PHYSIOLOGY OF THE BEE.
Nervous System--The Head--Eyes--Compound and Simple--Uses and
Powers--Sir John Lubbock's Experiments--The Antennæ--Structure
and Uses--Mouth--Detailed Description.
Before proceeding to detail the most important facts connected with the
internal economy of the hive, it will be desirable to describe with
some minuteness the physiology and anatomy of the inhabitants, so that
it may be more easy to understand the means by which various processes
are accomplished, and the most important events of the community are
brought about. Much that has been hitherto said will become more
readily comprehended by attention to the structure of the various
organs we are now about to describe.
It will hardly be necessary to enter into a more minute account, than
we have already given, of the egg, the larva, and the pupa. We shall,
therefore, confine ourselves to detailing the most interesting points
in the physiology of the perfect insect.
It has been noted, in an earlier chapter, that the members of this
division of the animal kingdom are characterised by having three very
distinct segments in their bodies the head, the thorax, and the
abdomen. As the nature and arrangement of the nervous system forms one
of the soundest bases of classification in the highest of the three
kingdoms in nature, we shall first direct attention, in each case, to
this all-important matter of detail.
[Illustration: Fig. 22.--Nervous System of Privet Hawk-moth.]
[Illustration: Fig. 23.--Nervous System of Larva of Bee.]
The general arrangement of the nerve-matter in the sub-kingdom
_Articulata_, to which all true insects belong, is that of a double
cord, with knot-like protuberances, called _ganglia_, at more or
less regular intervals. The two filaments are in some cases close
together: in others, quite distinct; while the larger nerve-masses--the
previously mentioned _ganglia_--also vary in juxtaposition, according
to the greater or less importance of the functions they regulate.
In the illustration of the larva of _Sphinx ligustri_ (the privet
hawk-moth) (Fig. 22), the nervous cord is nearly uniform throughout its
length, though at its upper portion a separation takes place into three
loops. The ganglia also occur at almost equal distances. A very similar
disposition of the nerve-structure is seen in the larval condition of
the bee; but we may note the absence of loops, the larger development
of the cephalic masses, without the separation of their filaments
to inclose the gullet together with a more plainly-defined distance
between the cords which run parallel through the rest of the body.
[Illustration: Fig. 24.--Nervous System of Perfect Insect.]
In the perfect insect we observe some decided modifications to have
taken place. The head portions have grown proportionally larger, and
show a loop for the passage of the œsophagus, while two large ganglia
in the thorax indicate the seat of impressions and impulses connected
with the organs of motion--wings and legs--which had no existence in
the larval condition. As the functions of the abdominal region, viz.,
those of digestion and circulation chiefly, remain much the same in the
different states through which the individual passes after the hatching
of the egg, we find, as we might expect, little change in the nervous
system of the posterior segment of the body.
From each nerve-mass will be observed filaments branching on either
side to the outer edges of the body. By means of these communication
is kept up between all parts of the frame. Sensations are received and
conveyed to the sensorial organs, and return-stimuli are sent to the
organs whose movements depend for regulation on the different ganglia.
This branching of the nerve-fibre is directly proportional to the
variety and force of the several functions subserved by the various
structures to which they proceed.
_The Head._--We will now describe in some detail the structure and
functions of the highly-important organs contained in the anterior
segment, or head. And first in order let us take the
_Eyes._--On either side of the head may be observed an oval lobe,
convexly rounded and immovable, brown in colour, covered with a horny
tunicle, and exhibiting to the unassisted eye a vast number of distinct
points. These points, under a high-power magnifying-glass, are seen to
be facets, hexagonal in shape, so as to occupy all available space,
without interstices, and each connected with a minute tube and a
thread of nerve-matter leading to the cephalic ganglia or brain. These
compound eyes, as they are called, are common to most true insects.
They may be easily seen in flies, bluebottles, moths, butterflies,
&c. The numbers of the facets vary greatly in different families of
the _Articulata_. In the common house-fly there are, it is stated,
about 4,000; in the white cabbage-butterfly, 17,000; in the dragon-fly,
24,000. It has been computed that in each compound eye of the bee there
are about 3,500 of them.
[Illustration: Fig. 25.--Eyes of a Bee, Greatly Magnified.]
