THE ANATOMY OF THE HONEY-BEE
A detailed discussion of the anatomy of the honey-bee does not fall
within the scope of this book; for such a discussion, special works on
insect anatomy must be consulted. But there are certain of the more
general features of the structure of the bee which the bee-keeper
should know; and a discussion of these, merits a place even in an
elementary book on bee-keeping.
In treating of insect anatomy it is customary to divide the subject
into two parts: first, external anatomy, which treats of the structure
of the body-wall; and, second, internal anatomy, which treats of the
parts found inside the body-wall.
I. EXTERNAL ANATOMY
_The body-wall._—Insects differ fundamentally from man and other
backboned animals. With us, the muscles and other soft parts are
supported by an internal skeleton; with the insects the body-wall, that
part which corresponds to our skin, is hard and serves as a skeleton.
In some respects this is a better arrangement than that which obtains
with us, for the skeleton of an insect serves as an armour to protect
the body as well as a support for the soft parts.
This arrangement of parts holds with the appendages of the body of
an insect as well as with the body itself; the legs, mouth-parts and
antennæ are all tubular organs, having a firm outer skeleton supporting
the inner parts.
Movement of the body and its appendages is provided for by narrow,
flexible, zone-like areas in the skeleton which encircle the body and
the appendages, at frequent intervals. This segmented condition of
the body is easily seen in the hind part or abdomen, which appears to
consist of a series of rings.
The microscopic structure of the body-wall is comparatively simple.
There is an inner cellular part which consists of a single layer of
cells: this is the hypodermis (Plate XXV, 2, _h_); and the outer or
hard part: this is the cuticle (Plate XXV, 2, _c_).
The hypodermis is the active living part; it produces the cuticle,
which receives additions from it constantly during the life of the
insect. On this account, when a section of the cuticle is examined with
a microscope it presents a layered appearance.
_Moulting of the cuticle._—From time to time during the growth of the
insect the outer layers of the cuticle are shed; this is known as
moulting. After a moult, the inner layers of the cuticle, which have
now become the outer layers, but which are still soft, stretch to
accommodate the increased size of the body, and then soon become hard.
This moulting, or shedding of the skin, takes place about six times
during the development of the bee. Several moults occur during the
larval life: one when the larva changes to a pupa, and the last one
when the pupa changes to the adult or winged form, just before leaving
the cell in which it has been developed.
_The head._—The segments of which the body of an insect is composed are
grouped into three regions: the head, the thorax, and the abdomen.
The head is the first of the three regions. It is formed of several
segments grown together so as to form a compact box. It bears the eyes,
the antennæ, and the mouth-parts.
The eyes are of two kinds, which are distinguished as the compound eyes
and the simple eyes.
The compound eyes are two in number, one on each side of the head;
they are the organs commonly recognised as the eyes. They are called
compound eyes because each consists of a great number of little
eyes closely pressed together. If a compound eye be examined with a
microscope, it will be seen to present the appearance of a honey-comb,
being composed of a great number of six-sided elements; each of these
is a separate eye.
In addition to the compound eyes, the bee has three simple eyes, or
_ocelli_, as they are termed. They are situated on the upper part of
the head between the compound eyes.
The antennæ are two slender, many-jointed organs projecting from the
front part of the head. Their use has not been fully elucidated.
They are doubtless sense organs; and it is believed that certain
microscopic pits, which occur in great numbers in their cuticle, are
the organs of smell. It is possible, also, that the antennæ function as
organs of touch, certain hairs with which they are furnished being the
tactile organs.
The mouth-parts are very complicated. They consist of an upper lip, a
lower lip, and two pairs of jaws between the lips.
The upper lip is known as the _labrum_. It is a flap-like projection
situated above, or in front of, the other mouth-parts (Plate XXV, _u_).
The first pair of jaws, those situated nearest the labrum, are the
mandibles (Plate XXV, 3, _md_). Each mandible consists of a single hard
piece. They are the biting organs. Certain wild bees, distantly related
to the honey-bee, dig holes in wood with their mandibles for nests
for their brood. The honey-bee uses its mandibles as tools for the
manipulation of wax and propolis, and as weapons in its combats.
The second pair of jaws, which are situated between the mandibles and
the lower lip, are the maxillæ (Plate XXV, 3, _mx_). Each maxilla is a
long blade. The maxillæ, combined with the lower lip, constitute what,
in popular language, is known as the tongue, the organ by means of
which the food is conveyed to the mouth, or the nectar extracted from a
flower.
[Illustration:
_Photograph by Ralph W. Curtis_
PLATE XXIV. SUMAC IN BLOSSOM AND BLOSSOM OF MOUNTAIN MAPLE
The Sumac is an excellent honey-producer.]
