Many descriptions have been written by others of the method by which pollen is gathered and moistened. Some of these are indefinite, some are incorrect, while others are, in part, at least, similar to my own interpretation of this process. A few citations will here be given:

The bee first strokes the head and the proboscis with the brushes of the forelegs and moistens these brushes with a little honey from the proboscis, so that with later strokes all of the pollen from the head is collected upon these brushes. Then the middle-leg brushes remove this honey-moistened pollen from the forelegs and they also collect pollen from the breast and the sides of the thorax.--[Translation from Alefeld, 1861.]

In his account of the basket-loading process Alefeld a.s.signs to the middle-leg brushes the function of a.s.sembling all of the pollen, even that from the plantar combs, and of placing it on the corbiculae, this latter act being accomplished by combing over the hairy edge of each basket with the middle-leg brush of the same side.

It appears probable that the bee removes the pollen from the head, breast, and abdomen by means of the hairy brushes which are located upon the medial sides of the tarsal segments of all of the legs, being most p.r.o.nounced upon the hind legs. The pollen is thus brought together and is carried forward to the mouth, where it is moistened with saliva and a little honey.--[Translation from Franz, 1906.]

Franz then says that this moistened pollen is pa.s.sed backward and loaded.

Since the pollen of many plants is sticky and moist it adheres to the surface of the basket. Dry pollen is moistened by saliva, so that it also sticks,--[Translation from Fleischmann and Zander.

1910.]

Pollen is taken from flowers princ.i.p.ally by means of the tongue, but at times, also, by the mandibles, by the forelegs, and middle legs. The brushes of the hind legs also load themselves, collecting from the hairs of the body. The pollen dust thus gathered is always transmitted to the mouth, where it is mixed with saliva.--[Translation from Hommell, 1906.]

Sladen considers the question of how pollen is moistened by the honey bee, humblebee (b.u.mblebee), and some other bees, but does not appear to reach definite conclusions. In one of his papers (1912, _c_) he states that the pollen of some plants may be found in the mouth cavity and in the region of the mouth, but he reaches the conclusion that this pollen is comparatively "dry," using the word in a "relative sense." He a.s.serts that "nowhere but on the corbicula and hind metatarsal brushes did I find the sticky pollen, except sometimes on the tips of the long, branched hairs on the back (upper) edges of the tibiae and femora of the middle legs, and then only in heavily laden bees, where it is reasonable to suppose it had collected accidentally as the result of contact with the hind metatarsal brushes."

These and other considerations lead Sladen to think that, in the case of the b.u.mblebee at least, the pollen "may be moistened on the hind metatarsus with the tongue." He states that the tongue of the b.u.mblebee is of sufficient length to reach the hind metatarsus (planta) and that it might rub over the brushes of the metatarsi or be caught between them when they are approximated and thus moisten the two brushes simultaneously. However, he has never seen the tongue of the collecting honey bee brought near to the hind legs, and it appears probable to him that it can not easily reach them. "Possibly the middle or front legs are used as agents for conveying the honey" (in the case of the honey bee). "In the humblebee the tongue is longer, and it could more easily moisten the hind legs in the way suggested."

In an earlier paper Sladen (1912, _a_) gives the following as his opinion of the "way in which pollen dust is moistened with nectar,"

although he states that this is one of the points "which still remains obscure":

The only satisfactory manner in which, it seems to me, this can be done is for the tongue to lick the tarsi or metatarsi of the forelegs, which are covered with stiff bristles, well suited for holding the nectar, the nectar being then transferred to the metatarsal brushes on the middle legs, and from these, again, to the metatarsal brushes on the hind legs. The latter being thus rendered sticky, the pollen dust would cling to them. The different pairs of legs were certainly brought together occasionally, but not after every sc.r.a.pe of the hind metatarsi, and their movements were so quick that it was impossible to see what was done. Still, several pollen-collecting bees that I killed had the tarsi and metatarsi of the forelegs and the metatarsal brushes of the middle and hind legs moistened with nectar, and I think it probable that the moistening process, as outlined, is performed, as a rule, during the flight from flower to flower.

Sladen (1912, _c_) also considers the possibility that the fluid which moistens the pollen might be secreted through the comb at the end of the tibia, through the tibio-tarsal joint, or from the surface of the auricle, but finds no evidence of glandular openings in these regions.

