CHAPTER XVII
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 from 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 at 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.
The lower lip, or labium (Plate XXV, 3, l), is the long central part of the so-called tongue; it bears on
PLATE XXIV. SUMAC IN BLOSSOM AND BLOSSOM OF MOUNTAIN MAPLE
The Sumac is an excellent honey-producer.
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, post-cerebral glands; 3 thoracic glands. Fig. 5.—The wax-plates (after Cheshire).
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 oesophagus 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 tracheae, which carry the air to all parts of the body. When the body of an insect is opened, the tracheae appear as silvery threads, on account of the contained air. In the adult honeybee certain of the tracheae are greatly expanded so as to form large air sacs. (Plate XXVI 6, 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 oesophagus 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 larvse during the first days of their development, to the queen larvae throughout their development, and to the adult queen during the egg-laying period. The food fed worker and drone larvae 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 oesophagus, 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.
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.
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 svstem of the honey-bee. s, s, s, spiracles; a, enlarged trachea or air sac (adapted from Leuckart).
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, hich 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.)