[Illustration:
PLATE XXV. FIG. 1.—Vertical longitudinal section of the body of
a larva of an insect; _s_, body-wall or skeleton; _m_, muscles;
_a_, alimentary canal; _h_, heart; _n_, nervous system; _r_,
reproductive organs. FIG. 2.—Section of the body-wall; _c_,
cuticle; _h_, hypodermis; _t_, trichogen or hair-forming cell.
FIG. 3.—Head of a bee and its appendages; _a_, antenna; _c_,
clypens; _u_, upper lip or labrum; _m_, mandible; _mx_, maxilla;
_l_, lower lip or labium; _p_, labial palpus. FIG. 4.—Glands
of a honey-bee (after Girard); _1_, supracerebral glands; _2_,
postcerebral glands; _3_, thoracic glands. FIG. 5.—The wax plates
(after Cheshire).]
The lower lip, or labium (Plate XXV, 3, _l_), is the long central part
of the so-called tongue; it bears on each side a long appendage;
these are the labial _palpi_ (Plate XXV, 3, _p_).
_The thorax._—The thorax is the central region of the body. It consists
of three body-segments, which are grown together so compactly in the
adult insect that it is difficult to distinguish them. The thorax bears
the organs of locomotion, the wings and the legs.
There are two pairs of wings; but the two wings of each side are so
closely united that they appear as one. The union is accomplished by
a row of hooks on the front edge of the hind wing, which fasten into
a fold in the hind edge of the fore wing. The wings are strengthened
by a framework of heavy lines, which extend in various but definite
directions. Between these lines the wing is a thin membrane.
There are three pairs of legs, a pair borne by each of the three
body-segments of which the thorax is composed.
Each leg consists of nine segments and a pair of claws at the tip
of the last segment. The first two segments, the _coxa_ and the
_trochanter_, are short; then follow the two principal segments, the
_femur_, or thigh, and the _tibia_, or shank; the five remaining
segments constitute the _tarsus_ or foot. A striking peculiarity in
the _tarsi_ of bees is that the first segment differs greatly in form
from the other segments and is much larger, approaching the _tibia_ in
size. This enlarged tarsal segment has received the special name of
_metatarsus_.
The legs serve several functions besides that of locomotion. Thus, on
each fore leg there is an organ for cleaning the antennæ. The antenna
cleaner consists of a circular notch near the base of the _metatarsus_,
which is furnished with teeth like a comb (Plate VII, F, _a_), and a
spur projecting back from the _tibia_ in such a way as to close this
notch when the leg is bent. The antenna to be cleaned is drawn through
this notch and thus the dirt is combed from it.
On the middle legs there is a strong spur at the distal end of the
_tibia_ which is used in loosing the pellets of pollen brought to the
hive on the hind legs.
The third pair of legs are furnished with three organs which deserve
mention here. _First_: the wax pincers. Both the _tibia_ and the
_metatarsus_ are wide; the joint uniting them is at one edge, hence by
alternately bending and straightening the leg at this joint, the space
between the two segments (Plate VII, B, _wp_) is opened and shut like
pincers. This organ is used to loosen from the abdomen the scales of
wax. _Second_: the pollen-combs. These are several comb-like series of
spines, borne on the inner surface of the _metatarsus_ (Plate VII, B,
_pc_). When a bee visits a flower the pollen is gathered by the tongue
and fore legs and some of it becomes entangled among the hairs on the
thorax. It is then combed from these parts by means of the pollen-combs
and transferred to the pollen baskets. _Third_: the pollen basket.
There is a pollen basket on the outer surface of the _tibia_ of each
hind leg. It consists of a fringe of hairs, surrounding a smooth,
concave area which occupies the greater part of the outer face of this
segment of the leg. In it the pollen is packed when combed from the
hairs, and transported to the hive.
_The abdomen._—The abdomen is the last of the three regions of the
body. It consists of a series of comparatively simple, overlapping
segments, without conspicuous appendages.
II. INTERNAL ANATOMY
_Relative position of the internal organs._—As has been shown in the
preceding pages, the body-wall serves as a skeleton, being hard and
giving support to the other organs of the body, which are contained
within it.
The accompanying diagram (Plate XXV, 1), which represents a vertical
longitudinal section of the body of the larva of an insect, will
enable the reader to gain an idea of the relative positions of some
of the more important organs. The parts shown in the diagram are the
following: The body-wall or skeleton (_s_); this is made up of a series
of overlapping segments; that part of it which is between the segments
is thinner, and is not hardened, this remaining flexible and allowing
for the movements of the body. Just within the body-wall, and attached
to it, are represented a few of the muscles (_m_); it will be seen
that these muscles are so arranged that the contraction of those on
the lower side of the body would bend it down, while the contraction
of those on the opposite side would act in the opposite direction. The
alimentary canal (_a_) occupies the centre of the body and extends
from one end to the other. The heart (_h_) is a tube open at both ends,
and lying between the alimentary canal and the muscles of the back.
