L07-Lab-seeley_95 - bee Apis mellifera the familiar bee...

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Unformatted text preview: bee, Apis mellifera, the familiar bee used for most of the world’s beekeeping. This remarkable social insect is native to Europe, the Middle East, and the whole of Africa, and has been introduced by bee- keepers to the Americas, Asia, Australia, and the Pacific Islands. Most of the information in this chapter applies to Apis mellifera across its immense range, but some ecological and sociological aspects pertain only to the cold temperate regions of the world, particularly parts of northern Europe and North America. In these seasonally cold regions, a honey bee colony must stockpile a large quantity—20 or more kg— of honey as fuel for keeping itself warm throughout the winter, and certain features of the social organization of temperate-zone colonies reflect this need to amass a huge energy reserve. The honey bee has been the subject of scientific observations since ancient times, and today there are scores of excellent books that de- scribe its basic biology. The most important of these are Ribbands’s Behaviour and Social Life of Honeybees (1953), Snodgrass’s Anatomy of the Honey Bee (1956), Lindauer’s Communication among Social Bees (1961), von Frisch’s Dance Language and Orientation of Bees (1967), Michener’s Social Behavior of the Bees (1974), Seeley’s Honeybee Ecology (1985), Erickson, Carlson, and Garment’s A Scanning Electron Micro- scope Atlas of the Honey Bee (1986), Winston’s Biology of the Honey Bee (1987), Ruttner’s Biogeography and Taxonomy of Honeybees (1988), Crane’s Bees and Beekeeping (1990), and Moritz and Southwick’s Bees as Superorganisms (1992). These publications should be consulted for In this chapter, I discuss the natural history of colonies of the honey detailed information on the topics touched on here. In this chapter, I make no attempt to provide thorough reviews of the subjects raised; rather I aim to provide readers with selected background information that is needed for a ready understanding of the subsequent chapters. 2.1. Worker Anatomy and Physiology Figure 2.1 shows an adult worker bee from the side. As in most other insects, the body consists of three anatomical sections: (1) the head, with mouthparts and sensory organs such as eyes and antennae; (2) the thorax, a locomotory center which is almost entirely filled with muscles that operate the membranous wings and jointed legs, and (3) the abdomen, nfore spacious than the other parts, which holds the organs for various functions, including digestion, circulation, and stinging. compound eye antenna The Honey Bee Colony Figure 2.1 External structure of the worker honey bee with the hairy covering removed. Upper right: detail showing the pollen basket on the outer surface of the hind legs. After Snodgrass 1956. 23 Figure 2.2 A worker honey bee foraging on buckwheat flowers. Note the proboscis, which is unfolded to probe for nectar, and the load of pollen packed on the outer surface of the hind leg. Photograph by T. D. Seeley. 24 2.1.1. EXTERNAL STRUCTURE The mouthparts of a bee comprise two sets of tools, one for chewing and one for sucking. The principal chewing structures are the rigid, jawlike mandibles. They are used to manipulate wax, masticate pollen pellets, gather plant resins, groom hivemates, cut open flow- ers to reach otherwise inaccessible nectar, and even grip an enemy to gain a firm purchase for implanting the sting. Sucking up liquids is accomplished with the proboscis, a folding structure built of several mouthparts that form a tube around the bee’s tongue. Liquids in this tube move upward toward the mouth (located at the base of the pro- boscis) as a result of the in—and-out movements of the bee’s tongue, suction from the mouth, and perhaps also capillary action. The pro— boscis evolved for the function of taking in nectar, but it is also used for gathering water, exchanging food with nestmates, licking sub— stances such as pheromones from other bees, and spreading nectar and water for rapid evaporation inside the hive. When not in use, the proboscis is folded out of the way in a large groove on the underside of the head. Solid food, mainly pollen, cannot be ingested through the proboscis, but is taken directly into the mouth after being broken up into small particles by the mandibles. The legs of a bee serve not only in locomotion, but also in food col- lection, for they bear special structures for transporting pollen, a dry, dustlike material. The outer side of the broad tibial segment of each hind leg is adapted to form a pollen-holding device, the so-called pollen basket. Its surface is smooth, slightly concave, and bordered by a fringe of long incurved hairs. Pollen, after being moistened with nectar, is packed into this basket and held in place by the hairs. Bees that are engaged in pollen collection are recognized instantly by the conspicuous balls of bright-colored pollen packed onto their hind legs (Figure 2.2). The