pollen within the corbiculæ depends very largely upon the character of
the flower from which the pollen grains are gathered. When little
pollen is obtained it is much more thoroughly moistened, and this is
particularly true in cases when the pollen is all, or nearly all,
collected in the region of the mouth, the forelegs, and head. When a
bee takes pollen from white or sweet clover practically all of it
first touches the bee in these regions. It immediately becomes moist,
and in this condition is passed backward until it rests within the
baskets. There is here no question of "dry" and "wet" pollen, or of
collecting movements to secure dry pollen from other regions of the
body, or of the ultimate method by which such free, dry pollen becomes
moist.
The sticky fluid which causes pollen grains to cohere is found upon
all of the legs, in the region of their brushes, although the pollen
combs and auricles of the hind legs are likely to show it in greatest
abundance, since nearly all of the pollen within each basket has
passed over the auricle, has been pressed upward and squeezed between
the auricle and the end of the tibia and the pollen mass above, and by
this compression has lost some of its fluid, which runs down over the
auricle and onto the combs of the planta. It is not necessary to
invoke any special method by which these areas receive their moisture.
The compressing action of the auricle squeezing heavily moistened
pollen upward into the basket is entirely sufficient to account for
the abundance of sticky fluid found in the neighborhood of each hind
tibio-tarsal joint. As has been noted, the brushes of the forelegs
acquire moisture directly by stroking over the proboscis and by
handling extremely moist pollen taken from the mouthparts. The
middle-leg brushes become moist by contact with the foreleg and
hind-leg brushes, probably also by touching the mouthparts themselves,
and by passing moist pollen backward. The hairy surface of the breast
is moistened by contact with the fore and mid leg brushes and with the
moist pollen which they bear.
The problem of the method of pollen moistening is somewhat more
complicated in the case of flowers which furnish an excessive supply.
Under such conditions the entire ventral surface of the collecting bee
becomes liberally sprinkled with pollen grains which either will be
removed and dropped or will be combed from the bristles and branching
hairs, kneaded into masses, transferred, and loaded. The question
naturally arises whether the movements here are the same as when the
plant yields but a small amount of pollen which is collected by the
mouthparts and anterior legs. In the opinion of the writer they are
essentially the same, except for the addition of cleansing movements,
executed chiefly by the middle and hind legs for the collection of
pollen which has fallen upon the thorax, upon the abdomen, and upon
the legs themselves. Indeed it is questionable as to just how much of
this plentiful supply of free pollen is really used in forming the
corbicular masses. Without doubt much of it falls from the bee and is
lost, and in cases where it is extremely abundant and the grains are
very small in size an appreciable amount still remains entangled among
the body-hairs when the bee returns to the hive. Yet it is also
evident that some of the dry pollen is mingled with the moistened
material which the mouthparts and forelegs acquire and together with
this is transferred to the baskets.
In all cases the pollen-gathering process starts with moist pollen
from the mouth region. This pollen is passed backward, and in its
passage it imparts additional moisture to those body regions which it
touches, the brushes of the fore and middle legs, the plantæ of the
hind legs, and the hairs of the breast which are scraped over by the
fore and middle leg brushes. This moist pollen, in its passage
backward, may also pick up and add to itself grains of dry pollen with
which it accidentally comes in contact. Some of the free, dry pollen
which falls upon the moist brushes or upon the wet hairs of the thorax
is also dampened. Some of the dry pollen which is cleaned from the
body by the action of all of the legs meets with the wet-brushes or
with the little masses of wet pollen and itself becomes wet by
contact. Pollen grains which reach the corbiculæ either dry or but
slightly moistened are soon rendered moist by contact with those
already deposited. Little pollen gets by the sticky surfaces of the
combs of the plantar or past the auricles without becoming thoroughly
moist.
Sladen (1912, _c_) very aptly compares the mixture of dry pollen with
wet to the kneading of wet dough with dry flour and suggests that the
addition of dry pollen may be of considerable advantage, since
otherwise the brushes, particularly those of the hind legs, would
become sticky, "just as the board and rolling pin get sticky in
working up a ball of dough if one does not add flour." The addition of
a considerable amount of dry pollen gives exactly this result, for the
corbiculæ then rapidly become loaded with pollen mixed with a minimum
supply of moisture and the brushes remain much dryer than would
otherwise be the case. However, if too much dry pollen is added the
resulting loads which the bees carry back to the hives are likely to
be irregular, for the projecting edges of the masses may crumble
through lack of a sufficient amount of the cohesive material by which
the grains are bound together.
