Bumblebee
A bumblebee 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 are known from fossils. They are found primarily 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 eusocial 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 called 'pile', making them appear and feel fuzzy. They have aposematic 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 bumblebees 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 bumblebees, the hind leg is hairy all around, 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
The English name bumblebee combines bumble, meaning to buzz or hum, with bee, both words imitative of the insect's sound. The generic name Bombus, introduced by Pierre André Latreille in 1802, derives from the Latin bombus, itself from Ancient Greek βόμβος.Charles Darwin referred to bumblebees as "humble-bees" in his 1859 book On the Origin of Species, emphasising their importance in pollinating clovers and other wildflowers. The term humblebee remained in use through the nineteenth century, but was gradually replaced by bumblebee in both scientific and common language during the twentieth century.
Evolution
Phylogeny
The tribe Bombini is one of four lineages of corbiculate bees in the Apidae. It is distinguished by having pollen baskets on their hind legs. The other three are the Apini, Euglossini, and Meliponini. Together they form a monophyletic group.Early analyses of morphology and behaviour suggested that complex eusociality evolved twice within the corbiculate bees, once in the ancestor of the Apini and once in the Meliponini. Molecular and morphological data indicate that the primitively eusocial Bombini are, despite appearances, closely related to the Meliponini, while the Apini and Euglossini form a separate branch. Cardinal and Danforth described this dual-origin model as consistent with earlier evidence from social and anatomical traits.
More recent phylogenomic studies based on the mRNA expression profile from more than 3,600 genes support instead a single origin of eusociality within the corbiculate bees. Romiguier et al. found that the Bombini, Meliponini, and Apini form a single clade, with Euglossini as their sister group, implying that advanced social organisation arose once in the common ancestor of these three tribes.
Revisiting the "corbiculate controversy", on the mismatch between morphological and molecular analyses in 2021, Diego Sasso Porto and Eduardo A. B. Almeida find that morphological analysis still suggests that Bombini are sister to a clade containing Apini and Meliponini, as shown in the phylogenetic tree. They propose "a more conciliatory" scenario involving "a diversification followed by several extinctions" so that Meliponini and Bombini shared "few apomorphic changes", while the similarities between Meliponini and Apini could be convergent evolution caused by "similar biology or similar eusocial behaviors".
The earliest divergence estimates suggest that the Bombini originated between 25 and 40 million years ago, whereas the Bombini–Meliponini clade dates to roughly 80–95 million years ago, around the time the corbiculate group as a whole diversified.
Fossil record
Fossil evidence of Bombini is limited. By 2019, about 14 potential fossil species had been described. The oldest known relatives include Calyptapis florissantensis from the Late Eocene Florissant Formation and Oligobombus cuspidatus from the Bembridge Marls.Several Oligocene and Miocene species have been assigned to modern subgenera, including Bombus beskonakensis and B. patriciae from Turkey, B. randeckensis from Germany, B. cerdanyensis from Spain, and B. trophonius from the Czech Republic.
These fossils indicate that the major subgenera of Bombus were already differentiated by the Miocene and that diversification of the tribe was well underway by the Oligocene.
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. The cuckoo bumblebees Psithyrus have sometimes been treated as a separate genus but are now considered to be part of Bombus.Description
Bumblebees vary in appearance, but are characteristically plump and densely furry. They are larger, broader and stouter-bodied than honeybees, and the tip of the abdomen is more rounded. Many species have broad bands of colour, whose patterns help 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, 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
Abundance and diversity
Bumblebees are most abundant across temperate and montane ecosystems of the Northern Hemisphere, with the greatest species richness found in Eurasia and North America. Bumblebees have a largely cosmopolitan distribution but are absent from Australia and occur 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.At the northern limits of their range, species such as Bombus polaris, B. alpinus, and the parasitic B. hyperboreus occupy Arctic tundra ecosystems that extend as far north as Greenland and Canada's Ellesmere Island. These populations endure some of the planet's shortest flowering seasons and coldest conditions, marking the northernmost boundary of eusocial insect life.
Physiological adaptations
One reason for bumblebee presence in such cold places is that they can regulate their body temperature through several physiological mechanisms. They use solar radiation to warm themselves, generate heat internally through "shivering" known as heterothermy, and employ countercurrent exchange to retain heat. Although other bees show similar forms of thermoregulation, these mechanisms are especially well developed and extensively studied in bumblebees. They also adapt to high-elevation environments by increasing their wing stroke amplitude to sustain flight in thin air.Biology
Feeding
The bumblebee tongue is a long, hairy structure that extends from a sheath-like modified maxilla. The primary action of the tongue is lapping or 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.