The instinct,
however, of the American ostrich, as in the case of the Molothrus bonarien-
sis, has not as yet been perfected; for a surprising number of eggs lie
strewed over the plains, so that in one day's hunting I picked up no less
than twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of other
kinds of bees. This case is more remarkable than that of the cuckoo; for
these bees have not only had their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the pollen-
collecting apparatus which would have been indispensable if they had
stored up food for their own young. Some species of Sphegidae (wasp-like
insects) are likewise parasitic; and M. Fabre has lately shown good reason
for believing that, although the Tachytes nigra generally makes its own
burrow and stores it with paralyzed prey for its own larvae, yet that, when
this insect finds a burrow already made and stored by another sphex, if
takes advantage of the prize, and becomes for the occasion parasitic. In this
case as with that of the Molothrus or cuckoo, I can see no difficulty in
natural selection making an occasional habit permanent, if of advantage to
the species, and if the insect whose nest and stored food are feloniously
appropriated, be not thus exterminated.


This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated father.
This ant is absolutely dependent on its slaves; without their aid, the species
would certainly become extinct in a single year. The males and fertile
females do no work of any kind, and the workers or sterile females, though
most energetic and courageous in capturing slaves, do no other work. They
are incapable of making their own nests, or of feeding their own larvae.
When the old nest is found inconvenient, and they have to migrate, it is
the slaves which determine the migration, and actually carry their masters
in their jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of food which they liked
best, and with their own larvae and pupae to stimulate them to work, they
did nothing; they could not even feed themselves, and many perished of
hunger. Huber then introduced a single slave (F. fusca), and she instantly
set to work, fed and saved the survivors; made some cells and tended the


larvae, and put all to rights. What can be more extraordinary than these
well-ascertained facts? If we had not known of any other slave-making ant,
it would have been hopeless to speculate how so wonderful an instinct
could have been perfected.

Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern parts
of England, and its habits have been attended to by Mr. F. Smith, of the
British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as any
one may well be excused for doubting the existence of so extraordinary an
instinct as that of making slaves. Hence, I will give the observations which
I made in some little detail. I opened fourteen nests of F. sanguinea, and
found a few slaves in all. Males and fertile females of the slave species
(F. fusca) are found only in their own proper communities, and have never
been observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in their appear-
ance is great. When the nest is slightly disturbed, the slaves occasionally
come out, and like their masters are much agitated and defend the nest:
when the nest is much disturbed, and the larvae and pupae are exposed, the
slaves work energetically together with their masters in carrying them away

; to a place of safety. Hence, it is clear that the slaves feel quite at home.
During the months of June and July, on three successive years, I watched
for many hours several nests in Surrey and Sussex, and never saw a slave
either leave or enter a nest. As, during these months, the slaves are very
few in number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June, and August, both in Surrey and Hamp-
shire, and has never seen the slaves, though present in large numbers in
August, either leave or enter the nest. Hence, he considers them as strictly
household slaves. The masters, on the other hand, may be constantly seen

! bringing in materials for the nest, and food of all kinds. During the year
i860, however, in the month of July, I came across a community with an
unusually large stock of slaves, and I observed a few slaves mingled with
their masters leaving the nest, and marching along the same road to a tall
Scotch fir-tree, twenty-five yards distant, which they ascended together,
probably in search of aphides or cocci. According to Huber, who had
ample opportunities for observation, the slaves in Switzerland habitually
work with their masters in making the nest, and they alone open and close
the doors in the morning and evening; and, as Huber expressly states, their
principal office is to search for aphides. This difference in the usual habits

i of the masters and slaves in the two countries, probably depends merely on
the slaves being captured in greater numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the mas-
ters carefully carrying their slaves in their jaws instead of being carried by