On the other hand, it does not appear at all necessary to mix much dry
pollen with the wet, nor do the brushes become sufficiently "sticky"
from the presence of an abundance of the moistening fluid to endanger
their normal functional activity. I have observed bees bringing in
pollen masses which were fairly liquid with moisture, and the pollen
combs also were covered with fluid, yet the baskets were fully and
symmetrically loaded.
Sladen's different interpretations of the pollen-moistening process
are rather confusing, and it is difficult to distinguish between what
he states as observed facts and what he puts forward as likely
hypotheses. He agrees with me in his observation that all of the legs
become moist in the region of their brushes and also in his
supposition that this moisture is transferred to them from the mouth.
In this moistening process my observations show that the fluid
concerned is passed backward by the contact, of the middle-leg brushes
with the wet foreleg brushes and that the middle-leg brushes in turn
convey moisture to the plantæ as they rub upon them. I am also
convinced that the wet pollen grains furnish additional moisture to
the brushes as they pass backward, and this is particularly true in
the case of the extremely moist surfaces of the auricles and the
pollen combs of the planta, since here moisture is pressed from the
pollen upon these areas. The pollen upon the fore and middle leg
brushes is not always "dry" even in "a relative sense."
In describing pollen manipulation several writers state that dry
pollen is picked up by the brushes of the legs and is carried forward
to the mouth, there moistened (according to some, masticated), and is
then carried backward by the middle legs for loading. Obviously such
accounts do not apply to cases in which all of the pollen is collected
by mouthparts and forelegs. Do they apply in cases where much pollen
falls on the body and limbs? Without doubt a certain amount of this
free pollen is brought forward when the middle legs, bearing some of
it, sweep forward and downward over the forelegs, mouthparts, and
breast. However, it does not appear to the writer that this dry pollen
is carried to the mouth for the specific purpose of moistening it, or
that it is essential to its moistening that it be brought in contact
with the mouth. Some of it touches the moist hairs on the forelegs and
breast and is moistened by contact. All that remains on the brushes of
the middle legs secures moisture from these brushes or from wet pollen
which the brushes collect from the mouthparts or forelegs. The
supposed necessity of carrying forward pollen to the mouth for
moistening is a delusion. Some is accidentally brought forward and
into contact with the mouth and gets wet, but the process is not
essential.
If the pollen which bees transport to their hives has been moistened
with some fluid substance which causes the grains to cohere, this
addition should be indicated by differences in the results of an
analysis of pollen from a plant as compared with that found in the
corbiculæ of a bee which has been working on this plant. For the sake
of determining this difference and in an endeavor to ascertain, if
possible, the approximate nature of the added fluid, analyses were
made of three kinds of pollen, as follows: (1) Pollen collected by
hand from the corn plant itself; (2) pollen taken from the corbiculæ
of bees which had secured their supply from corn; (3) pollen stored in
the cells of the hive. In the first two cases pollen from the same
species of plant (corn) was used. The material from the cells of the
hive was composed largely of corn pollen, but contained an admixture
of some other pollens.
The writer is indebted to Dr. P. B. Dunbar, of the Bureau of
Chemistry, for the following analyses:
Pollen Corn pollen Stored
direct from pollen
from corbicula. from
corn. hive.
-----------------------------------------------------
Total solids 53.47 66.94 79.66
Moisture 46.53 33.06 20.34
Reducing sugar
before inversion 2.87 11.07 17.90
Sucrose 2.77 3.06 2.25
Total reducing sugar
after inversion 5.79 14.29 20.27
===== ===== =====
Dry basis:
Reducing sugar 5.37 16.54 22.47
Sucrose 5.18 4.57 2.82
----- ----- -----
10.55 21.11 ·····
These analyses show conclusively that a very large amount of sugar has
been added to the pollen by the time it reaches the corbiculæ.
Calculated on a dry basis just about twice as much sugar is present in
the basket pollen as in that from the corn plant. Not only is this so,
but the additional fact is disclosed that over three times as much
reducing sugar is present in the corbicular pollen as sucrose. This
latter result indicates that honey (largely a reducing sugar) rather
than nectar (containing more sucrose) is the chief sugar ingredient of
the corbicular pollen. The additional amount of sugar (here again a
reducing sugar) in the stored pollen of the hive is what might be
expected, since it is supposed that the workers add honey and possibly
other ingredients to the pollen within the storage cells.