The central part of the nervous system (_n_) is a series of small
masses of nervous matter, connected by two longitudinal chords; one of
these masses, the brain, lies above the alimentary canal; the others
are situated, one in each segment, between the alimentary canal and
the ventral wall of the body; the two chords connecting these masses,
or ganglia, pass one on each side of the œsophagus to the brain. The
reproductive organs (_r_) lie in the cavity of the abdomen and open
near the hind end of the body. The respiratory organs are omitted from
this diagram for the sake of simplicity.
_The respiratory system._—The most striking peculiarity in the
structure of insects is the form of their organs for breathing, for
they do not breathe through the mouth as we do. If an insect be
carefully examined, there can be found along the sides of the body, a
series of openings; these are the openings through which the air passes
into the respiratory system, and are termed spiracles. The spiracles of
the honey-bee are small, and are not easily found by one not trained
to look for such things; but if the reader will examine the sides of
one of our larger caterpillars, he will have no trouble in seeing them.
Typically, there is a pair of spiracles, one on each side of the body,
in each of the body-segments, but they are lacking in the head and in
some of the other segments. The spiracles lead into a system of air
tubes, termed tracheæ, which carry the air to all parts of the body.
When the body of an insect is opened, the tracheæ appear as silvery
threads, on account of the contained air. In the adult honey-bee
certain of the tracheæ are greatly expanded so as to form large air
sacs. (Plate XXVI _b_, 2).
_The glands._—Those glands found in the body of the honey-bee that are
of most interest to the practical bee-keeper are the following:
In the larva there is a pair of long, tubular glands, which secrete the
silk of which the cocoon is made. These glands open through a common
duct, which has its outlet near the mouth.
In the adult worker bee there are four pairs of glands opening into
the mouth, which have been much discussed by students of this subject.
These glands are designated both by number and by name as follows:
System I. or supracerebral glands; system II. or postcerebral glands;
system III. or thoracic glands; and system IV. or mandibulary glands.
The supracerebral glands or system I. (Plate XXV, Fig. 4, 1) are
situated in the head above the brain. They open by two openings in the
floor of the mouth cavity, one on each side.
The postcerebral glands or system II. (Plate XXV, Fig. 4, 2) are
situated in the head behind the brain; their outlets unite into a
common duct which opens on the middle line of the anterior end of the
œsophagus at the base of the tongue.
The thoracic glands or system III. (Plate XXV, Fig. 4, 3) are situated
in the thorax; their outlets unite into a common duct, which joins the
ducts from the postcerebral glands, the two systems of glands opening
through a common opening.
The mandibulary glands or system IV. are two small glands one on each
side opening at the base of the mandible.
There has been much discussion regarding the function of these
different glands; and even now any statement of conclusions must be
regarded as provisional.
The supracerebral glands are large in nurse bees and shrunken in the
old bees that no longer nurse the brood; they are normally found only
in the workers. It is therefore believed that they secrete the milky
food, commonly called royal jelly, which is fed to all larvæ during
the first days of their development, to the queen larvæ throughout
their development, and to the adult queen during the egg-laying period.
The food fed worker and drone larvæ during the latter part of their
development is produced in the chyle-stomach of the nurse bees, and is
semi-digested food.
The other systems of glands enumerated above produce the saliva, which
is supposed to perform a great variety of functions. “It helps the
digestion; it changes the chemical condition of the nectar harvested
from the flowers; it helps to knead the scales of wax of which the
combs are built, and perhaps the propolis with which the hives are
varnished. It is used also to dilute the honey when too thick, to
moisten the pollen grains, to wash the hairs when daubed with honey,
etc.”
The wax glands are found only in the worker. There are four pairs of
them. They are situated on the ventral wall of the second, third,
fourth and fifth abdominal segments, and on that part of the segment
which is overlapped by the preceding segment. Each gland is simply a
disc-like area of the hypodermis, the cells of which take nourishment
from the blood and transform it into wax. The cuticle covering each
gland is smooth and delicate, and is known as a wax plate. The wax
exudes through these plates and accumulates, forming little scales.
(Plate VI, X, also Plate XXV, Fig. 5.)
_The alimentary canal._—The form of the alimentary canal of the adult
honey-bee is shown in Plate XXVI. The following parts can easily be
recognised: the œsophagus, a slender tube, beginning at the mouth and
extending through the head and thorax to the base of the abdomen.
