The Behavior of the Honey Bee in Pollen Collection.

by D. B. Casteel.

LETTER OF TRANSMITTAL.

U. S. Department of Agriculture,

Bureau of Entomology,

_Washington, D. C, September 23, 1912_.

Sir: I have the honor to transmit herewith a ma.n.u.script ent.i.tled "The Behavior of the Honey Bee in Pollen Collecting," by Dr. Dana B.

Casteel, of this bureau. The value of the honey bee in cross pollinating the flowers of fruit trees makes it desirable that exact information be available concerning the actions of the bee when gathering and manipulating the pollen. The results recorded in this ma.n.u.script are also of value as studies in the behavior of the bee and will prove interesting and valuable to the bee keeper. The work here recorded was done by Dr. Casteel during the summers of 1911 and 1912 at the apiary of this bureau.

I recommend that this ma.n.u.script be published as Bulletin No. 121 of the Bureau of Entomology.

Respectfully,

L. O. Howard,

_Entomologist and Chief of Bureau_.

INTRODUCTION.

While working upon the problem of wax-scale manipulation during the summer of 1911 the writer became convinced that the so-called wax shears or pinchers of the worker honey bee have nothing whatever to do with the extraction of the wax scales from their pockets, but rather that they are organs used in loading the pollen from the pollen combs of the hind legs into the corbiculae or pollen baskets (Cast eel, 1912). Further observations made at that time disclosed the exact method by which the hind legs are instrumental in the pollen-loading process and also the way in which the middle legs aid the hind legs in patting down the pollen ma.s.ses. During the summer of 1912 additional information was secured, more particularly that relating to the manner in which pollen is collected upon the body and legs of the bee, how it is transferred to the hind legs, how it is moistened, and finally the method by which it is stored in the hive for future use. In the present paper a complete account will be given of the history of the pollen from the time it leaves the flower until it rests within the cells of the hive. The points of more particular interest in the description of pollen manipulation refer to (1) the movements concerned in gathering the pollen from the flowers upon the body and legs, (2) the method by which the baskets of the hind legs receive the loads which they carry to the hive, and (3) the manner in which the bee moistens pollen and renders it sufficiently cohesive for packing and transportation.

THE STRUCTURES CONCERNED IN THE MANIPULATION OF POLLEN.

The hairs which cover the body and appendages of the bee are of the utmost importance in the process of pollen gathering. For the purposes of this account these hairs may be cla.s.sified roughly as (1) branched hairs and (2) unbranched hairs, the latter including both long, slender hairs and stiff, spinelike structures.

Of these two cla.s.ses the branched hairs are the more numerous. They make up the hairy coat of the head, thorax, and abdomen, with the exception of short sensory spines, as those found upon the antennae and perhaps elsewhere, and the stiff unbranched hairs which cover the surfaces of the compound eyes (Phillips, 1905). Branched hairs are also found upon the legs; more particularly upon the more proximal segments. A typical branched hair is composed of a long slender main axis from which spring numerous short lateral barbs. Grains of pollen are caught and held in the angles between the axis and the barbs and between the barbs of contiguous hairs. The hairy covering of the body and legs thus serves as a collecting surface upon which pollen grains are temporarily retained and from which they are later removed by the combing action of the brushes of the legs. Although, as above noted, some unbranched hairs are located upon the body of the bee, they occur in greatest numbers upon the more distal segments of the appendages.

They are quite diverse in form, some being extremely long and slender, such as those which curve over the pollen baskets, others being stout and stiff, as those which form the collecting brushes and the pecten spines.

The mouthparts of the bee are also essential to the proper collection of pollen. The mandibles are used to sc.r.a.pe over the anthers of flowers, and considerable pollen adheres to them and is later removed.

The same is true of the maxillae and tongue. From the mouth comes the fluid by which the pollen grains are moistened.

The legs of the worker bee are especially adapted for pollen gathering. Each leg bears a collecting brush, composed of stiff, unbranched hairs set closely together. These brushes are located upon the first or most proximal tarsal segment of the legs, known technically as the palmae of the forelegs and as the plantae of the middle and hind pair. The brush of the foreleg is elongated and of slight width (fig. 1), that of the middle leg broad and flat (fig. 2), while the brush upon the planta of the hind leg is the broadest of all, and is also the most highly specialized. In addition to these well-marked brushes, the distal ends of the tibiae of the fore and middle legs bear many stiff hairs, which function as pollen collectors, and the distal tarsal joints of all legs bear similar structures.

[Ill.u.s.tration: Fig. 1.--Left foreleg of a worker bee. (Original.)]

The tibia and the planta of the hind leg of the worker bee are greatly flattened. (See figs. 3, 4.) The outer surface of the tibia is marked by an elongated depression, deepest at its distal end, and bounded laterally by elevated margins. From the lateral boundaries of this depression spring many long hairs, some of which arch over the concave outer surface of the tibia and thus form a kind of receptacle or basket to which the name corbicula or pollen-basket is given. The lower or distal end of the tibia articulates at its anterior edge with the planta. The remaining portion of this end of the tibia is flattened and slightly concave, its surface sloping upward from the inner to the outer surface of the limb. Along the inner edge of this surface runs a row of short, stiff, backwardly directed spines, from 15 to 21 in number, which form the pecten or comb of the tibia. The lateral edge of this area forms the lower boundary of the corbicula r depression and is marked by a row of very fine hairs which branch at their free ends. Immediately above these hairs, springing from the floor of the corbicula, are found 7 or 8 minute spines, and above them one long hair which reaches out over the lower edge of the basket.

The broad, flat planta (metatarsus or proximal tarsal segment of the hind leg) is marked on its inner surface by several rows of stiff, distally directed spines which form the pollen combs. About 12 of these transverse rows may be distinguished, although some of them are not complete. The most distal row, which projects beyond the edge of the planta, is composed of very strong, stiff spines which function in the removal of the wax scales (Casteel, 1912). The upper or proximal end of the planta is flattened and projects in a posterior direction to form the auricle. The surface of the auricle is marked with short, blunt spines, pyramidal in form, and a fringe of fine hairs with branching ends extends along its lateral edge. This surface slopes upward and outward.

[Ill.u.s.tration: Fig. 2.--Left middle leg of a worker bee. (Original.)]

THE POLLEN SUPPLY.

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 cla.s.s 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 ill.u.s.trated by the case of white clover. If bees are watched while working upon clover flowers, the observer will soon perceive some which bear pollen ma.s.ses 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.

[Ill.u.s.tration: Fig. 3.--Outer surface of the left hind leg of a worker bee. (Original.)]

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.

[Ill.u.s.tration: Fig. 4.--Inner surface of the left hind leg of a worker bee. (Original.)]

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, a.s.sisted 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 ta.s.sel 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 ma.s.ses 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 ma.s.ses 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 corbiculae in a manner to be described later.