Part 4

When bees collect pollen from flowers they may be engaged in this occupation alone or may combine it with nectar gathering. From some flowers the bees take only nectar, from others only pollen; a third class of flowers furnishes an available supply of both of these substances. But even where both pollen and nectar are obtainable a bee may gather nectar and disregard the pollen. This is well illustrated by the case of white clover. If bees are watched while working upon clover flowers, the observer will soon perceive some which bear pollen masses upon their hind legs, while others will continue to visit flower after flower, dipping into the blossoms and securing a plentiful supply of nectar, yet entirely neglecting the pollen.

The supply of pollen which is available for the bees varies greatly among different flowers. Some furnish an abundant amount and present it to the bee in such a way that little difficulty is experienced in quickly securing an ample load, while others furnish but little. When flowers are small and when the bee approaches them from above, little, if any, pollen is scattered over the bee's body, all that it acquires being first collected upon the mouth and neighboring parts, of a Very different conditions are met with when bees visit such plants as corn and ragweed. The flowers of these plants are pendent and possess an abundant supply of pollen, which falls in showers over the bodies of the bees as they crawl beneath the blossoms. The supply of pollen which lodges upon the body of the bee will thus differ considerably in amount, depending upon the type of flower from which the bee is collecting, and the same is true regarding the location upon the body of a bee of pollen grains which are available for storage in the baskets. Moreover, the movements concerned in the collection of the pollen from the various body parts of the bee upon which it lodges will differ somewhat in the two cases, since a widely scattered supply requires for its collection additional movements, somewhat similar in nature to those which the bee employs in cleaning the hairs which cover its body.

GENERAL STATEMENT OF THE POLLEN-COLLECTING PROCESS.

A very complete knowledge of the pollen-gathering behavior of the worker honey bee may be obtained by a study of the actions of bees which are working upon a plant which yields pollen in abundance. Sweet corn is an ideal plant for this purpose, and it will be used as a basis for the description which follows.

In attempting to outline the method by which pollen is manipulated the writer wishes it to be understood that he is recounting that which he has seen and that the description is not necessarily complete, although he is of the opinion that it is very nearly so. The movements of the legs and of the mouthparts are so rapid and so many members are in action at once that it is impossible for the eye to follow all at the same time. However, long-continued observation, assisted by the study of instantaneous photographs, gives confidence that the statements recorded are accurate, although some movements may have escaped notice.

To obtain pollen from corn the bee must find a tassel in the right stage of ripeness, with flowers open and stamens hanging from them. The bee alights upon a spike and crawls along it, clinging to the pendent anthers. It crawls over the anthers, going from one flower to another along the spike, being all the while busily engaged in the task of obtaining pollen. This reaches its body in several ways.

As the bee moves over the anthers it uses its mandibles and tongue, biting the anthers and licking them and securing a considerable amount of pollen upon these parts. This pollen becomes moist and sticky, since it is mingled with fluid from the mouth. A considerable amount of pollen is dislodged from the anthers as the bee moves over them. All of the legs receive a supply of this free pollen and much adheres to the hairs which cover the body, more particularly to those upon the ventral surface. This free pollen is dry and powdery and is very different in appearance from the moist pollen masses with which the bee returns to the hive. Before the return journey this pollen must be transferred to the baskets and securely packed in them.

After the bee has traversed a few flowers along the spike and has become well supplied with free pollen it begins to collect it from its body, head, and forward appendages and to transfer it to the posterior pair of legs. This may be accomplished while the bee is resting upon the flower or while it is hovering in the air before seeking additional pollen. It is probably more thoroughly and rapidly accomplished while the bee is in the air, since all of the legs are then free to function in the gathering process.

If the collecting bee is seized with forceps and examined after it has crawled over the stamens of a few flowers of the corn, its legs and the ventral surface of its body are found to be thickly powdered over with pollen. If the bee hovers in the air for a few moments and is then examined very little pollen is found upon the body or upon the legs, except the masses within the pollen baskets. While in the air it has accomplished the work of collecting some of the scattered grains and of storing them in the baskets, while others have been brushed from the body.

