The Bumblebee

 
Bombus terrestris

A bumblebee is any member of the bee genus Bombus, in the family Apidae. There are over 250 known species,  existing primarily in the Northern Hemisphere although they also occur in South America. They have been introduced to New Zealand and the Australian state of Tasmania.
Bumblebees are social insects that are characterised by black and yellow body hairs, often in bands. However, some species have orange or red on their bodies, or may be entirely black.  Another obvious (but not unique) characteristic is the soft nature of the hair (long, branched setae), called pile, that covers their entire body, making them appear and feel fuzzy. They are best distinguished from similarly large, fuzzy bees by the form of the female hind leg, which is modified to form a corbicula: a shiny concave surface that is bare, but surrounded by a fringe of hairs used to transport pollen (in similar bees, the hind leg is completely hairy, and pollen grains are wedged into the hairs for transport).

Like their relatives the honey bees, bumblebees feed on nectar and gather pollen to feed their young.

Biology

A bumblebee Bombus terrestris with pollen in its pile: the contrasting colours in the pile are a warning to predators.

The blood or hemolymph, as in other arthropods, is carried in an open circulatory system.  The body organs, "heart" (dorsal aorta), muscles, etc. are surrounded in a reservoir of blood. The dorsal aorta does pulse blood through its long tube, though, so there is a circulation of sorts.

In fertilised queens the ovaries are activated when the queen lays her egg. It passes along the oviduct to the vagina. In the vagina there is a chamber called the spermatheca. This is where the queen stores sperm from her mating. The queen, depending on need, may allow her egg to be fertilised. Non-fertilised eggs become males, and only fertilised eggs grow into females and queens.

As in all animals, hormones play a significant role in the growth and development of the bumblebee. The hormones that stimulate the development of the ovaries are suppressed in female worker bees, while the queen remains dominant. Salivary glands in the head secrete saliva, which mixes with the nectar and pollen. Saliva is also mixed into the nest materials to soften them. The body fat is a nutritional store; before hibernation, queens eat as much as they can to enlarge their fat body, and the fat in the cells is used up during hibernation.

A bumblebee Bombus pascuorum extending its tongue towards a Heuchera inflorescence

Like all bee tongues, the bumblebee tongue (the proboscis) is a long hairy structure that extends from a sheath-like modified maxilla. The primary action of the tongue is lapping, i.e. repeated dipping of the tongue into liquid.  During lapping, nectar is drawn up the proboscis by capillary action. When at rest or flying, the proboscis is kept folded under the head. The exoskeleton of the abdomen is divided into plates called dorsal tergites and ventral sternites. Wax is secreted from glands on the sternites.

The brightly coloured pile of the bumblebee is a form of aposematic signal. Depending on the species and morph, these colours can range from entirely black, to bright yellow, red, orange, white, and pink. Thick pile can also act as insulation to keep the bee warm in cold weather. Further, when flying, a bee builds up an electrostatic charge, and as flowers are usually well grounded, pollen is attracted to the bee's pile when it lands. When a pollen-covered bee enters a flower, the charged pollen is preferentially attracted to the stigma because it is better grounded than the other parts of the flower.

Bumblebees do not have ears; however, they can feel the vibrations of sounds through nearby materials.

Habitat

Bumblebees are typically found in higher latitudes and/or high altitudes, though exceptions exist (there are a few lowland tropical species).  A few species (Bombus polaris and B. alpinus) range into very cold climates where other bees might not be found; B. polaris can be found in northern Ellesmere Island—the northernmost occurrence of any eusocial insect—along with its parasite, B. hyperboreus.  One reason for this is that bumblebees can regulate their body temperature, via solar radiation, internal mechanisms of "shivering" and radiative cooling from the abdomen (called heterothermy). Other bees have similar physiology, but the mechanisms have been best studied in bumblebees.

Nests

Bombus terrestris inside an artificial pollination nest, the queen rocks her wings.

Bumblebees form colonies, which are usually much less extensive than those of honey bees. This is due to a number of factors including the small physical size of the nest cavity, the responsibility of a single female for the initial construction and reproduction that happens within the nest, and the restriction of the colony to a single season (in most species). Often, mature bumblebee nests will hold fewer than 50 individuals. Bumblebee nests may be found within tunnels in the ground made by other animals, or in tussock grass as opposed to Carpenter Bees that burrow into wood. Bumblebees sometimes construct a wax canopy ("involucrum") over the top of their nest for protection and insulation. Bumblebees do not often preserve their nests through the winter, though some tropical species live in their nests for several years (and their colonies can grow quite large, depending on the size of the nest cavity). In temperate species, the last generation of summer includes a number of queens who overwinter separately in protected spots. The queens can live up to one year, possibly longer in tropical species.

