Bumblebees

A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related genera (e.g., ''Calyptapis'') are known from fossils. They are found primarily in higher altitudes or latitudes in the Northern Hemisphere, although they are also found in South America, where a few lowland tropical species have been identified. European bumblebees have also been introduced to New Zealand and Tasmania. Female bumblebees can sting repeatedly, but generally ignore humans and other animals.

Most bumblebees are social insects that form colonies with a single queen. The colonies are smaller than those of honey bees, growing to as few as 50 individuals in a nest. Cuckoo bumblebees are brood parasitic and do not make nests or form colonies; their queens aggressively invade the nests of other bumblebee species, kill the resident queens and then lay their own eggs, which are cared for by the resident workers. Cuckoo bumblebees were previously classified as a separate genus, but are now usually treated as members of ''Bombus''.

Bumblebees have round bodies covered in soft hair (long branched setae) called 'pile', making them appear and feel fuzzy. They have aposematic (warning) coloration, often consisting of contrasting bands of colour, and different species of bumblebee in a region often resemble each other in mutually protective Müllerian mimicry. Harmless insects such as hoverflies often derive protection from resembling bumblebees, in Batesian mimicry, and may be confused with them. Nest-making bumblebees can be distinguished from similarly large, fuzzy cuckoo bees by the form of the female hind leg. In nesting bumblebees, it is modified to form a pollen basket, a bare shiny area surrounded by a fringe of hairs used to transport pollen, whereas in cuckoo bees, the hind leg is hairy all round, and they never carry pollen.

Like their relatives the honeybees, bumblebees feed on nectar, using their long hairy tongues to lap up the liquid; the proboscis is folded under the head during flight. Bumblebees gather nectar to add to the stores in the nest, and pollen to feed their young. They forage using colour and spatial relationships to identify flowers to feed from. Some bumblebees steal nectar, making a hole near the base of a flower to access the nectar while avoiding pollen transfer. Bumblebees are important agricultural pollinators, so their decline in Europe, North America, and Asia is a cause for concern. The decline has been caused by habitat loss, the mechanisation of agriculture, and pesticides.

Etymology and common names

The word "bumblebee" is a compound of "bumble" and "bee"—'bumble' meaning to hum, buzz, drone, or move ineptly or flounderingly. The generic name ''Bombus'', assigned by Pierre André Latreille in 1802, is derived from the Latin word for a buzzing or humming sound, borrowed from Ancient Greek ().

According to the Oxford English Dictionary (OED), the term "bumblebee" was first recorded as having been used in the English language in the 1530 work ''Lesclarcissement'' by John Palsgrave, "I bomme, as a bombyll bee dothe." However the OED also states that the term "humblebee" predates it, having first been used in 1450 in ''Fysshynge wyth Angle'', "In Juyll the greshop & the humbylbee in the medow." The latter term was used in ''A Midsummer Night's Dream'' () by William Shakespeare, "The honie-bags steale from the humble Bees." Similar terms are used in other Germanic languages, such as the German (Old High German ), Dutch or Swedish .

An old provincial name, "dumbledor", also denoted a buzzing insect such as a bumblebee or cockchafer, "dumble" probably imitating the sound of these insects, while " dor" meant "beetle".

In ''On the Origin of Species'' (1859), Charles Darwin speculated about "humble-bees" and their interactions with other species:

However, "bumblebee" remained in use, for example in '' The Tale of Mrs. Tittlemouse'' (1910) by Beatrix Potter, "Suddenly round a corner, she met Babbitty Bumble--"Zizz, Bizz, Bizzz!" said the bumble bee." Since World War II "humblebee" has fallen into near-total disuse.

Phylogeny

The bumblebee tribe Bombini is one of four groups of corbiculate bees (those with pollen baskets) in the Apidae, the others being the Apini (honey bees), Euglossini (orchid bees), and Meliponini (stingless bees). The corbiculate bees are a monophyletic group. Advanced eusocial behaviour appears to have evolved twice in the group, giving rise to controversy, now largely settled, as to the phylogenetic origins of the four tribes; it had been supposed that eusocial behaviour had evolved only once, requiring the Apini to be close to the Meliponini, which they do not resemble. It is now thought that the Apini (with advanced societies) and Euglossini are closely related, while the primitively eusocial Bombini are close to the Meliponini, which have somewhat more advanced eusocial behaviour. Sophie Cardinal and Bryan Danforth comment that "While remarkable, a hypothesis of dual origins of advanced eusociality is congruent with early studies on corbiculate morphology and social behavior." Their analysis, combining molecular, morphological and behavioural data, gives the following cladogram:

On this hypothesis, the molecular data suggest that the Bombini are 25 to 40 million years old, while the Meliponini (and thus the clade that includes the Bombini and Meliponini) are 81 to 96 million years old, about the same age as the corbiculate group.