them, as in the case of F. rufescens. Another day my attention was struck j
by about a score of the slave-makers haunting the same spot, and evidently
not in search of food; they approached and were vigorously repulsed by
an independent community of the slave-species (F. fusca) ; sometimes as
many as three of these ants cHnging to the legs of the slave-making F.
sanguinea. The latter ruthlessly killed their small opponents and carried
their dead bodies as food to their nest, twenty-nine yards distant; but they
were prevented from getting any pupae to rear as slaves. I then dug up a -i
small parcel of the pupae of F. fusca from another nest, and put them down
on a bare spot near the place of combat; they were eagerly seized and
carried ofT by the tyrants, who perhaps fancied that, after all, they had been
victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae of ^
another species, F. flava, with a few of these little yellow ants still clinging
to the fragments of their nest. This species is sometimes, though rarely,
made into slaves, as has been described by Mr. Smith. Although so small a
species, it is very courageous, and I have seen it ferociously attack other
ants. In one instance I found to my surprise an independent community of
F. flava under a stone beneath a nest of the slave-making F. sanguinea; and
when I had accidentally disturbed both nests, the little ants attacked their
big neighbors with surprising courage. Now I was curious to ascertain
whether F. sanguinea could distinguish the pupae of F. fusca, which they
habitually make into slaves, from those of the little and furious F. flava,
which they rarely capture, and it was evident that they did at once dis-
tinguish them; for we have seen that they eagerly and instantly seize the
pupae of F. fusca, whereas they were much terrified when they came across
the pupae, or even the earth from the nest, of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little yellow
ants had crawled away, they took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying the
dead bodies of F. fusca (showing that it was not a migration) and numer- i
ous pupae. I traced a long file of ants burdened with booty, for about forty I
yards back, to a very thick clump of health, whence I saw the last individual
of F. sanguinea emerge, carrying a pupa; but I was not able to find the
desolated nest in the thick heath. The nest, however, must have been close
at hand, for two or three individuals of F. fusca were rushing about in the "
greatest agitation, and one was perched motionless with its own pupa in
its mouth on the top of a spray of heath, an image of despair over its
ravaged home.

Such are the facts, though they did not need confirmation by me, in i
regard to the wonderful instinct of making slaves. Let it be observed what
a contrast the instinctive habits of F. sanguinea present with those of the 3
continental F. rufescens. The latter does not build its own nest, does not t
determine its own migrations, does not collect food for itself or its young,
and cannot even feed itself: it is absolutely dependent on its numerous


slaves. Formica sanguinea, on the other hand, possesses much fewer slaves,
and in the early part of the summer extremely few: the masters determine
when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves seem
to have the exclusive care of the larvae, and the masters alone go on slave-
making expeditions. In Switzerland the slaves and masters work together,
making and bringing materials for the nest; both, but chiefly the slaves,
tend and milk, as it may be called, their aphides; and thus both collect
food for the community. In England the masters alone usually leave the
nest to collect building materials and food for themselves, their slaves and
larvae. So that the masters in this country receive much less service from
their slaves than they do in Switzerland.

, By what steps the instinct of F. sanguinea originated I will not pretend
ito conjecture. But as ants which are not slave-makers will, as I have seen,
carry off the pupae of other species, if scattered near their nests, it is possible
that such pupae originally stored as food might become developed; and
the foreign ants thus unintentionally reared would then follow their proper
instincts, and do what work they could. If their presence proved useful to
the species which had seized them — if it were more advantageous to this
species, to capture workers than to procreate them — the habit of collecting
pupae, originally for food, might by natural selection be strengthened and
rendered permanent for the very different purpose of raising slaves. When
the instinct was once acquired, if carried out to a much less extent even
than in our British F. sanguinea, which, as we have seen, is less aided by
'its slaves than the same species in Switzerland, natural selection might
increase and modify the instinct — always supposing each modification to be
of use to the species — until an ant was formed as abjectly dependent on
its slaves as is the Formica rufescens.


I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from mathe-
maticians that bees have practically solved a recondite problem, and have
made their cells of the proper shape to hold the greatest possible amount
of honey, with the least possible consumption of precious wax in their con-
struction. It has been remarked that a skilful workman with fitting tools
and measures, would find it very difficult to make cells of wax of the true
form, though this is effected by a crowd of bees working in a dark hive.
Granting whatever instincts you please, it seems at first quite inconceivable
how they can make all the necessary angles and planes, or even perceive
when they are correctly made. But the difficulty is not nearly so great as it
first appears: all this beautiful work can be shown, I think, to follow from
a few simple instincts.


