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
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.
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:
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