Colony cycle

Bumblebee nests are first constructed by over-wintered queens in the spring (in temperate areas). Upon emerging from hibernation, the queen collects pollen and nectar from flowers and searches for a suitable nest site. The characteristics of the nest site vary among bumblebee species, with some species preferring to nest in underground holes and others in tussock grass or directly on the ground. Once the queen finds a site, she prepares wax pots to store food, and wax cells to lay eggs in. These eggs then hatch into larvae, which cause the wax cells to expand isometrically into a clump of brood cells.

A bumblebee Bombus terrestris enlarging her nest hole

To develop, these larvae must be fed both nectar for carbohydrates and pollen for protein. Bumblebees feed nectar to the larvae by chewing a small hole in the brood cell into which they regurgitate nectar. Larvae are fed pollen in one of two ways, depending on the bumblebee species. So-called "pocket-maker" bumblebees create pockets of pollen at the base of the brood-cell clump that the larvae feed themselves from. Conversely, "pollen-storers" store pollen in separate wax pots and feed it to the larvae in the same fashion as nectar.  Bumblebees are incapable of trophallaxis (direct transfer of food from one bee to another).

With proper care, the larvae progress through four instars, becoming successively larger with each moult. At the end of the fourth instar, the larvae spin silk cocoons under the wax covering the brood cells, changing them into pupal cells. The larvae then undergo an intense period of cellular growth and differentiation and become pupae. These pupae then develop into adult bees, and chew their way out of the silk cocoon. When adult bumblebees first emerge from their cocoons, the hairs on their body are not yet fully pigmented and are a greyish-white colour. The bees are referred to as "callow" during this time, and they will not leave the colony for at least 24 hours. The entire process from egg to adult bee can take as long as five weeks, depending on the species and the environmental conditions.

After the emergence of the first or second group of workers, workers take over the task of foraging and the queen spends most of her time laying eggs and caring for larvae. The colony grows progressively larger and at some point will begin to produce males and new queens. The point at which this occurs varies among species and is heavily dependent on resource availability and environmental factors. Unlike the workers of more advanced social insects, bumblebee workers are not physically reproductively sterile and can lay haploid eggs that develop into viable male bumblebees. Only fertilised queens can lay diploid eggs that mature into workers and new queens.

Early in the colony cycle, the queen bumblebee compensates for potential reproductive competition from workers by suppressing their egg-laying by way of physical aggression and pheromonal signals.^[9] Thus, the queen will usually be the mother of all of the first males laid. Workers eventually begin to lay male eggs later in the season when the queen's ability to suppress their reproduction diminishes.  The reproductive competition between workers and the queen is one reason that bumblebees are considered "primitively eusocial".

New queens and males leave the colony after maturation. Males in particular are forcibly driven out by the workers. Away from the colony, the new queens and males live off nectar and pollen and spend the night on flowers or in holes. The queens are eventually mated (often more than once) and search for a suitable location for diapause (dormancy).

Foraging Behaviour

 

A bumblebee loaded with pollen in its pollen baskets

Bumblebees generally visit flowers exhibiting the bee pollination syndrome. They can visit patches of flowers up to 1–2 kilometres from their colony.  Bumblebees will also tend to visit the same patches of flowers every day, as long as they continue to find nectar and pollen,   a habit known as pollinator or flower constancy. While foraging, bumblebees can reach ground speeds of up to 15 metre per second (54 km/h).^[13]
Experiments have shown that bumblebees use a combination of colour and spatial relationships to learn which flowers to forage from.  Bumblebees can also detect both the presence and the pattern of electric fields on flowers, which occur due to the positive static charges that are generated when bees fly through the air (see Atmospheric electricity), and take a while to leak away into the ground. They use this information to find out if a flower has been recently visited by another bee.  After arriving at a flower, they extract nectar using their long tongue ("glossa") and store it in their crop. Many species of bumblebee also exhibit what is known as "nectar robbing": instead of inserting the mouthparts into the flower normally, these bees bite directly through the base of the corolla to extract nectar, avoiding pollen transfer.  These bees obtain pollen from other species of flowers that they "legitimately" visit.