I was led to investigate this subject by Mr. Waterhouse, who has shown ii
that the form of the cell stands in close relation to the presence of adjoining «
cells; and the following view may, perhaps, be considered only as a modifi-
cation of his theory. Let us look to the great principle of gradation, and see (
whether Nature does not reveal to us her method of work. At one end of a
short series we have humble-bees, which use their old cocoons to hold
honey, sometimes adding to them short tubes of wax, and likewise making
separate and very irregular rounded cells of wax. At the other end of the
series we have the cells of the hive-bee, placed in a double layer: each cell,
as is well known, is an hexagonal prism, with the basal edges of its six sides
bevelled so as to join an inverted pyramid, of three rhombs. These rhombs
have certain angles, and the three which form the pyramidal base of a
single cell on one side of the comb enter into the composition of the bases
of three adjoining cells on the opposite side. In the series between the
extreme perfection of the cells of the hive-bee and the simplicity of those
of the humble-bee we have the cells of the Mexican Melipona domestica,
carefully described and figured by Pierre Huber. The Melipona itself is
intermediate in structure between the hive and humble-bee, but more nearly
related to the latter; it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly spherical and of nearly
equal sizes, and are aggregated into an irregular mass. But the important
point to notice is, that these cells are always made at that degree of nearness
to each other that they would have intersected or broken into each other if
the spheres had been completed; but this is never permitted, the bees build-
ing perfectly flat walls of wax between the spheres which thus tend to in-
tersect. Hence, each cell consists of an outer spherical portion, and of two,
three, or more flat surfaces, according as the cell adjoins two, three, or
more other cells. When one cell rests on three other cells, which, from the
spheres being nearly of the same size, is very frequently and necessarily the
case, the three flat surfaces are united into a pyramid; and this pyramid,
as Huber has remarked, is manifestly a gross imitation of the three-sided
pyramidal base of the cell of the hive-bee. As in the cells of the hive-bee,
so here, the three plane surfaces in any one cell necessarily enter into the
construction of three adjoining cells. It is obvious that the Melipona saves .
wax, and what is more important, labor, by this manner of building; for
the flat walls between the adjoining cells are not double, but are of the
same thickness as the outer spherical portions, and yet each flat portion i
forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of 1
equal sizes, and had arranged them symmetrically in a double layer, the re -
suiting structure would have been as perfect as the comb of the hive-bee..
Accordingly I wrote to Professor Miller of Cambridge, and this geometer''
has kindly read over the following statement, drawn up from his informa
tion, and tells me that it is strictly correct : —


If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of radius
X V 2j or radius X 141 421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and parallel layer;
then, if planes of intersection between the several spheres in both layers be
formed, there will result a double layer of hexagonal prisms united together
by pyramidal bases formed of three rhombs; and the rhombs and the sides
of the hexagonal prisms will have every angle identically the same with
the best measurements which have been made of the cells of the hive-bee.
But I hear from Professor Wyman, who has made numerous careful
measurements, that the accuracy of the workmanship of the bee has been
greatly exaggerated; so much so, that whatever the typical form of the cell
may be, it is rarely, if ever, realized.

Hence we may safely conclude that, if we could slightly modify the in-
stincts already possessed by the Melipona, and in themselves not very won-
derful, this bee would make a structure as wonderfully perfect as that of
the hive-bee. We must suppose the Melipona to have the power of forming
her cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and seeing
what perfectly cylindrical burrows many insects make in wood, apparently
by turning round on a fixed point. We must suppose the Melipona to
arrange her cells in level layers, as she already does her cylindrical cells;
and we must further suppose, and this is the greatest difficulty, that she can
somehow judge accurately at what distance to stand from her fellow-
laborers when several are making their spheres; but she is already so far
enabled to judge of distance, that she always describes her spheres so as to
intersect to a certain extent; and then she unites the points of intersection
by perfectly flat surfaces. By such modifications of instincts which in them-
selves are not very wonderful — hardly more wonderful than those which
guide a bird to make its nest — I believe that the hive-bee has acquired,
through natural selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, rectangular strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made them
wider and wider until they were converted into shallow basins, appearing
to the eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to observe that, wherever several bees had be-
gun to excavate these basins near together, they had begun their work at
such a distance from each other that by the time the basins had acquired
the above-stated width {i.e.y about the width of an ordinary cell), and
were in depth about one-sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or broke into each other.
As soon as this occurred, the bees ceased to excavate, and began to build
up flat walls of wax on the lines of intersection between the basins, so that