The total solid percentages, corn 53.47, corbicula 66.94, stored
pollen 79.66, also show that the fluid substance which is added is one
highly charged with solids, a condition which honey amply fulfills.
In the descriptions which have been cited of the pollen-gathering
process in which the mouth is supposed to supply the requisite fluid
three substances are mentioned: Nectar, honey, and saliva. The
analyses herein given indicate that reducing sugar is mingled with the
pollen, and in the case of corn it is indicated that honey is used in
greater abundance. Without doubt a certain amount of saliva also finds
its way to the pollen, but the proportion of this substance has not
been determined. This salivary fluid may have adhesive qualities, but
this is scarcely necessary, since honey alone is amply sufficient for
this purpose.
It appears probable that the fluid which a bee adds to the pollen
which it is collecting varies somewhat in amount, since the pollen of
different plants differs considerably in moisture content and that of
the same plant will differ in this respect at different times. Pollen
collected in the early morning before the dew has left the plant is
much more moist than that found upon the same plant later in the day,
and the grains, if taken when moist, have a natural tendency to become
aggregated and form small masses. Moreover, this may explain the fact
that bees make their pollen-collecting trips during the morning hours,
rather than in the afternoon, although some may be seen upon the
flowers throughout the whole day.
STORING POLLEN IN THE HIVE.
When the bee has fully loaded its baskets and before it returns to the
hive it often spends a little time upon the plant from which it has
been collecting, occupied with the task of cleaning scattered grains
of pollen from its body and of patting down securely the loads which
it has obtained. Upon its return to the hive it hurries within and
seeks for a suitable place in which to deposit the pollen. Some
returning bees walk leisurely over the combs and loiter among their
sister workers, while others appear to be greatly agitated, shaking
their bodies and moving their wings as though highly excited. Many
pollen-bearing bees appear eager to receive food upon their return to
the hive, and they will solicit it from other workers or take it from
the honey-storage cells. The workers of the hive at times take a
little of the fresh pollen from the baskets of the laden bee, nibbling
it off with their mandibles or rasping off grains with their tongues.
If the combs of a colony are examined, stored pollen will be found in
various parts of the hive. In the brood frames the greatest amount is
located above and at the sides of the brood and between this and the
stored honey. Cells scattered through the brood from which young bees
have lately emerged may also contain pollen. In the outer frames of
the hive, where brood is less likely to be found, nearly all of the
cells may be packed with pollen, or honey-storage cells may be found
interspersed with those filled with pollen. As a rule pollen is not
stored in drone comb, although this occasionally happens.
As the pollen-bearing bee crawls over the combs it appears to be
searching for a suitable cell in which to leave its load. It sticks
the head into cell after cell until finally one is located which meets
its requirements, although it is an open question as to why any one of
a group should be chosen rather than another. This selected cell may
already contain some pollen or it may be empty. If partly filled, the
pollen which it contains is likely to be from the same species of
plant as that which the bee carries, although different kinds of
pollen are often stored in the same cell.
In preparation for the act of unloading the bee grasps one edge of the
cell with its forelegs and arches its abdomen so that the posterior
end of the abdomen rests upon the opposite side of the cell. The body
is thus held firmly and is braced by these two supports with the head
and anterior thoracic region projecting over one of the neighboring
cells. The hind legs are thrust down into the cell and hang freely
within it, the pollen masses being held on a level with the outer edge
of the cell, or slightly above it. The middle leg of each side is
raised and its planta is brought into contact with the upper
(proximal) end of the tibia of the same side and with the pollen mass.
The middle leg now presses downward upon the pollen mass, working in
between it and the corbicular surface, so that the mass is shoved
outward and downward and falls into the cell. As the pollen masses
drop, the middle legs are raised and their claws find support upon the
edge of the cell. The hind legs now execute cleansing movements to
remove small bits of pollen which still cling to the corbicular
surfaces and hairs. After this is accomplished the bee usually leaves
the cell without paying further attention to the two pellets of pollen
although some collecting bees will stick the head into the cell,
possibly to assure themselves that the pollen is properly deposited.
It has been stated by some (Cheshire, for example) that the spur upon
the middle leg is used to help pry the pollen mass from the corbicula.