In attempting to describe the movements by which this result is accomplished it will be best first to sketch briefly the roles of the three pairs of legs. They are as follows:

(a) The first pair of legs remove scattered pollen from the head and the region of the neck, and the pollen that has been moistened by fluid substances from the mouth.

(b) The second pair of legs remove scattered pollen from the thorax, more particularly from the ventral region, and they received the pollen that has been collected by the first pair of legs.

(c) The third pair of legs collect a little of the scattered pollen from the abdomen and they receive pollen that has been collected by the second pair. Nearly all of this pollen is collected by the pollen combs of the hind legs, and is transferred from the combs to the pollen baskets or corbiculæ in a manner to be described later.

It will thus be seen that the manipulation of pollen is a successive process, and that most of the pollen at least passes backward from the point where it happens to touch the bee until it finally reaches the corbiculæ or is accidentally dislodged and falls from the rapidly moving limbs.

ACTION OF THE FORELEGS AND MOUTHPARTS.

Although the pollen of some plants appears to be somewhat sticky, it may be stated that as a general rule pollen can not be successfully manipulated and packed in the baskets without the addition of some fluid substance, preferably a fluid which will cause the grains to cohere. This fluid, the nature of which will be considered later, comes from the mouth of the bee, and is added to the pollen which is collected by the mouthparts and to that which is brought into contact with the protruding tongue and maxillæ, and, as will appear, this fluid also becomes more generally distributed upon the legs and upon the ventral surface of the collecting bee.

When a bee is collecting from the flowers of corn the mandibles are actively engaged in seizing, biting, and scraping the anthers as the bee crawls over the pendent stamens. Usually, but not always, the tongue is protruded and wipes over the stamens, collecting pollen and moistening the grains thus secured. Some of the pollen may possibly be taken into the mouth. All of the pollen which comes in contact with the mouthparts is thoroughly moistened, receiving more fluid than is necessary for rendering the grains cohesive. This exceedingly wet pollen is removed from the mouthparts by the forelegs (fig. 5), and probably the middle legs also secure a little of it directly, since they sometimes brush over the lower surface of the face and the mouth. In addition to removing the very moist pollen from the mouth the forelegs also execute cleansing movements over the sides of the head and neck and the anterior region of the thorax, thereby collecting upon their brushes a considerable amount of pollen which has fallen directly upon these regions, and this is added to the pollen moistened from the mouth, thereby becoming moist by contact. The brushes of the forelegs also come in contact with the anterior breast region, and the hairs which cover this area become moist with the sticky exudation which the forelegs have acquired in the process of wiping pollen from the tongue, maxillæ, and mandibles.

ACTION OF THE MIDDLE LEGS.

The middle legs are used to collect the pollen gathered by the forelegs and mouthparts, to remove free pollen from the thoracic region, and to transport their load of pollen to the hind legs, placing most of it upon the pollen combs of these legs, although a slight amount is directly added to the pollen masses in the corbiculæ. Most of the pollen of the middle legs is gathered upon the conspicuous brushes of the first tarsal segments or plantæ of these legs.

In taking pollen from a foreleg the middle leg of the same side is extended in a forward direction and is either grasped by the flexed foreleg or rubbed over the foreleg as it is bent downward and backward. In the former movement the foreleg flexes sharply upon itself until the tarsal brush and coxa nearly meet. The collecting brush of the middle leg is now thrust in between the tarsus and coxa of the foreleg and wipes off some of the pollen from the foreleg brush. The middle leg brush is then raised and combs down over the flexed foreleg, thus removing additional pollen from the outer surface of this leg. The middle leg also at times reaches far forward, stroking down over the foreleg before it is entirely flexed and apparently combing over with its tarsal brush the face and mouthparts themselves. When the middle leg reaches forward to execute any of the above movements the direction of the stroke is outward, forward, and then back toward the body, the action ending with the brush of the leg in contact with the long hairs of the breast and with those which spring from the proximal segments of the forelegs (coxa, trochanter, femur). As a result of the oft-repeated contact of the brushes of the middle and forelegs with the breast, the long, branched hairs which cover this region become quite moist and sticky, since the brushes of these two pair of legs are wet and the pollen which they bear possesses a superabundance of the moistening fluid. Any dry pollen which passes over this region and touches these hairs receives moisture by contact with them. This is particularly true of the free dry pollen which the middle pair of legs collect by combing over the sides of the thorax.