Pollen is removed from flowers deliberately or incidentally by bumblebees. Incidental removal occurs when bumblebees come in contact with the anthers of a flower while collecting nectar. The bumblebee's body hairs receive a dusting of pollen from the anthers, which is then groomed into the corbicula ("pollen basket"). Bumblebees are also capable of buzz pollination.

In at least a few species, once a bumblebee has visited a flower, it leaves a scent mark on the flower. This scent mark deters visitation of the flower by other bumblebees until the scent degrades.  It has been shown that this scent mark is a general chemical bouquet that bumblebees leave behind in different locations (e.g. nest, neutral, and food sites),  and they learn to use this bouquet to identify both rewarding and unrewarding flowers.   In addition, bumblebees rely on this chemical bouquet more when the flower has a high handling time (i.e. it takes a longer time for the bee to find the nectar).



Once they have collected nectar and pollen, bumblebees return to the nest and deposit the harvested nectar and pollen into brood cells, or into wax cells for storage. Unlike honey bees, bumblebees only store a few days' worth of food and so are much more vulnerable to food shortages.

Cuckoo bumblebees

The cuckoo bumblebee Bombus vestalis, a parasite of Bombus terrestris

Bumblebees of the subgenus Psithyrus (known as cuckoo bumblebees, and formerly considered a separate genus) are a lineage that live parasitically in the colonies of other bumblebees and have lost the ability to collect pollen. Before finding and invading a host colony, a Psithyrus female (there is no caste system in these species) will feed directly from flowers. Once she has infiltrated a host colony, the Psithyrus female will kill or subdue the queen of that colony and forcibly (using pheromones and/or physical attacks) "enslave" the workers of that colony to feed her and her young.  The female Psithyrus also has a number of morphological adaptations, such as larger mandibles and a larger venom sac that increase her chances of taking over a nest. Upon hatching, the male and female Psithyrus disperse and mate. Like non-parasitic bumblebee queens, female Psithyrus find suitable locations to spend the winter and enter diapause upon being mated.

Reproduction

In temperate zone species, in the autumn, young queens ("gynes") mate with males (drones) and diapause during the winter in a sheltered area, whether in the ground or in a man-made structure. In the early spring, the queen comes out of diapause and finds a suitable place to create her colony. Then she builds wax cells in which to lay her fertilised eggs from the previous winter. The eggs that hatch develop into female workers, and in time the queen populates the colony, with workers feeding the young and performing other duties similar to honey bee workers. New reproductives are produced in autumn, and the queen and workers die, as do the males.

Sting

Queen and worker bumblebees can sting. Unlike a honey bee's stinger, a bumblebee's stinger lacks barbs, so it can sting more than once.  Bumblebee species are not normally aggressive, but will sting in defence of their nest, or if harmed. Female cuckoo bumblebees will aggressively attack host colony members, and sting the host queen, but will ignore other animals (e.g. humans) unless disturbed.

Bumblebees and people

Bumblebees are important pollinators of both crops and wildflowers.

Comments by Charles Darwin

In his first (1859) edition of On the Origin of Species,  Charles Darwin wrote of "humble-bees" (a now-disused term for bumblebees; see the etymology section below in this article for more information) and their interactions with other species:

Bumblebee Bombus pascuorum on red clover

plants and animals, most remote in the scale of nature, are bound together by a web of complex relations. [...] I have [...] reason to believe that humble-bees are indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. From experiments which I have tried, I have found that the visits of bees, if not indispensable, are at least highly beneficial to the fertilisation of our clovers; but humble-bees alone visit the common red clover (Trifolium pratense), as other bees cannot reach the nectar. Hence I have very little doubt, that if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests; and Mr. H. Newman, who has long attended to the habits of humble-bees, believes that 'more than two thirds of them are thus destroyed all over England.' Now the number of mice is largely dependent, as every one knows, on the number of cats; and Mr. Newman says, 'Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.' Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district!

Agricultural use

Bumblebees are increasingly cultured for agricultural use as pollinators because they can pollinate plant species that other pollinators cannot by using a technique known as buzz pollination. For example, bumblebee colonies are often placed in greenhouse tomato production, because the frequency of buzzing that a bumblebee exhibits effectively releases tomato pollen.