Behind the horny covering, or cornea, which consists of two
plano-convex lenses, is a layer of dark pigment, which gives the
characteristic colour to these eyes. This is pointed like the neck of
a vase, and serves the purpose of the iris in the higher animals. This
is traversed by a minute aperture or pupil, through which the rays pass
by a longer conical lens to the optic nerve. A vertical section shows
that each ocellus (or _little eye_) is the frustum of a pyramid, the
large end or base of which is bounded by the cornea, while the other
and pointed end terminates against an expansion of the optic nerve.
The eminent physiologist, Dr. W. B. Carpenter, says, in describing
the minute structure of these organs: "The interior of this pyramid
is occupied by a transparent substance, which represents the vitreous
humour (of the eyes of vertebrates), and the pyramids are separated
from each other by a layer of dark pigment, which completely incloses
them, save at the pupillary apertures, and also at a corresponding set
of apertures at their smaller ends, where the pigment is perforated by
the fibres of the optic nerve, of which one proceeds to each separate
eye.
[Illustration: Fig. 26.--Facets of Eye of a Bee.]
"Each facet, or 'corneule' of the common cornea, is convex on both
its surfaces, and thus acts as a lens, the focus of which has been
ascertained, by experiment, to be equivalent to the length of the
transparent pyramid behind it; so that the image produced by the lens
will fall upon the extremity of the filament of the optic nerve, which
passes to its truncated end. The rays which have passed through the
several 'corneules' are prevented from mixing with each other by means
of the layer of black pigment which surrounds each cone; and thus, no
rays, except those which correspond with the axis of the cone, can
reach the fibres of the optic nerve. Hence it is evident that each
separate eye must have an extremely limited range of vision, being
adapted to receive but a very small pencil of rays proceeding from a
single point in any object; and as these eyes are immovable, they would
afford but very imperfect information of the position of surrounding
objects, were it not for their enormous multiplication, by which a
separate eye, so to speak, is provided for each point to be viewed. No
two of these, save those upon the opposite sides of the head, which
are directed exactly forwards, can form an image of the same point
at the same time; but the combined action of all of them may give
to the insect, it may be imagined, as distinct a picture as that we
obtain by a very different organisation." We venture to suggest that
another reason for the vast multiplication of the numbers of "ocelli"
is to enable the insects to see in what would be to us darkness.
Nearly all the operations carried on in the interior of the hives are
done, during the day-time, in very dim light; and in the night-time,
when work is by no means intermitted, there would, to _our_ eyes, be
absolute darkness. To the bees, however, the scanty rays received by
so many sensitive points may be sufficient to enable them to see with
considerable clearness. If the simple enlargement of a single pupil,
such as takes place in us on emerging from a strong into a dim light,
makes so great a difference in our power of vision a fact with which
we are all familiar on going from a well-lighted room into what seems
for the first few seconds complete darkness we may well believe that
the permanent means of entry into the sensorium of an immense number
of separate rays may give greatly enlarged powers of seeing scantily
illuminated objects.
Still, an opposite view is held by many naturalists, for it seems very
doubtful whether there is any power in the bee of focusing these eyes,
so as to adapt their range to different distances. The probability is
that no such faculty of adjustment exists in them. We should expect
this from the structure of the visual apparatus. Yet it seems possible
that the compound eyes act as telescopes, and serve for great range of
vision, but not for near objects. For, while bees dart homewards from
far-off fields with the directness of an arrow, they will frequently
fly against persons or things in the direct line of their course,
without apparently having seen them at a little distance off. Moreover,
when they have alighted within an inch or two of the entrance to
their hives, they often fail to perceive its position, and constantly
wander to one side or the other, searching for their way in. We might
conclude, therefore, that these compound eyes confer distinctness of
vision afar, and possibly ability to use up scanty light, rather than
any great discernment of objects near at hand.
In addition to these "facetted" eyes, bees have, on the top of the
head, three simple ones, called by some writers "coronets," by others
"stemmata." Their position and arrangement are shown at _g_ in Fig.
27, p. 98. The focal length of their lens is said to be short, and
they are supplied with numerous filaments from the optic ganglia.
The special purpose of these simple organs is not well ascertained.