This structure is in close proximity with the mass while the middle
leg is pushing downward upon it, but its small size renders difficult
an exact estimate of its value in this connection. It is certainly
true that the entire planta of the middle leg is thrust beneath the
upper end of the pollen mass, but the spur may be used as an entering
wedge.
Pollen masses which have been dropped by the collecting bee may remain
for some time within the cell without further treatment, but usually
another worker attends to the packing of the pollen shortly after it
has been deposited. To accomplish this the worker enters the cell head
first, seizes the pollen pellets with its mandibles, breaks them up
somewhat or flattens them out, probably mingles additional fluid with
the pollen, and tamps down the mass securely in the bottom of the
cell. As is shown by the analyses of corbicular pollen and of stored
pollen, certain substances are added to the pollen after the
collecting bee leaves it in the cell. Sugar is certainly added, and it
is generally supposed that secretions from some of the salivary glands
are mixed with the pollen after deposition. It appears probable that
the stored pollen or "beebread" is changed somewhat in chemical
composition through the action of the fluids which have been added to
it, either during the process of collection, at the time of packing,
or later.
SUMMARY.
Pollen may be collected by the worker bee upon its mouthparts, upon
the brushes of its legs, and upon the hairy surface of its body. When
the bee collects from small flowers, or when the supply is not
abundant, the mouthparts are chiefly instrumental in obtaining the
pollen.
The specialized leg brushes of the worker are used to assemble the
pollen, collecting it from the body parts to which it first adheres
and transporting it to the pollen baskets or corbiculæ of the hind
legs. In this manipulation the forelegs gather pollen from the
mouthparts and head; the middle legs, from the forelegs and from the
thorax; the hind legs, from the middle legs and from the abdomen.
The pollen baskets are not loaded by the crossing over of one hind leg
upon the other or to any great extent by the crossing of the middle
legs over the corbiculæ. The middle legs deposit their loads upon the
pollen combs of the hind plantæ, and the plantæ, in turn, transfer the
pollen of one leg to the pecten comb of the other, the pecten of one
leg scraping downward over the pollen comb of the opposite leg. (See
fig. 7.) A little pollen is loaded directly from the middle legs into
the baskets when these legs are used to pat down the pollen masses.
(See fig. 6.)
Aside from the foregoing exception, all of the pollen which reaches
the baskets enters them from below, since it is first secured by the
pecten combs, and is then pushed upward by the impact of the rising
auricles, which squeeze it against the distal ends of the tibiæ and
force it on into the baskets to meet that which has gone before.
The long hairs which form the lateral boundaries of the baskets are
not used to comb out pollen from the brushes of any of the legs. They
serve to retain the accumulating masses within the baskets and to
support the weight of the pollen, as it projects far beyond the
surfaces of the tibiæ.
Pollen grains are moistened and rendered cohesive by the addition to
them of fluid substances which come from the mouth. Analyses show that
honey forms a large part of this moistening fluid, although nectar and
secretions from the salivary glands are probably present also.
In the process of pollen manipulation this fluid substance becomes
well distributed over the brushes of all of the legs. The forelegs
acquire moisture by brushing over the mouthparts, and they transfer
this to the hairs of the breast and to the middle-leg brushes when
they come in contact with them. The middle-leg brushes transmit their
moisture to the pollen combs of the hind legs when they rub upon them.
All of these brushes also transport wet pollen which has come from the
mouthparts and thereby acquire additional moisture. The auricles and
the plantæ of the hind legs become particularly wet from this source,
since fluid is squeezed from the wet pollen when it is compressed
between the auricles and the distal ends of the tibiæ. Dry pollen
which falls upon the body hairs becomes moist when brought into
contact with the wet brushes or with wet pollen.
During the process of manipulation pollen passes backward from its
point of contact with the bee toward its resting place within the
baskets.
Pollen which the collecting bee carries to the hive is deposited by
this bee within one of the cells of the comb. As a rule, this pollen
is securely packed in the cell by some other worker, which flattens
out the rounded masses and adds more fluid to them.
BIBLIOGRAPHY.
Alefeld, Dr.--Vol. 5. Nos. 15 and 16. Eichstädt Bienen Zeitung.
Summarized in "Die Bienenzeitung in neuer, geschichteter und
systematische geordneter Ausgabe." Herausgegeben vom Schinid
und Kleine: Erste Band, Theoretischer Theile. 1861.