Here there is a sac-like enlargement of the canal, which is termed
the honey-stomach; it is in this that the nectar accumulates as it
is collected by the bee, and is carried to the hive. Behind the
honey-stomach lies the true stomach, the chief digestive organ. Closely
applied to the true stomach are several small tubes, which open into
it; which are known as the Malpighian tubes; they were named after
one of the early anatomists who described them; they are the urinary
organs. Next to the true stomach is the small intestine; and behind
this, the large intestine.
_The reproductive organs._—The internal reproductive organs are
situated in the abdomen; there is a set on each side, but the two sets
open by a common duct, whose outlet is at the hind end of the body.
The reproductive organs of the female are shown on Plate XXVI (_a_),
Fig. 1. There are two ovaries (_o_), one on each side of the body.
Each ovary consists of a large number of parallel egg-tubes, within
which the eggs are developed. The egg-tubes of each ovary open into an
oviduct (_od_). The two oviducts unite and form a single tube on the
middle line of the body; this is the _vagina_; the _vagina_ leads to
the external opening of the system. Communicating with the _vagina_,
there is a sac-like pouch, the _spermatheca_, which is the reservoir
for the seminal fluid; this is filled at the time of pairing, and the
_spermatozoa_ may remain alive in it for several years.
Each egg-tube produces many eggs. As the eggs increase in size, they
pass towards the oviduct. When the egg is fully developed a shell is
formed about it. This shell has a minute opening through it at one end;
this is the micropyle. At the time the egg is laid a _spermatozoon_ may
pass from the _spermatheca_, where thousands of them are stored, into
the egg through the micropyle, and thus the egg is fertilised.
With most animals, the egg must be fertilised in order that it may
develop. But with bees, both fertilised and unfertilised eggs develop,
the former into females, that is, workers or queens, the latter into
males, that is, drones.
[Illustration:
PLATE XXVI. (_a._) The reproductive organs of the honey-bee.
(From Leuckart, slightly modified.) FIG. 1.—Reproductive organs
of a queen; _o_, _o_, ovaries; _od_, oviduct; _s_, spermatheca;
_g_, gland; _p_, poison sac connected with the sting. FIG.
2.—Reproductive organs of a drone; _t_, _t_, testes; _v_, vas
deferens; _s_, _s_, seminal sacs; _m_, _m_, mucous glands; _ed_,
ejaculatory duct; _b_, pouch or bulb. The bulb and the following
parts are everted through the outer opening at the time of
pairing.]
[Illustration:
PLATE XXVI. (_b._) FIG. 1.—The internal anatomy of the
honey-bee (after Cheshire). _o_, œsophagus; _hs_, honey sac;
_p_, stomach-mouth; _cs_, true stomach or chyle-stomach; _m_,
malpighian tubes; _si_, small intestine; _li_, large intestine;
_h_, heart or dorsal vessel; _n_, central nervous system; _1_,
supracerebral glands; _2_, postcerebral glands; _3_, thoracic
glands. The respiratory system and the reproductive organs
are not shown in this figure (after Cheshire). FIG. 2.—The
respiratory system of the honey-bee. _s_, _s_, _s_, spiracles;
_a_, enlarged trachea or air sac (adapted from Leuckart).]
[Illustration:
_Courtesy of Ginn & Company_
PLATE XXVII. AN OLD-FASHIONED APIARY]
Closely associated with the reproductive organs of the female is the
sting; this is a barbed dart connected with a poison gland, whose use
is well known.
In the abdomen of the male there is a pair of organs, the testes, in
which the _spermatozoa_ are developed. These correspond in position
to the ovaries of the female, but are much smaller. From each testis
there extends a tube corresponding to the oviduct, this is the _vas
deferens_. The two _vasa deferentia_ unite and form the single
ejaculator duct. Each _vas deferens_ is enlarged just before it joins
the ejaculatory duct, forming a reservoir for the accumulation of
_spermatozoa_; these reservoirs are termed the seminal sacs. Appended
to each seminal sac there is a large glandular sac, which adds mucus to
the seminal fluid. Near the outer end of the ejaculatory duct there is
a pouchlike enlargement into which the _spermatozoa_ pass. Here they
are massed into a compact body, known as the spermatophore, which is
transferred to the female at the time of pairing. The terminal part
of the reproductive organs of the male, the intermittent organ, has
several appendages, which are firmly grasped in the opening of the
reproductive organs of the female and are torn from the male when the
two pairing individuals separate. This causes the death of the male.
The male has no sting. (Plate XXVI, Fig. 2.)