If their focal length is short, this would seem to imply that their
range of vision is also very limited. But it is very possible they may
possess a focusing power, which would adapt them for seeing at all
distances. Réaumur thinks they may, with their hemispherical lens, act
as microscopes. This point needs further investigation, as the subject
of the uses of these two kinds of visual apparatus is, at present, very
far from satisfactorily elucidated. One remarkable fact relating to
the "stemmata" must be mentioned. It is that, if they be covered with
a little opaque paint, the bee, on being let go, will fly continually
upwards. Dr. W. B. Carpenter considers this curious fact due to
automatic movements initiated by the ganglia connected with flight,
uncontrolled by the visual impressions which the simple eyes convey in
their natural condition. Neither kind of eye has a lid, but both are
protected from dust by numerous small hairs growing round them, and in
the points of junction of the facets.
How far the eyes of bees enable them to distinguish colours is still
a moot point. On _a priori_ grounds we should expect that one very
definite object in the hues of flowers is to attract the notice of
insects, just as we have strong reason to believe that odours exhaled
in the vegetable world serve this purpose. Sir John Lubbock has
detailed a series of experiments on this point, the following summary
of which is abstracted from his work on _Ants, Bees, and Wasps_. He
says, p. 304: "In recording the results I marked down successively the
order in which the bee went to the different-coloured glasses (on which
honey was placed). For instance, in the first journey from the nest,
as recorded below, the bee lit first on the blue, which accordingly
I marked I; when the blue was removed, she flew about a little, and
then lit on the white; when the white was removed she settled on the
green; and so on successively on the orange, yellow, plain, and red.
I repeated the experiment a hundred times, using two different hives
one in Kent, and one in Middlesex and spreading the observations over
some time, so as to experiment with different bees, and under varied
circumstances. Adding the numbers together, it, of course, follows that
the greater the preference shown for each colour, the lower will be the
number standing against it.
"The following table gives the first day's observations _in extenso_:--
+---------+------+-------+-------+--------+-----+-------+--------+
| | | | Plain | | | | |
|Journeys.| Blue.| Green.| Glass.| Orange.| Red.| White.| Yellow.|
+---------+------+-------+-------+--------+-----+-------+--------+
| 1 | 1 | 3 | 6 | 4 | 7 | 2 | 5 |
| 2 | 5 | 4 | 7 | 6 | 1 | 2 | 3 |
| 3 | 1 | 4 | 7 | 6 | 5 | 3 | 2 |
| 4 | 2 | 4 | 6 | 7 | 5 | 1 | 3 |
| 5 | 1 | 4 | 7 | 2 | 6 | 5 | 3 |
| 6 | 1 | 2 | 3 | 6 | 5 | 4 | 7 |
| 7 | 2 | 1 | 4 | 7 | 3 | 5 | 6 |
| 8 | 3 | 4 | 6 | 2 | 7 | 5 | 1 |
| 9 | 5 | 1 | 7 | 4 | 6 | 3 | 2 |
| 10 | 1 | 6 | 7 | 5 | 3 | 2 | 4 |
| 11 | 4 | 6 | 5 | 2 | 7 | 3 | 1 |
| +------+-------+-------+--------+-----+-------+--------+
| | 26 | 39 | 65 | 51 | 55 | 35 | 37 |
+---------+------+-------+-------+--------+-----+-------+--------+
"In the next series of experiments the bees had been trained for three
weeks to come to a particular spot on a large lawn, by placing from
time to time honey on a piece of plain glass. This naturally gave the
plain glass an advantage; nevertheless, as will be seen, the blue
still retained its pre-eminence. It seems hardly necessary to give the
observations in detail. The following table shows the general result:--
+-----------+------+-----+------+-------+------+-----+-------+-------+
| |No. of| | | | | | | |
| Series. | Exp. |Blue.|Green.|Orange.|Plain.| Red.| White.|Yellow.|
+-----------+------+-----+------+-------+------+-----+-------+-------+
|1st. | 11 | 26 | 39 | 51 | 65 | 55 | 35 | 37 |
|2nd. May 30| 15 | 38 | 57 | 59 | 72 | 66 | 58 | 70 |
|3rd. July 2| 16 | 44 | 76 | 82 | 73 | 53 | 53 | 67 |
|4th. " 4| 15 | 43 | 61 | 64 | 80 | 66 | 50 | 56 |
|5th. " 5| 10 | 36 | 47 | 39 | 40 | 40 | 36 | 42 |
|6th. " 6| 2 | 2 | 8 | 9 | 10 | 14 | 6 | 7 |
|7th. " 20| 11 | 33 | 39 | 50 | 47 | 49 | 41 | 49 |
|8th. " 23| 10 | 31 | 46 | 48 | 52 | 37 | 35 | 31 |
|9th. " 25| 10 | 22 | 54 | 38 | 52 | 33 | 35 | 46 |