However, a more recent phylogeny using transcriptome data from 3,647 genes of ten corbiculate bee species supports the single origin of eusociality hypothesis in the corbiculate bees. They find that Bombini is in fact sister to Meliponini, corroborating that previous finding from Sophie Cardinal and Bryan Danforth (2011). However, Romiguier et al. (2015) shows that Bombini, Meliponini, and Apini form a monophyletic group, where Apini shares a most recent common ancestor with the Bombini and Meliponini clade, while Euglossini is most distantly related to all three, since it does not share the same most recent common ancestor as Bombini, Meliponini, and Apini. Thus, their analysis supports the single origin of eusociality hypothesis within the corbiculate bees, where eusociality evolved in the common ancestor of Bombini, Apini, and Meliponini.

The fossil record for bees is limited, with around 14 species that might possibly be Bombini having been described by 2019. The only ''Bombus'' relatives in Bombini are the late Eocene ''Calyptapis florissantensis'' from the Florissant Formation, USA, and ''Oligobombus cuspidatus'' from the Bembridge Marls of the Isle of Wight. Two species of ''Bombus'' have been described from the Oligocene of Beşkonak, Bucak Turkey: '' Bombus (Mendacibombus) beskonakensis'' and '' Bombus (Paraelectrobombus) patriciae''. Both species were originally placed in genera considered at the time of description as outside of ''Bombus'', being initially named ''Oligoapis beskonakensis'' and ''Paraelectrobombus patriciae'' respectively, however reexaminiation of the fore-wings lead to both being considered as ''Bombus'' species In 2012 a fossil bumblebee from the Miocene was found in Germany's Randeck Maar and classified as '' Bombus (Bombus) randeckensis''. In 2014, another species, '' Bombus cerdanyensis'', was described from Late Miocene lacustrine beds of La Cerdanya, Spain, but not initially placed into any subgenus, The species '' Bombus trophonius'' was described in October 2017 and placed in ''Bombus'' subgenus ''Cullumanobombus''. A redescription of the Bombini fossil record by Dehon ''et al'' ( 2019) resulted in the synonymization of the genus ''Oligoapis'' with ''Bombus'' subgenus ''Mendacibombus'', and the placement of genus ''Paraelectrobombus'' as ''Bombus'' subgenus ''Paraelectrobombus'', rather than as a genus in Electrobombini. The subgenus ''Cullumanobombus'' was expanded to include not only ''Bombus trophonius'' but also ''Bombus randeckensis'' which was moved from subgenus ''Bombus'' and '' Bombus pristinus'', first described by Unger ( 1867). Within the subgenus ''Melanobombus'' only ''Bombus cerdanyensis'' is present from the fossil record. An additional three species, '' "Bombus" luianus'', '' "Bombus" anacolus'' and '' "Bombus" dilectus'' have been attributed to ''Bombus'' from the Middle Miocene Shanwang formation of China by Zhang, (1990) and Zhang ''et al'' (1994). Due to not being able to study Zhang's type specimens, but only illustrations of the fossils, Dehon ''et al'' did not place the three species within any specific subgenera, and considered all three as "species inquirenda", needing fuller re-examination. Two other species were not examined at all by Dehon ''et al'', '' Bombus? crassipes'' of the Late Miocene Krottensee deposits in the Czech Republic, and '' Bombus proavus'' from the Middle Miocene Latah Formation, USA.

Taxonomy

The genus ''Bombus'', the only one extant genus in the tribe Bombini, comprises over 250 species; for an overview of the differences between bumblebees and other bees and wasps, see characteristics of common wasps and bees. The genus has been divided variously into up to 49 subgenera, a degree of complexity criticised by Williams (2008). The cuckoo bumblebees ''Psithyrus'' have sometimes been treated as a separate genus but are now considered to be part of ''Bombus'', in one or more subgenera.

Examples of ''Bombus'' species include ''Bombus pauloensis'', '' Bombus dahlbomii'', '' Bombus fervidus'', ''Bombus lapidarius'', '' Bombus ruderatus'', and ''Bombus rupestris.''

:::Subgenera of the genus ''Bombus''

General description

Bumblebees vary in appearance, but are generally plump and densely furry. They are larger, broader and stouter-bodied than honeybees, and their abdomen tip is more rounded. Many species have broad bands of colour, the patterns helping to distinguish different species. Whereas honeybees have short tongues and therefore mainly pollinate open flowers, some bumblebee species have long tongues and collect nectar from flowers that are closed into a tube. Bumblebees have fewer stripes (or none), and usually have part of the body covered in black fur, while honeybees have many stripes including several grey stripes on the abdomen. Sizes are very variable even within species; the largest British species, ''B. terrestris'', has queens up to long, males up to long, and workers between long. The largest bumblebee species in the world is '' B. dahlbomii'' of Chile, up to about long, and described as "flying mice" and "a monstrous fluffy ginger beast".