pollen baskets are also used for transporting resin, which is gathered from sticky tree buds and used in nest construc- tion. The sting apparatus lies tucked inside a special sting chamber within the last abdominal segment. It is a modified ovipositor, or egg- laying tube. The shaft of the sting consists of two barbed lancets and a stylet which fit together to form a venom canal inside the sting’s shaft. Venom is produced in a poison gland, which widens to form a sac in which venom is stored. When the bee stings, she forces venom Introduction W4» ‘HWW mmqrminxWWWn—mmma...” .. .. .. . into the venom canal and the sharp lancets are pushed into the tissue of the animal under attack. When the bee tries to retract her sting from tough skin, or the enemy tries to brush off a stinging bee, the barbed lancets ensure that the sting apparatus remains embedded. 2.1.2. INTERNAL ORGANS The alimentary canal of a honey bee is shown in Figure 2.3. Just in— side the mouth is the cibarium, or pump, which the bee can dilate and contract to draw liquid food up the proboscis and into the esopha- gus. Food then passess through the thorax via the esophagus and into the honey stomach (or crop), which is tremendously expandable. When a forager has gathered a full load of nectar, the honey stomach is stretched until its walls are transparent and its bulk presses the rest of the viscera to the rear of the abdomen. The contents of the honey stomach are voluntarily regurgitated when the bee applies pressure to the distended crop by contracting the telescoping abdominal seg— ments. Pollen grains are transported to the honey stomach in solu- tion, and then are removed from the honey stomach by a special valve, the proventriculus, which passes them and some of the liquid food into the midgut. Here is where enzymes are added and most of thoracic flight muscle salivary gland honey head salivary gland hypopharyngeal gland mouth proventriculus esophagus Figure 2.3 Some of the internal organs of a worker honey bee. After Michener 1974. The Honey Bee Colony midgut wax glands venom 38C Nasanov's gland poison gland 25 26 the digestion and absorption occur. Posterior to this is the rectum, where water and feces are stored until the bee can fly from the hive to defecate. tually fill the thorax. These tissues, The large flight muscles vir g the highest known, not only pro- whose metabolic activity is amon vide the pOWer for flights to flowers outside the hive but also serve several functions inside the hive. Their action produces fanning of the wings by bees on the combs, to produce air currents which ventilate the hive. They also serve to generate heat. By uncoupling the flight muscles from the flight mechanism, and contracting them isometri- cally, a bee inside the hive can produce heat but no locomotion. The flight muscles also produce small Vibrations of the wings to make sounds during the communication dances of bees (see Section 2.5). Adult bees possess numerous se external secretions are either building materials, foo ces (pheromones). Here I describe the locations and functions of the ex— ternally secreting glands which play a role in a colony’s collection and storage of food. The hypopharyngeal glands are paired structures, one in each side of the head, which discharge just inside the mouth. The main ducts of these glands are massive discharge from individual cells along their entir duce two quite different secretions: in young bees (nurses), protein- aceous food for both the larval brood and older adults; and in older bees (food storers and foragers), enzymes which break down the su- crose in nectar, an important step in the honey—making process. Both the head and the thorax have salivary glands, which discharge through a common duct near the base of the proboscis. Their secretion is used to clean the queen’s body and is added to wax for softening when it is manipulated. In the abdomen, wax glands produce beeswax from which a colony’s combs are built. The wax is secreted as a scale, d mixed with salivary gland secretions un— whereupon it is chewed an til it is malleable. The Nasanov’s gland, located on the upper surface of the abdomen, produces several volatile compounds that, when dis- nestmates. For example, these persed by fanning the wings, attract secretions are used outside the hive to advertise the location of a rich food source. glands who d, or communication substan 2.1.3. SENSORY ORGANS A worker honey bee is exquisit ment needed to perceive a wi ely endowed with the sensory equip— de range of the mechanical, visual, Introduction chemical, and temperature stimuli in its world. Many of the mechanoreceptors are sensory hairs (trichoid sensilla) which respond to specific patterns of deflection or vibration. They are distributed widely over the body and appendages. Those located at the tip of each antenna, for instance, enable foragers to detect differences in the sur— faces of flower petals, which can be useful in locating the nectar in flowers. Other sensory hairs are grouped in