The agricultural use of bumblebees is limited to pollination. Because bumblebees do not overwinter the entire colony, they are not obliged to stockpile honey, and are therefore not useful as honey producers.

Endangered status

Bumblebees are in danger in many developed countries due to habitat destruction and collateral pesticide damage. In Britain, until relatively recently, 19 species of native true bumblebee were recognised along with six species of cuckoo bumblebees. Of these, three have been extirpated,  eight are in serious decline, and only six remain widespread.  Similar declines in bumblebees have been reported in Ireland, with 4 species being designated endangered, and another two species considered vulnerable to extinction.  A decline in bumblebee numbers could cause large-scale changes to the countryside, resulting from inadequate pollination of certain plants. The world's first bumblebee sanctuary was established at Vane Farm in the Loch Leven National Nature Reserve in Scotland in 2008.

Some bumblebees native to North America are also vanishing, such as Bombus terricola, Bombus affinis and Bombus occidentalis, with one, Bombus franklini, that may even be extinct.



In 2011, the International Union for the Conservation of Nature set up the Bumblebee Specialist Group to review the threat status of all bumblebee species worldwide using the IUCN Red List criteria.

Misconceptions

Flight

A widely believed falsehood holds that scientists proved that bumblebees are incapable of flight

According to 20th century folklore, the laws of aerodynamics prove that the bumblebee should be incapable of flight, as it does not have the capacity (in terms of wing size or beats per second) to achieve flight with the degree of wing loading necessary. The origin of this claim has been difficult to pin down with any certainty. John McMasters recounted an anecdote about an unnamed Swiss aerodynamicist at a dinner party who performed some rough calculations and concluded, presumably in jest, that according to the equations, bumblebees cannot fly.


In later years McMasters has backed away from this origin, suggesting that there could be multiple sources, and that the earliest he has found was a reference in the 1934 book Le vol des insectes by French entomologist Antoine Magnan (1881–1938); they had applied the equations of air resistance to insects and found that their flight was impossible, but that "One shouldn't be surprised that the results of the calculations don't square with reality".



The following passage appears in the introduction to Le Vol des Insectes:

“

Tout d'abord poussé par ce qui se fait en aviation, j'ai appliqué aux insectes les lois de la résistance de l'air, et je suis arrivé avec M. Sainte-Laguë à cette conclusion que leur vol est impossible.

”

This translates to:

“

First prompted by what is done in aviation, I applied the laws of air resistance to insects, and I arrived, with Mr. Sainte-Laguë, at this conclusion that their flight is impossible.

”

Magnan refers to his assistant André Sainte-Laguë.


Some credit physicist Ludwig Prandtl (1875–1953) of the University of Göttingen in Germany with popularizing the idea. Others say it was Swiss gas dynamicist Jacob Ackeret (1898–1981) who did the calculations.

The calculations that purported to show that bumblebees cannot fly are based upon a simplified linear treatment of oscillating aerofoils. The method assumes small amplitude oscillations without flow separation. This ignores the effect of dynamic stall, an airflow separation inducing a large vortex above the wing, which briefly produces several times the lift of the aerofoil in regular flight. More sophisticated aerodynamic analysis shows that the bumblebee can fly because its wings encounter dynamic stall in every oscillation cycle.


Additionally, John Maynard Smith, a noted biologist with a strong background in aeronautics, has pointed out that bumblebees would not be expected to sustain flight, as they would need to generate too much power given their tiny wing area. However, in aerodynamics experiments with other insects he found that viscosity at the scale of small insects meant that even their small wings can move a very large volume of air relative to the size, and this reduces the power required to sustain flight by an order of magnitude.

Another description of a bee's wing function is that the wings work similarly to helicopter blades, "reverse-pitch semirotary helicopter blades".

Bees beat their wings approximately 200 times a second. Their thorax muscles do not expand and contract on each nerve firing but rather vibrate like a plucked rubber band.

Buzz

One common, yet incorrect, assumption is that the buzzing sound ( listen (help·info)) of bees is caused by the beating of their wings. The sound is actually the result of the bee vibrating its flight muscles, and this can be achieved while the muscles are decoupled from the wings. This is especially pronounced in bumblebees, as they must warm up their bodies considerably to get airborne at low ambient temperatures.  Bumblebees have been known to reach an internal thoracic temperature of 30 °C (86 °F) using this method.

Source: Internet