The queen broods the eggs.
The pollen stimulates
the ovaries to produce eggs, which the queen lays in batches of 4 -16 on the
ball of pollen, this is then covered with wax. The eggs are pearly white and slightly curved like a sausage, and are about 2.5 - 4.0 mm long.
The ball of pollen with the eggs
is placed within reach of the honey pot; this enables the queen to brood the
eggs and drink honey at the same time (see the drawing above right of a queen
brooding eggs). Like birds queen bumblebees brood their eggs to keep them warm.
Bumblebees are very hairy, but the underside of the abdomen has a bare patch,
and so the heat from the queen's body can pass directly to the clump of
wax-covered eggs. During this stage the queen rarely leaves the eggs for long and she
keeps them at about 30 oC (see temperature
regulation). So you see it is not really correct to say that the bumblebee
is cold blooded. If the temperature does fall below 30 oC the larvae's growth will be stunted.
After about 4 days the eggs hatch. In the early days of the nest it is estimated that a Bombus terrestris queen may have to visit as many as 6000 flowers per day in order to get enough nectar to maintain the heat needed to brood her eggs. And during every foraging trip the brood will cool down, so the trips should be short. This is why it is vital that the nest is located close to rewarding flowers.
The grubs feed and grow
After this stage the
various species differ slightly in the way the larvae are fed, but in all cases their main food is pollen. The larvae grow
and pupate then emerge as adults. The larva usually have 13 segments with a pair of spiracles (air holes) in each segment (see above right). The antennae are nothing more than buds, they have no eyes or legs, and very limited powers of movement. They are eating machines. However though they eat and grow they do not defecate. They have what is known as a blind gut. This means the end of their gut is closed, so all the waste material remains inside the larva. Because of this the larval cell and food remain uncontaminated with waste. The waste is expelled just before the pupal stage, and stays in the cocoon.
The cocoon is made from silk produced in glands near the mouth. The faeces is smeared on to the inside wall of the cocoon becoming part of its structure. The outer walls of pollen grains are very tough and not digested by the grub. So by examining the faeces in the cocoon the pollen husks can be identified to show which flower species the adult bumblebees have been foraging from. Bumblebees usually have 4 or 5 instars. An instar is the period between moults.
At this stage it is only the queen who lays eggs, and all the eggs she lays are fertilized, and so will develop into females (workers). Fertilized eggs can develop into workers or queens. Queens are produced at a later stage in the life of the colony. It is thought that fertilized larvae develop into queens and not workers when the queen stops secreting a certain pheromone. These larvae destined to be queens are fed more frequently and for longer than larvae destined to be workers.
The queen usually lays another batch of eggs
while the first batch is still in the larval stage. Bumblebee larvae are milky
white and look a bit like maggots (see the drawing on the right). It takes 4 -5
weeks from egg to adult bumblebee. About half the time is spent as a larva
feeding, then the rest of the time as a pupa in a cocoon.
In a series of studies
by A. D. Brian it was found that of the eggs laid by the queen 71% hatch, of
these larvae 75% go on to become pupae, and of the pupae 90% hatch out as adult
bumblebees. So only about 47% of the eggs laid go on to produce adult
Next, the bumblebee colony develops