Distribution and habitat

Bumblebees are typically found in temperate climates, and are often found at higher latitudes and altitudes than other bees, although a few lowland tropical species exist. A few species ('' B. polaris'' and '' B. alpinus'') range into very cold climates where other bees might not be found; ''B. polaris'' occurs in northern Ellesmere Island in the high Arctic, along with another bumblebee '' B. hyperboreus'', which parasitises its nest. This is the northernmost occurrence of any eusocial insect. One reason for their presence in cold places 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 seem best developed and have been most studied in bumblebees. They adapt to higher elevations by extending their wing stroke amplitude. Bumblebees have a largely cosmopolitan distribution but are absent from Australia (apart from Tasmania where they have been introduced) and are found in Africa only north of the Sahara. More than a hundred years ago they were also introduced to New Zealand, where they play an important role as efficient pollinators.

Biology

Feeding

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, that is, repeated dipping of the tongue into liquid.
The tip of the tongue probably acts as a suction cup and during lapping, nectar may be drawn up the proboscis by capillary action. When at rest or flying, the proboscis is kept folded under the head. The longer the tongue, the deeper the bumblebee can probe into a flower and bees probably learn from experience which flower source is best-suited to their tongue length. Bees with shorter proboscides, like ''Bombus bifarius'', have a more difficult time foraging nectar relative to other bumblebees with longer proboscides; to overcome this disadvantage, ''B. bifarius'' workers were observed to lick the back of spurs on the nectar duct, which resulted in a small reward.

Wax production

The exoskeleton of the abdomen is divided into plates called dorsal tergites and ventral sternites. Wax is secreted from glands on the abdomen and extruded between the sternites where it resembles flakes of dandruff. It is secreted by the queen when she starts a nest and by young workers. It is scraped from the abdomen by the legs, moulded until malleable and used in the construction of honeypots, to cover the eggs, to line empty cocoons for use as storage containers and sometimes to cover the exterior of the nest.

Coloration

The brightly coloured pile of the bumblebee is an aposematic (warning) signal, given that females can inflict a painful sting. Depending on the species and morph, the warning colours range from entirely black, to bright yellow, red, orange, white, and pink. Dipteran flies in the families Syrphidae (hoverflies), Asilidae (robber flies), Tabanidae (horseflies), Oestridae (bot or warble flies) and Bombyliidae (bee flies, such as '' Bombylius major'') all include Batesian mimics of bumblebees, resembling them closely enough to deceive at least some predators.

Many species of ''Bombus'', including the group sometimes called ''Psithyrus'' (cuckoo bumblebees), have evolved Müllerian mimicry, where the different bumblebees in a region resemble each other, so that a young predator need only learn to avoid any of them once. For example, in California a group of bumblebees consists of largely black species including '' B. californicus'', '' B. caliginosus'', '' B. vandykei'', '' B. vosnesenskii'', '' B. insularis'' and '' B. fernaldae''. Other bees in California include a group of species all banded black and yellow. In each case, Müllerian mimicry provides the bees in the group with a selective advantage. In addition, parasitic (cuckoo) bumblebees resemble their hosts more closely than would be expected by chance, at least in areas like Europe where parasite-host co-speciation is common; but this too may be explained as Müllerian mimicry, rather than requiring the parasite's coloration to deceive the host ( aggressive mimicry).

Temperature control

Bumblebees are active under conditions during which honeybees stay at home, and can readily absorb heat from even weak sunshine.Macdonald, 2003. p. 6 The thick pile created by long setae (bristles) acts as insulation to keep bumblebees warm in cold weather; species from cold climates have longer setae (and thus thicker insulation) than those from the tropics. The temperature of the flight muscles, which occupy much of the thorax, needs to be at least before flight can take place. The muscle temperature can be raised by shivering. It takes about five minutes for the muscles to reach this temperature at an air temperature of .

Chill-coma temperature

The chill-coma temperature in relation to flying insects is the temperature at which flight muscles cannot be activated. Compared to honey bees and carpenter bees, bumblebees have the lowest chill-coma temperature. Of the bumblebees '' Bombus bimaculatus'' has the lowest at . However, bumblebees have been seen to fly in colder ambient temperatures. This discrepancy is likely because the chill-coma temperature was determined by tests done in a laboratory setting. However, bumblebees live in insulated shelters and can shiver to warm up before venturing into the cold.

Communication and social learning

Bumblebees do not have ears, and it is not known whether or how well they can hear. However, they are sensitive to the vibrations made by sound travelling through wood or other materials.