Casteel, D. B., 1912.--The manipulation of the wax scales of the honey
bee, Circular 161, Bureau of Entomology, U. S. Dept. Agriculture,
pp. 15.
Cheshire, F. R., 1886.--Bees and bee-keeping; scientific and practical.
Vol. I, scientific; II, practical. London.
Fleischmann und Zander, 1910.--Beiträge zur Naturgeschichte der
Honigbiene.
Franz, A., 1906.--In "Unsere Bienen," herausgegeben von Ludwig, A.,
Berlin, pp. [viii]+831.
Hommell, R., 1906.--Apiculture, Encyclopedic Agricola, Paris.
Phillips, E. F., 1905.--Structure and development of the compound eye
of the bee. Proc. Acad. Nat. Sci. Philadelphia, vol. 57,
pp. 123-157.
Sladen, F. W. L., 1911.--How pollen is collected by the social bees, and
the part played in the process by the auricle. British Bee Journal,
vol. 39, pp. 491-493, Dec. 14.
Sladen, F. W. L., 1912.--(_a_) How pollen is collected by the honey bee.
Nature, vol. 88, pp. 586, 587, Feb. 29.
1912.--(_b_) Further notes on how the corbicula is loaded with pollen.
British Bee Journal, vol. 40, pp. 144, 145, Apr. 11.
1912.--(_c_) Pollen collecting. British Bee Journal, vol. 40,
pp. 164-166, Apr. 25.
1912.--(_d_) How propolis is collected. Some further notes on
pollen-collecting. Gleanings in Bee Culture, vol. 40,
pp. 335, 336, June 1.
1912.--(_e_) Hind legs of the worker honey bee. Canadian Bee Journal,
vol. 20, p. 203. July.
Wolff, O. J. B., 1873.--Das Pollen-Einsammeln der Biene. Eichstädt
Bienen-Zeitung. 29 Jahrg. Nrs. 22 u. 23, pp. 258-270.
INDEX.
Page.
Alefeld on pollen moistening by worker bee 23
Antenna cleaner of worker bee, figure S
Auricle of hind planta of worker bee, definition 9
figure 11
role and action in pollen collecting 16-17, 19, 20-22
Basket, pollen. (See Corbicula.)
Brush of foreleg of worker bee, action and role in
pollen collecting 13
figure 8
hind leg of worker bee, action and role in pollen
collecting 16
middle leg of worker bee, action and role in pollen
collecting 14-16
figure 9
Brushes of legs of worker bee, use in pollen collecting 8-9
Bumblebee, moistening of pollen, views of Sladen 23-21
Cheshire on process of loading pollen baskets by worker bee 17
Comb or pecten of hind tibia of worker bee, definition 9
figure 77
role and action in pollen collecting 16-19
Corbicula of worker bee, definition 9
figure 10
process of loading 15-22
Corn, sweet, pollen collecting therefrom by honey bee 11-13
Coxæ of worker bee, figures 8, 9
Dunbar, Dr. P. B., analyses of corn pollen from plant,
from corbiculæ of bees, and from hive cells 28
Femora of worker bee, figures 8, 9, 10, 11
Fleischmann and Zander on process of loading pollen
baskets by worker bee 15
Flowers, variable amounts of pollen from different plants 10-11
Franz on pollen moistening of worker bee 23
process of loading pollen baskets by worker bee 17
Hairs, branched, of honey bee, use in pollen collecting 7, 8
fringing pollen basket, function 20
unbranched, of honey bee, use in pollen collecting 7, 8
Hommell on pollen moistening of worker bee 23
process of loading pollen baskets by worker bee 18
Honey, use by worker bee for moistening pollen 24, 28-29
Leg, hind, of worker bee, loaded with pollen, figure 22
Legs, fore, of worker bee, action and role in pollen
collecting 12, 33
hind, of worker bee, action and role in pollen
collecting 13, 16-18
stages in basket-loading process, figure 19
middle, of worker bee, action and role in pollen
collecting 13, 14-16
of worker bee, action in unloading pollen 30-31
structures used in pollen collecting 7-9
Mandibles of honey bee, action and role in pollen collecting 8, 13
worker bee, use in packing pollen in the cell 31
Maxilæ of honey bee, action and role in pollen collecting 8, 13