| +------+-----+------+-------+------+-----+-------+-------+
| | 100 | 275 | 427 | 440 | 491 | 413 | 349 | 405 |
+-----------+------+-----+------+-------+------+-----+-------+-------+
"The precautions taken seem to me to have placed the colours on an
equal footing; while the number of experiments appears sufficient to
give a fair average." As this table differs in form from the other, it
may be as well to explain the first _line_ of figures in illustration
of the whole. The first series consisted of eleven experiments. The
preferences were noted as before, and when the numbers indicating
these were added up, the results were that twenty-six represented the
total for the blue glass, thirty-nine for the green, fifty-one for
the orange, sixty-five for the plain glass, fifty-five for the red,
thirty-five for the white, and thirty-seven for the yellow the blue
being again manifestly the most attractive colour to the bees. Some
practical bee-keepers consider the question by no means settled. The
field is doubtless open for further exploration.
_The Antennæ._ In the front part of the head are two organs,
which appear to supplement, in some remarkable way, probably by
touch-sensations, the power of vision, and also to possess other
capabilities constituting a sense to which _we_ have nothing strictly
analogous. These organs are called _antennæ_. They spring from origins
near together, at equal distances from the medial and anterior point of
the head, and are connected, by distinct and somewhat large filaments,
with the nerve matter forming the cephalic ganglia. Externally they
consist, first, of one segment nearest the head, much longer than the
rest. This part is called the _scape_. Then, forming a sort of elbow
with it, is the _flagellum_, consisting of eleven joints in queens and
workers, and of twelve in the drones. These segments are tubular, and
so attached to each other as to give the greatest possible freedom of
motion. Their extremities are wonderfully sensitive, and it is probable
that there is a very delicate power of feeling in each of the joints.
For the cleansing of these organs, special provision is made in the
construction of the fourth and fifth joints of the most forward pair of
legs. At the anterior part of the tibia, or fourth joint, is a spur,
within, and at the base of, which is a small angular projection, called
the _velum_ or _sail_. At the base of the next joint, and opposite
the play of this _velum_, is found a deep notch. From the fact of its
being fringed with hairs, this is called the _curry-comb_. Upon this
notch the velum can act at the will of the insect, and, when shut over
one another, they form a circular orifice, just large enough to take
the antennæ. When the latter organ needs cleansing, it is laid within
the notch: the velum is pressed over it, and being drawn through the
round space, dust and other soilures are removed from its surface.
So particular are the bees about keeping their antennæ thoroughly
clean, that they may often be observed continuing this operation of
drawing them through the curry-comb till perfectly satisfied with their
condition. Doubtless, the delicate nature of the impressions to which
these organs are susceptible, supplies the reason for the care taken in
freeing them from all extraneous substances.
The uses served by the antennæ are various and very remarkable. Their
first function seems to be to supplement vision. Endowed with exceeding
flexibility, they are kept by the insects in constant motion; and when
their eyes fail to guide them to particular spots, such as the entrance
to the hive, or as to the nature of objects with which they come into
contact, the antennæ appear to supply the necessary information.
There is little doubt that these "horns" or "feelers," as they are
commonly called, are sensitive, also, to impressions from objects at
some distance. Vibrations of the air too feeble to affect our organs
affect them. It may even be that other qualities of the atmosphere are
apprehended by them. The shape of the cells; the suitability of these
for brood of various kinds, for honey or for bee-bread, is ascertained
by the antennæ. Every want and every duty is recognised by them; the
presence or absence of the queen is discovered by their use, and
intelligence is conveyed from one individual to another by means of
them.