The pollen upon the middle legs is transferred to the hind legs in at least two ways. By far the larger amount is deposited upon the pollen combs which lie on the inner surfaces of the plantæ of the hind legs. To accomplish this a middle leg is placed between the plantæ of the two hind legs, which are brought together so as to grasp the brush of the middle leg, pressing it closely between them, but allowing it to be drawn toward the body between the pollen combs of the two hind legs. (See fig. 5.) This action results in the transference of the pollen from the middle-leg brush to the pollen combs of the hind leg of the opposite side, since the combs of that leg scrape over the pollen-laden brush of the middle leg. This action may take place while the bee is on the wing or before it leaves the flower.

The middle legs place a relatively small amount of pollen directly upon the pollen masses in the corbiculæ. This is accomplished when the brushes of the middle legs are used to pat down the pollen masses and to render them more compact. (See fig. 6.) The legs are used for this purpose quite often during the process of Loading the baskets, and a small amount of pollen is incidentally added to the masses when the brushes come into contact with them. A misinterpretation of this action has led some observers into the erroneous belief that all or nearly all of the corbicular pollen is scraped from the middle-leg brushes by the hairs which fringe the sides of the baskets. The middle legs do not scrape across the baskets, but merely pat downward upon the pollen which is there accumulating.

It is also possible that, in transferring pollen from the middle leg of one side to the planta of the opposite hind leg, the middle-leg brush may touch and rub over the pecten of the hind leg and thus directly place some of its pollen behind the pecten spines. Such a result is, however, very doubtful.

ACTION OF THE HIND LEGS.

The middle legs contribute the major portion of the pollen which reaches the hind legs, and all of it in cases where all of the pollen first reaches the bee in the region of the mouth. However, when much pollen falls upon the body of the bee the hind legs collect a little of it directly, for it falls upon their brushes and is collected upon them when these legs execute cleansing movements to remove it from the ventral surface and sides of the abdomen. All of the pollen which reaches the corbiculæ, with the exception of the small amount placed there by the middle legs when they pat down the pollen masses, passes first to the pollen combs of the plantæ.

When in the act of loading pollen from the plantar brushes to the corbiculæ the two hind legs hang beneath the abdomen with the tibio-femoral joints well drawn up toward the body. (See fig. 7.) The two plantæ lie close together with their inner surfaces nearly parallel to each other, but not quite, since they diverge slightly at their distal ends. The pollen combs of one leg are in contact with the pecten comb of the opposite leg. If pollen is to be transferred from the right planta to the left basket, the right planta is drawn upward in such a manner that the pollen combs of the right leg scrape over the pecten spines of the left. By this action some of the pollen is removed from the right plantar combs and is caught upon the outer surfaces of the pecten spines of the left leg.

This pollen now lies against the pecten and upon the flattened distal end of the left tibia. At this moment the planta of the left leg is flexed slightly, thus elevating the auricle and bringing the auricular surface into contact with the pollen which the pecten has just received. By this action the pollen is squeezed between the end of the tibia and the surface of the auricle and is forced upward against the distal end of the tibia and on outward into contact with the pollen mass accumulating in the corbicula. As this act, by which the left basket receives a small contribution of pollen, is being completed, the right leg is lowered and the pecten of this leg is brought into contact with the pollen combs of the left planta, over which they scrape as the left leg is raised, thus depositing pollen upon the lateral surfaces of the pecten spines of the right leg. (See fig. 7.)

Right and left baskets thus receive alternately successive contributions of pollen from the planta of the opposite leg. These loading movements are executed with great rapidity, the legs rising and falling with a pump-like motion. A very small amount of pollen is loaded at each stroke and many strokes are required to load the baskets completely.