bristle fields located at the neck and the base of the abdomen. They are deflected by any ten- dency of the head or abdomen to hang downward, and in this way serve in the perception of the gravitational force. Accordingly, the bee can know which way is down, and so can construct vertical hanging combs and perform communication dances—which are oriented with respect rto the vertical—in an appropriate fashion. Other mechanoreceptors are spindle—shaped stretch receptors (chordotonal sensilla), located in the joints of appendages, which register vibra- tions and positions of the body. For example, as a recent study (Dreller and Kirchner 1993) has revealed, bees following a dancing nestrnate perceive the airborne sounds produced by the dancer by sensing in- duced vibrations of their antennae with chordotonal sensilla (John- ston’s organ) located in the second antennal segment. Similar receptors in the legs enable bees to respond to the substrate-borne vi- brations used in other communication signals. The bee’s sense of smell is based on olfactory receptors located on the antennae. Each one contains some 3000 pore plates (sensilla pla— codea) which electrophysiological study has shown function in odor perception. There are also several other types of sensilla, some of which are no doubt involved in temperature perception, which is most acute in the antennae. Experimental studies, based mainly on testing the bee’s ability to discriminate between odors to locate food, indicate that bees are often far more sensitive to certain odors than are humans, especially floral odors and bee pheromones, and are roughly equal to humans in the task of discriminating between dif- ferent odors. Other chemoreceptors are located on the mouthparts, where they provide a sense of taste. The best studied are those in- volved in the perception of sweetness. Bees are not highly sensitive to sugars, for even in starved bees the behavioral response threshold is approximately 1/16 mol/ L. This is not surprising, however, since the sugar solutions which bees deal with in nature when gathering nectar are generally quite concentrated, in the range of 0.5 to 2.5 mol/ L. The Honey Bee Colony 27 embryo, 3rd day larval instars prepupa sass“ ”’lll‘t Figure 2.4 Stages in the development of a worker honey bee, from egg to pupa. Workers develop in the nearly horizontal cells that form the combs inside a beehive. There are five larval instars, or stages, each one sepa- rated by a molt in which the larva sheds its old skin and begins a new phase of growth. The so-called prepupa is merely the pupa in its early developmental stages within the skin of the fifth instar larva. When this larval skin is finally cast off, the insect appears in the form of an adult bee, and is called a pupa. After Dade 1977. 28 The bee’s principal visual receptors consist of two compound eyes which cover large parts of the sides of the head. Each eye is made up of a sheet of some 6900 visual units called ommatidia, spread over a convex surface so that each covers a different portion of the bee’s visual field. The divergence between the Visual fields of adjacent ommatidia is 1—4°, which is at least 100 times greater than the an- gular divergence between adjacent cones in the foveal region of the human eye. However, as Wehner and Srinivasan (1984) recently pointed out, the reduction in the bee’s Visual acuity that results from the high angular divergence of its visual units is largely compen- sated for by the small interaction distances between a bee and its subjects of visual scrutiny, such as a flower on which it is about to land. The bee presumably perceives its light environment by inte- grating in the nervous system the information received by the pho- toreceptor mosaic of the thousands of ommatidia in each compound eye. Each ommatidium registers both color and intensity, so that the bee is endowed with color vision as well as form vision. The bee’s visible spectrum differs, however, from our own, for a bee is highly sensitive to ultraviolet radiation (with wavelengths as short as 300 nm) but basically insensitive to red light (wavelengths greater than about 650 nm). 2.2. Worker Life History 2.2.1. DEVELOPMENT The development of worker bees is typical for an insect that under- goes complete metamorphosis. Each individual passes through four stages: egg, larva, pupa, and adult. The changes in appearance of a worker as it develops from an egg to an adult are shown in Figure 2.4 (see also Figure 2.6). The embryo grows inside the egg for 3 days, con- suming the protein—rich egg yolk. The larva, a whitish grub, then hatches from the egg and begins an intensive feeding stage, with its food supplied by the adult bees—a mixture of honey, pollen, and brood food secreted from the hypopharyngeal glands of the adult nurse bees. Larvae grow enormously, undergoing four molts (shed- dings of old cuticle or skin) and multiplying their weight by a factor of more than 2000 during the 6-day-long larval stage. A larval bee’s feeding ceases when