each hexagonal prism was built upon the scalloped edge of a smooth basin,
instead of on the straight edges of a three-sided pyramid as in the case of
ordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax, a
thin and narrow, knife-edged ridge, colored with vermilion. The bees in-
stantly began on both sides to excavate little basins near to each other, in
the same way as before; but the ridge of wax was so thin, that the bottoms
of the basins, if they had been excavated to the same depth as in the
former experiment, would have broken into each other from the opposite
sides. The bees, however, did not suffer this to happen, and they stopped
their excavations in due time; so that the basins, as soon as they had been
a little deepened, came to have flat bases; and these flat bases, formed by
thin little plates of the vermilion wax left ungnawed, were situated, as far
as the eye could judge, exactly along the planes of imaginary intersection
between the basins on the opposite side of the ridge of wax. In some parts,
only small portions, in other parts, large portions of a rhombic plate were
thus left between the opposed basins, but the work, from the unnatural
state of things, had not been neatly performed. The bees must have worked
at very nearly the same rate in circularly gnawing away and deepening the
basins on both sides of the ridge of vermilion wax, in order to have thus
succeeded in leaving flat plates between the basins, by stopping work at the
planes of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, while at work on the two sides of a strip of wax, per-
ceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that
the bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half -completed rhombs at the base of a
just commenced cell, which were slightly concave on one side, where I I
suppose that the bees had excavated too quickly, and convex on the opposed
side where the bees had worked less quickly. In one well-marked instance, I
put the comb back into the hive, and allowed the bees to go on working for '
a short time, and again examined the cell, and I found that the rhombic
plate had been completed, and had become perfectly fiat: it was absolutely
impossible, from the extreme thinness of the little plate, that they could
have eff'ected this by gnawing away the convex side; and I suspect that
the bees in such cases stand on opposite sides, and push and bend the
ductile and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax we can see that, if
the bees were to build for themselves a thin wall of wax, they could make
their cells of the proper shape, by standing at the proper distance from
each other, by excavating at the same rate, and by endeavoring to make
equal spherical hollows, but never allowing the spheres to break into each
other. Now bees, as may be clearly seen by examining the edge of a growing
comb, do make a rough, circumferential wall or rim all round the comb;


and they gnaw this away from the opposite sides, always working circularly
as they deepen each cell. They do not make the whole three-sided pyramidal
base of any one cell at the same time, but only that one rhombic plate
iwhich stands on the extreme growing margin, or the two plates, as the case
may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements differ
from those made by the justly celebrated elder Huber, but I am convinced
of their accuracy; and if I had space, I could show that they are con-
formable with my theory.

Ruber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly correct; the
first commencement having always been a little hood of wax; but I will not
here enter on details. We see how important a part excavation plays in the
construction of the cells; but it would be a great error to suppose that the
bees cannot build up a rough wall of wax in the proper position — that is,
along the plane of intersection between two adjoining spheres. I have
several specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position to the planes of the rhom-
bic basal plates of future cells. But the rough wall of wax has in every case
to be finished off, by being largely gnawed away on both sides. The man-
ner in which the bees build is curious ; they always make the first rough wall
from ten to twenty times thicker than the excessively thin finished wall of the
cell, which will ultimately be left. We shall understand how they work, by
supposing masons first to pile up a broad ridge of cement, and then to begin

Online LibraryCharles DarwinThe origin of species → online text (page 25 of 50)