Of these facts, Huber has given the following striking evidence. He
divided a stock hive into two parts by metal network, sufficiently
fine to prevent the passage of the bees, but with meshes wide enough
to allow the antennæ to be passed through. At first, by a pair of such
gratings at a little distance apart, he separated the two portions,
so that no communication whatever could take place between them.
Very soon that half from which the queen was excluded showed signs
of commotion and distress, and even began to prepare queen-cells, to
supply themselves with a new sovereign; but when, by the removal of one
grating, Huber allowed the feelers to be used to convey intelligence
between the bees on opposite sides of the remaining division, he saw
the insects by hundreds making inquiries as to what had happened. Then
the queen was observed on the grating, and the bees being assured, by
crossing antennæ with her, that their mother was still in the hive,
though shut off from free access to one set of her subjects, they
all quieted down, left off making the royal cells, and resumed their
various avocations.
Huber tried the further experiment of depriving two queens of their
antennæ, and introducing both into the same hive. The population did
not seem able to recognise their own sovereign from the stranger, and
both were let alone; but, directly he put in a third queen, unmutilated
in these organs, the workers fell upon her, and slaughtered her.
The antennæless queens lost all purpose, laid eggs at random, and
wandered about the hives as if they had "lost their heads."
Another very curious fact is, that if a worker is deprived of her
feelers, and then allowed to fly, she becomes incapable of recognising
her hive, even when near to it, and is hopelessly lost as to her
whereabouts. From this circumstance we are inclined to conclude that
the antennæ are possessed of sensibilities to which we have nothing
strictly analogous--that, in fact, there resides in them a sense, or
senses, with which mankind is not endowed, one of which we are disposed
to call the "homing-sense."
Numerous observations show that by the antennæ, also, distinct
information can be given. We have ourselves tried the following
experiment in confirmation of this point. Having placed near the
entrance of a hive a dead humble-bee, we first noticed one of the
sentinels rush to the body, and with her feelers investigate its
nature. Finding it was a lifeless creature, and one, therefore, simply
to be got rid of, she began to tug at it, to move it towards the edge
of the floor-board. At once discovering that the weight was too great
for her strength, she went to the entrance, and meeting a friend, by
crossing their feelers, the one was made aware of the difficulty of the
other. The second then went to the aid of the first; but, as the body
was too great a burden for their united efforts, the new-comer gave up
her attempts to move it, as if the duty did not concern her much. The
first bee, however, would not be baffled till she had fetched several
other individuals, one at a time, to the work in hand. But, at length,
as she could get no combined action, and as no two were sufficiently
strong to haul away the large carcase of their distant relative, she
gave up the task in despair, and retired to the hive in apparent
disgust.
On a moonlight night the sentries may be observed marching eagerly
about the entrances of their abodes, and vigorously moving their
antennæ, to ascertain whether moths, or other unwelcome intruders,
are trying to get inside the hives. The presence of an enemy being
detected, he is soon chased away.
By some naturalists the feelers have been thought to afford the
capacity of smell. It is, however, more probable that this sense
resides in the mouth itself, or in its immediate neighbourhood.
Whether or not bees appreciate sound, is another moot point. It is,
indeed, doubted by many observers whether hearing is possessed at
all by insects. Sir John Lubbock records a series of experiments
which he conducted on this point, to which we shall make reference a
little later on. Those writers, who credit bees with the ability to
distinguish sound waves, incline to the belief that the power resides
in the antennas. As modern science has shown that all our physical
impressions are modifications of vibration, variously interpreted,
according to the means by which they are conveyed to the sensorium,
we may readily imagine that more than one faculty may reside in these
jointed organs of which we have been speaking, and that each separate
part may possibly have its own specific function; while, by combined
action, such differences may be made as are analogous to chords, and
harmonies, or discords in music, as compared with the striking of
single notes.
We have dwelt at considerable length on the subject of the antennæ,
not simply because what is known of them is so remarkable, but because
we wish to draw attention to the fact that there is here a most
interesting field for further investigation. Much remains to be done
to clear up the mysteries still unsolved, and to harmonise the various
observations already made respecting the nature and properties of these
organs, which, not only in bees, but in many other families of insects,
play such an important part in their life-history.