she has lived about 8 days (5 of which have been spent as a larva), at which time the adult workers construct a wax Introduction capping that seals the larva in its cell. The fully grown larva then spins a cocoon of silk inside its cell, and orients itself with the head out- ward. A few days after cocoon spinning, the insect sheds its skin once again, now appearing as a fully formed pupa. The intricate process of pupal development actually starts before the last larval skin is shed, while the bee is a ”prepupa.” Pupal development is a recon- struction process in which a second set of cells, which had remained inactive in the larva, suddenly starts to divide rapidly. Their nour- ishment comes from the large larval cells, which are digested. This group of newly active cells forms the adult tissue, eventually replac— ing all the larval tissue, to give rise to the pupa, with its appearance of an adult bee. Finally the newly formed adult worker gnaws through the waxcap of the cell with her mandibles and emerges as a soft, young bee. 2.2.2. ADULT ACTIVITIES When a worker emerges from her cell in the comb, her anatomical features are fixed, but the full development of her glandular system takes places only afterward, in a complex pattern which mirrors the changes in the bee’s behavior over her life, Typically, brood food is secreted by young workers, beeswax by middle—aged bees, and en— zymes for converting nectar into honey by older workers. Figure 2.5 portrays the sequence of activities that unfolds over the lives of bees, as determined by monitoring the activities of one cohort of bees liv— ing in an observation hive. During the first few days of adult life, a worker functions primarily as a cell cleaner, cleaning and polishing recently vacated brood cells. She also devotes time to eating some of the pollen that is stored nearby, which favors the rapid activation of her hypopharyngeal glands. The worker also spends some 20% of her time resting—standing motionless on the combs or in a cell— and another 20% patrolling—walking about the combs, as if search- ing for work. By the time she reaches 3 days of age, she functions as a nurse, for her hypopharyngeal glands have begun secreting brood food and she has started spending much time feeding the brood. She also performs the other tasks that arise within the broodnest, including tending the queen, capping brood, and grooming and feeding nestrnates. This pattern continues for the next 10 days or so, or until she is about 12 days old. At this point she leaves the cen- tral broodnest to work primarily in the peripheral, food-storage region of the hive. Here she functions mainly as a food storer. Her The Honey Bee Colony 29 Figure 2.5 The behavioral changes of worker bees as they grow older. At each age, individ— uals specialize on a subset of the tasks needed to maintain the colony’s well-being. Typically, young workers concentrate on the jobs occur- ring in the central broodnest, such as cleaning cells, feeding brood, and tending the queen. Middle-aged bees work mainly on the periph— ery of the combs, receiving and storing nectar, packing pollen, and ventilating. The old work- ers function almost entirely outside the hive as foragers. Based on the data in figure 1 of Seeley 1982. 30 4O 20 Cleaning cells 10 Tending brood 10 Tending queen 10 Eating pollen 2‘ E 10 Feeding & grooming nestmates 6 cu . A ‘5 10 Ventilating nest 8 Sha in comb E 10 p g E, . % 10 Storing nectar E 10 Packing pollen ‘6 F 'n g 80 Greg: 9 £9 0:, 60 E g: 40 20 4° Patrolling 20 4O Resting 20 1 4 7 10 13 16 19 22 25 Age of bee (days) hypopharyngeal glands are secreting the enzymes needed for pro- ducing honey, and her poison gland has filled the venom sac. Shut— tling between the hive entrance and the upper honeycombs, she receives nectar from the returning nectar foragers, converts it to honey, and deposits this in the storage cells. She also packs pollen in cells, ventilates the hive by fanning her wings, helps guard the hive entrance, and continues grooming and feeding her hivemates. Also, Introduction if additional comb is needed for honey storage, these middle—aged bees will activate their wax glands and build comb. Finally, from the age of about 20 days until the end of life, a worker toils outside the hive as a forager, gathering nectar, pollen, water, resin, or some com- bination of these substances. The general sequence of activities depicted in Figure 2.5—from cell cleaner to nurse bee to food storer and finally to forager—is more or less fixed for worker bees, but there is tremendous variation among individuals in the effort expended on the different activities within each of the four sets of tasks. One worker may never undertake a cer- tain activity, while another may specialize in it for several days. For example, some food-storer bees never guard the hive entrance, while others spend a week or more specializing as guards. Likewise, some forager bees concentrate on pollen foraging wh...
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