Bumblebees do not exhibit the " bee dances" used by honeybees to tell other workers the locations of food sources. Instead, when they return from a successful foraging expedition, they run excitedly around in the nest for several minutes before going out to forage once more. These bees may be offering some form of communication based on the buzzing sounds made by their wings, which may stimulate other bees to start foraging. Another stimulant to foraging activity is the level of food reserves in the colony. Bees monitor the amount of honey in the honeypots, and when little is left or when high-quality food is added, they are more likely to go out to forage.

Bumblebees have been observed to partake in social learning. In a 2017 study involving ''Bombus terrestris'', bees were taught to complete an unnatural task of moving large objects to obtain a reward. Bees who first observed another bee complete the task were significantly more successful in learning the task than bees who observed the same action performed by a magnet, indicating the importance of social information. The bees did not copy one another exactly: in fact, the study suggested that the bees were instead attempting to emulate one another's goals.

Reproduction and nesting

Nest size depends on species of bumblebee. Most form colonies of between 50 and 400 individuals, but colonies have been documented as small as ~20 individuals and as large as 1700. These nests are small compared to honeybee hives, which hold about 50,000 bees. Many species nest underground, choosing old rodent burrows or sheltered places, and avoiding places that receive direct sunlight that could result in overheating. Other species make nests above ground, whether in thick grass or in holes in trees. A bumblebee nest is not organised into hexagonal combs like that of a honeybee; the cells are instead clustered together untidily. The workers remove dead bees or larvae from the nest and deposit them outside the nest entrance, helping to prevent disease. Nests in temperate regions last only for a single season and do not survive the winter.

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 eggs which were fertilised the previous year. 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 honeybee workers. In temperate zones, young queens ( gynes) leave the nest in the autumn and mate, often more than once, with males (drones) that are forcibly driven out of the colony. The drones and workers die as the weather turns colder; the young queens feed intensively to build up stores of fat for the winter. They survive in a resting state (diapause), generally below ground, until the weather warms up in the spring with the early bumblebee being the species that is among the first to emerge. Many species of bumblebee follow this general trend within the year. '' Bombus pensylvanicus'' is a species that follows this type of colony cycle. For this species the cycle begins in February, reproduction starts in July or August, and ends in the winter months. The queen remains in hibernation until spring of the following year in order to optimize conditions to search for a nest.

In fertilised queens, the ovaries only become active when the queen starts to lay. An egg passes along the oviduct to the vagina where there is a chamber called the spermatheca, in which the sperm from the mating is stored. Depending on need, she may allow her egg to be fertilised. Unfertilised eggs become haploid males; fertilised eggs grow into diploid females and queens. The hormones that stimulate the development of the ovaries are suppressed in female worker bees, while the queen remains dominant.

To develop, the 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. Pocket-making bumblebees create pockets of pollen at the base of the brood-cell clump from which the larvae feed themselves. Pollen-storing bumblebees keep pollen in separate wax pots and feed it to the larvae.

After the emergence of the first or second group of offspring, 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 eventually begins to produce males and new queens.Goulson, 2013. pp. 16–24
Bumblebee workers can lay unfertilised haploid eggs (with only a single set of chromosomes) that develop into viable male bumblebees. Only fertilised queens can lay diploid eggs (one set of chromosomes from a drone, one from the queen) that mature into workers and new queens.

In a young colony, the queen minimises reproductive competition from workers by suppressing their egg-laying through physical aggression and pheromones. Worker policing leads to nearly all eggs laid by workers being eaten. Thus, the queen is usually 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. Because of the reproductive competition between workers and the queen, bumblebees are considered "primitively eusocial".

Although a large majority of bumblebees follow such monogynous colony cycles that only involve one queen, some select ''Bombus'' species (such as ''Bombus pauloensis'') will spend part of their life cycle in a polygynous phase (have multiple queens in one nest during these periods of polygyny).

Foraging behaviour

Bumblebees generally visit flowers that exhibit the bee pollination syndrome and these patches of flowers may be up to 1–2 km from their colony. They tend to visit the same patches of flowers every day, as long as they continue to find nectar and pollen there, a habit known as pollinator or flower constancy. While foraging, bumblebees can reach ground speeds of up to .

Bumblebees use a combination of colour and spatial relationships to learn which flowers to forage from. They can also detect both the presence and the pattern of electric fields on flowers, which occur due to 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. Bumblebees can detect the temperature of flowers, as well as which parts of the flower are hotter or cooler and use this information to recognise flowers. After arriving at a flower, they extract nectar using their long tongues (" glossae") and store it in their crops. Many species of bumblebees also exhibit "nectar robbing": instead of inserting the mouthparts into the flower in the normal way, these bees bite directly through the base of the corolla

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