_The Mouth._--Passing next to the mouth, we find a somewhat complex
structure; for it consists of many parts, each of which has its
ascertained function. We find first, the _labrum_, or upper lip;
the _epipharynx_, or valve closing the aperture of the gullet; the
_pharynx_, or gullet, forming the true mouth, as well as the entrance
to the _œsophagus_, or food-pipe; the _hypopharynx_, lying just
below the gullet; the _labium_, or lower lip; and the _proboscis_,
or true tongue. These are all single parts; but there are also pairs
of _mandibles_, or upper jaws, and _maxillæ_, or lower jaws, besides
_palpi_--certain jointed, sensiferous organs, whose functions are not
well understood, but which are possibly connected with the sensation of
taste.
The _labrum_, or upper lip, has a vertical motion, and when not in use
falls over the organs beneath it; while it is covered, in its turn, by
the _mandibles_, which are jointed on to the cheeks, and act laterally.
The _pharynx_ is a cavity lying beneath the _epipharynx_, and can be
closed by the latter, over which the two previously described parts
lap, so that the entrance to the œsophagus is trebly protected.
The _labium_, or lower lip, is capable of being pushed forward and
retracted, and lies, when not in use, within the under cavity of the
head.
On either side are the _maxillæ_, or so-called jaws, which form the
under sheath of the rest of the lingual structures when in repose.
[Illustration: Fig. 27.--Head of Bee, With Antennæ.
_a._ Antennæ._b._ Compound eyes. _c._ Jaws. _d._ Maxillae. _e._ Labial
palpi. _f._ Ligula, or tongue. _g._ Stemmata.]
The true tongue is attached to the middle point of the lower lip,
having the labial palpi at its sides. It is much elongated when thrust
out in use. While at rest, the anterior part folds back upon the
posterior portion, when it is covered by the maxillæ, which seem then
like a part of the tongue itself. The back is much larger than the
front. The whole is flattened, when not sipping liquid. It is then much
broader than its thickness, but its edges are rounded. It narrows
from its base to its extremity, at which there is a slight inflation,
which seems to have a perforation in its centre, and is surrounded
by hairs. The tongue has also a large number of cartilaginous rings,
each bordered with minute hairs, which appear to be the means used
for sweeping up the last remains of any fluid which has been almost
exhausted. The act of imbibition is performed, not so much by suction,
as by lapping. Its motions being free in all directions, it can easily
draw liquid into the mouth on all sides. We notice, however, that when
the supply of food being taken is very considerable, the segments
of the abdomen have a vibratory motion, or, rather, are alternately
lengthened and shortened, as if fluid were being pumped into the body.
It is, therefore, possible that, under some circumstances, suction as
well as lapping may go on. Still, it is remarkable that a bee does
not insert the tip of its proboscis into a drop of honey or other
saccharine material, as it would do if it intended to draw liquid
through a tube. It much rather uses the middle of the upper surface
of the tongue, curving round the point as if not to employ it. If,
however, the honey or syrup be very thick, the fore-part of the tongue
is thrust into it, possibly to dilute the liquid with saliva, and thus
to render it fit for lapping. In all cases the insect tries to load
the upper surface, whence the fluid passes backward under the sheaths
to the gullet; and we see no reason to believe that the proboscis
constitutes a tube for imbibition. A further confirmation of this
conclusion is given by Shuckard, who says, "By pressing towards its
origin, I have detected the liquid which gave it its extension; but
all my pressing would never make the liquid pass through the extremity,
although the pressure has sometimes made it almost rend the membranes
to give it an opening to escape by."
A further use of the tongue is for shaping the pliant wax in
comb-building; and it appears to be employed much as a trowel is by
a bricklayer, or, perhaps, we should rather say, like a finger by a
moulder of plaster of Paris.
As we have mentioned, the jaws open vertically; but the mandibles
and maxillæ work horizontally. They are thus enabled to seize and
tightly hold any object they can grasp. The mandibles of the drone and
the queen have two notches or teeth. Those of workers are not thus
furnished, probably because, for shaping and smoothing the cells, an
unbroken edge is much more convenient than a notched one. These organs
are, however, very strong, and enable their possessor to grasp enemies,
drones or queens; to nibble hard kinds of food; to break away pieces of
damaged comb; and to mould wax for building purposes. In the last of
these operations they are, doubtless, aided by the shear-like maxillæ.