A suggestion of a similar nature, apparently unknown to Sladen, was made by Wolff (1873), who describes "sweat-glands" which, he claims, are located within the hind tibia and the planta, and which pour a secretion upon the surface of the corbicula and upon the upper end of the planta through many minute openings located at the bases of hairs, particularly those which arise from the lateral margins of the corbicula. Wolff is convinced that the fluid thus secreted is the essential cohesive material by which the grains of pollen are bound together to form the solid ma.s.s which fills each fully loaded basket.

He noticed that the mouthparts are used to collect pollen, and that some of it is moistened with "honey" or "nectar," but he does not consider that the fluid thus supplied is sufficient to explain adequately the facility with which the collecting bee brings together the scattered grains of pollen and packs them away securely in the baskets. Wolff's description of the basket-loading process itself is strikingly similar to that advocated later by Cheshire.

The writer is not prepared to deny the possibility that the surface of the chitin of the hind legs of worker bees may be moistened by the secretion of glands which lie beneath it, but he is convinced that any fluid thus secreted bears little or no relation to the cohesion of the pollen grains within the baskets. Sections and dissected preparations of the hind legs of worker bees show certain large cells which lie within the cavity of the leg and which may function as secreting gland cells; but similar structures occur in even greater numbers within the hind legs of the drone and they are found within the hind legs of the queen.

As has been noted, the extreme moisture of the plantar combs and of the tibio-tarsal articulation of the hind leg is readily understood when one recalls the manner in which moist pollen is compressed between the auricle and the tibial surface above it.

From the account already given it is evident that, in the opinion of the writer, the mouth is the source from which the pollen-moistening fluid is obtained. It is extremely difficult to determine with absolute accuracy the essential steps involved in the process of adding moisture to the pollen. In an endeavor to solve this problem the observer must of necessity consider a number of factors, among which may be noted (1) the location upon the body of the collecting bee of "moist" and of comparatively "dry" pollen, (2) the movements concerned in the pollen-gathering and pollen-transferring processes, (3) the relative moisture of those parts which handle pollen, (4) the chemical differences between the natural pollen of the flower and that of the corbiculae and of the cells of the hive, and (5) the observer must endeavor to distinguish between essential phenomena and those which are merely incidental or accidental.

In the first place it should be noted that the relative dampness of pollen within the corbiculae 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 pa.s.sed 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 pa.s.sed over the auricle, has been pressed upward and squeezed between the auricle and the end of the tibia and the pollen ma.s.s 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 pa.s.sing 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 ma.s.ses, 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 ma.s.ses. 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 pa.s.sed backward, and in its pa.s.sage it imparts additional moisture to those body regions which it touches, the brushes of the fore and middle legs, the plantae of the hind legs, and the hairs of the breast which are sc.r.a.ped over by the fore and middle leg brushes. This moist pollen, in its pa.s.sage 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 ma.s.ses of wet pollen and itself becomes wet by contact. Pollen grains which reach the corbiculae 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 corbiculae 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 ma.s.ses 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 ma.s.ses 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 pa.s.sed 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 plantae as they rub upon them. I am also convinced that the wet pollen grains furnish additional moisture to the brushes as they pa.s.s 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 a.n.a.lysis of pollen from a plant as compared with that found in the corbiculae 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, a.n.a.lyses were made of three kinds of pollen, as follows: (1) Pollen collected by hand from the corn plant itself; (2) pollen taken from the corbiculae 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 a.n.a.lyses:

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 a.n.a.lyses show conclusively that a very large amount of sugar has been added to the pollen by the time it reaches the corbiculae.

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 a.n.a.lyses 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 ma.s.ses. 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 ma.s.ses 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 ma.s.s.

The middle leg now presses downward upon the pollen ma.s.s, working in between it and the corbicular surface, so that the ma.s.s is shoved outward and downward and falls into the cell. As the pollen ma.s.ses 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 a.s.sure 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 ma.s.s from the corbicula.

This structure is in close proximity with the ma.s.s 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 ma.s.s, but the spur may be used as an entering wedge.

Pollen ma.s.ses 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 ma.s.s securely in the bottom of the cell. As is shown by the a.n.a.lyses 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 a.s.semble the pollen, collecting it from the body parts to which it first adheres and transporting it to the pollen baskets or corbiculae 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.