Common Name: Bumblebee – An onomatopoeia for a bee that bumbles, which can mean either to blunder or to make a humming sound, the latter definition inferred. Known as humblebee in the United Kingdom.
Scientific Name: Bombus spp – The generic name is Latin for ‘a deep, hollow sound.’ The use of the abbreviation for species (spp) is a general reference to all of the species of the genus rather than to any one in particular.
Potpourri: Bumblebees are generally characterized by a robust body covered with alternating bands of yellow and black hairs called setae, their apparent corpulence due to the hirsute pile covering their wasp-waist midsection. Globally, there are some 250 species primarily in the northern hemisphere of which about 50 are native to North America. They are well adapted to cold climates, with colonies above the Arctic Circle on Ellesmere Island in northern Canada. In addition to the insulation layer that their hirsute covering provides, bumblebees have a mechanism to raise their internal temperature to the 86 degrees F temperature necessary for flight. In essence the bee shivers, using the flight muscles decoupled from the wings to generate heat. It takes about 5 minutes to warm up from 55 degrees and 15 minutes from 40 degrees, one reason you may find bumblebees on the ground shivering to get warm enough to fly in colder weather. Once warmed, they can maintain flight at temperatures as low as 50 degrees. The buzzing sound for which the bumblebee gets its name is the result of the vibration of the flight muscles and not, as is commonly thought, due to the beating of the wings.
The two other most notable features of the bumblebee are its long and feathery tongue called a glossa (the family is sometimes referred to as ‘the long-tongued bees’) and its ‘hairbrush’ legs which are equipped with a basket-like appendage called a corbicula. Both of these evolutionary adaptations are related to the collection and storage of their sole food sources, the nectar and pollen of flowers; the tongue collects nectar and the legs store pollen. Although it is axiomatic that bumblebees must therefore have flowers to survive just as most flowers must have the pollinators to survive, it is not as obvious that the bees are obligate (dependent). That bumblebees are highly efficient in collecting pollen and nectar is a necessity of the energetic nature of their physiology. The sugary nectar is more important than pollen for immediate survival, as it can be both stored and consumed as the situation demands.
Collection of nectar is facilitated by the long glossa, as the bumblebee is able to penetrate to the deepest depths of the flower beyond the reach of most other insects. Energy management is the province of the honeystomach, which is a sac in the abdomen lined with cuticle (a tough, outer epidermal layer) that has a volume of about one tenth of a milliliter. When it is filled with nectar, it takes up almost the entire abdomen and comprises about 90 percent of the bees weight. It is estimated that a bumblebee can fly for 40 minutes with a full honeystomach. Flying takes a lot of energy as the wings beat about 200 times a second; the metabolic rate of the bumblebee is almost twice that of the hummingbird. Nectar gathering is a prodigious task since most flowers only contain about a thousandth of a milliliter of nectar. It therefore takes about 100 flowers to fill the honeystomach, and, since the nectar is likely to have been already extracted from some flowers by other pollinators, it may take twice that many. Bumblebees forage as far as 2 kilometers from their nests and fly at about 15 meters per second (about 35 mph) which means the roundtrip takes about 5 minutes. If the bumblebee only needs 5 minutes to find 100 flowers with nectar, then as much as one quarter of the usable nectar energy would be depleted in the food gathering process; however, the average is estimated to be about 10 percent. The remainder of the nectar goes back to the nest to feed the brood.
Pollen is the lifeblood of the reproductive cycle that sustains the bumblebee species, it is a protein and vitamin B rich food source with many minerals including potassium, calcium, and magnesium. Bee pollen is marketed as a health food supplement with the usual hyperbole as to its efficacy for human health. Its collection by bumblebees is incident to the collection of nectar, as the pollen of the flowers’ anthers is deposited on the head and on the setae of the thorax during ingress and egress. The pollen is pushed down with the forelegs, where it is collected in a specialized section of the upper leg or tibia that consists of a shiny concave surface surrounded by bristles. This structure is called the pollen basket or corbicula (corbula means little basket in Latin). The pollen baskets are filled from the bottom up and each can hold about one million individual pollen grains which amounts to about one tenth of a gram. When the baskets are full, the rear legs appear swollen with yellow or orange coloration according to the nature of the pollen.
The queen bumblebee hibernates in the fall after having mated to emerge in the spring to form an individual colony that will last only until the beginning of winter and eventually consist of about fifty members. The first step in colonization is the location of a suitable nest site, normally a preexisting opening in the ground such as the abandoned burrow of a hypogeal (below ground) mammal. The search process is characterized by what appears to be a meander; one frequently encounters bumblebees flying into the dark corners of a garage or into a coat pocket, only to emerge when stymied on a new vector to the next likely looking spot, continuing with instinctual due diligence until an appropriate location is found. The colonization process continues with the construction of a honey pot made from wax which the bee produces as an exudate that emerges from between the abdominal segments. The honey pot is filled with nectar to provide nourishment during the laying and brooding of the eggs. Pollen collection follows; when one encounters a bumblebee with corbiculae filled with pollen in the spring, it is likely a queen who has found a nesting site. The pollen is moistened with nectar and formed into a ball that is known as ‘bee bread.’ Ingestion of some of the pollen by the queen bee stimulates the ovaries to produce eggs, which are subsequently deposited in groups ranging from 4 to 16 on the bee bread ball which is then covered by a protective layer of wax. The proximity of the nectar filled honey pot allows the queen to brood the eggs and to use the wing muscle shivering to keep them above a temperature of 86 degrees F which is necessary for the proper growth of the larvae. During the four-day period required for gestation, the queen leaves the nest only to gather enough nectar to keep warm. It is estimated that over five thousand flowers need to be visited daily to sustain the brood at the proper temperature.
Bumblebees have a complete metamorphic cycle that includes egg, larva, pupa and adult stages. The larvae that hatch from the eggs after about four days have dietary practices that vary somewhat according to the species. They subsist primarily on bee bread that is either provided for them to eat on their own or fed to them by the queen bee. Nectar is regurgitated into the larval cell cavity as a supplement to the pollen. The larvae eat voraciously but do not defecate, as they have no anal opening (also known as a blind gut). The fecal waste matter is maintained internally so that the cell and the food that is supplied to it remain uncontaminated. When the larva pupates, the collected feces are deposited into the cocoon and are applied to the inside of the silk enclosure as a protective layer of sealant. It takes about 5 weeks during which there are 4 or 5 instars between molts for the bumblebee to completely metamorphose into the imago or adult stage. Statistically, about half of the eggs laid reach adulthood. The queen starts a second brood while the first is in the larva-pupa stage to establish a sustainable colony size.
The first adult bumblebees to emerge are female workers or gynes who take over the process of food gathering and larval feeding from the queen so that she can concentrate on egg laying and brooding. The ovary of the queen bumblebee contains several hundred ovarioles that each contain about 60 eggs. When the eggs move down the oviduct to be laid, they pass by a sperm reservoir called the spermatheca where sperm from mating with the male drones the previous fall is stored. The queen decides on the sex of the egg by relaxing a muscular ring about the sperm duct. If a sperm is released, the egg is fertilized and a female results. If a sperm is not released, the egg is unfertilized and a male results. Male bumblebees produced by the queen do not have a full set of chromosomes and are referred to as haploid, as this is the number (n) of chromosomes in each of the male and female gametes. The female bumblebees are diploid, having 2n chromosomes, the result of a union of n chromosomes from the male gamete sperm and the n chromosomes of the female gamete ovum.
The determination of sex is made according to the age of the colony, worker females predominate in the beginning to do the work of building the colony with an eventual shift to the otiose male drones whose only function is to mate with the queens for the next generation. New queen bumblebees are also produced later in the life of the colony, their production triggered by a change in the pheromones of the original queen. The new queens are fed more frequently and for a longer period than the other larvae. The mating process begins with the departure of the males from the nest, their path marked by pheromones they secrete from their mandibles according to the specific species of bumblebee. The new queen bumblebees follow, attracted to the appropriate venue for tryst. Mating takes up to an hour, most of which is needed for the hardening of the genital plug that is provided by the male to seal in the sperm after deposition. The fertilized new queen bumblebees seek shelter for winter hibernation to start new colonies in the following year; all of the drones, gynes and “reigning” old queens expire with the onset of winter.
Bumblebees are in the order Hymenoptera along with the ants, wasps and in the family Apidae with the other bees (Apis is Latin for bee). The hymenopterans are among the most prolific of insects groupings with more than 100,000 species. They are named for their most common characteristic, membranous wings for the Greek hymen meaning membrane and ptero meaning wings, or, since Hymen is also the Greek god of marriage, the name could refer to the fact that the two wings on each side of the bees and the wasps are linked together with small hooks called hamuli so that they operate in harmony. There is no universal characteristic of hymenopterans, though they for the most part have diaphanous wings, chitinous exoskeletons and a narrow waist between the thorax and the abdomen. The specialization of the hymenopterans (bees, wasps and ants) is clearly of benefit to the colony but it is not necessarily of benefit to the individual, is referred to as eusociality, the highest level of social organization. It is characterized by reproductive division of labor (the queen), overlapping generations, and mutual care of the young. Bumblebees are considered weakly eusocial because the female workers can and do produce male offspring, though they are discouraged by the queen through a combination of aggression and pheromones.