I stopped by the comfrey patch this morning. The comfrey has
now been in blossom for a good month. A Small White butterfly was feeding on comfrey, but I couldn't snap her. A ginger queen Bombus pascuorum fed on the blossom. A queen wasp got comfortable on a leaf and
basked in the sun, as did some hoverflies. While I watched a couple of
Anthophora plumipes males patrolling and feeding, a tiny worker of the tree
bumblebee Bombus hypnorum, the first of the year, turned up (above). As I compared the
sizes of these two bees – the bumblebee worker was smaller than A. plumipes - I
reflected on the different life cycles of these two bee species. A. plumipes, a
solitary bee, has males and females, while bumblebees, in addition of males and females (queens), have a
worker caste. In bumblebees there are no males for most of the year: A male has a relatively
short - if hectic - life. Upon emergence in the summer, he will find a queen and
mate with her. The queen will store his sperm and use it
to fertilise her eggs the following spring, when she will emerge from overwintering, build a nest
and lay fertilised eggs, which will hatch into worker larvae. She will be busy collecting nectar and pollen from early flowering trees and plants to rear these larvae, and once the workers emerge, they will
take over from the queen in collecting more nectar and pollen for subsequent
eggs, although they won't reproduce themselves. In the summer, the last batch of eggs of the queen will produce new queens and males, which the workers will help rear. Once they leave the nest, the
founder queen and workers will die and the cycle starts again. Wasps, ants, bees,
and bumblebees (hymenopterans) share a weird way to determine the sex of the
offspring: fertilised eggs become females and unfertilised eggs become males: a
system called haplodiploidy.
Most sexual organisms have two sets of chromosomes: a set
they inherit from their mother and another that comes from their father, but as
bumblebees and other hymenopterans do not have a dad they just have a single
set of chromosomes, the set coming from their mother.
How strange is that? If you think about it, the system means
that bumblebee males don’t have dads, they also cannot have sons, only
daughters, although they can have grandsons and have granddads. In addition,
this arrangement causes strange relationships between the family members. We
are equally related to our parents than to our kids – on average – but due to
haplodiploidy, bumblebee sisters are more strongly related than they are to
their mums. This is because full sisters have received an identical set of
chromosomes from their dad, in addition to the set they receive from their mum, which is a mixture from the sets the queen received from her parents.
This unbalanced
genetic relationship between mother-daughters and sisters is thought to
underlay the evolution of the worker caste, which do not reproduce, but help
their mother to rear their own sisters.
The solitary bee Anthophora plumipes
A wasp? No, the exquisite wasp mimic hoverfly Myothropa florea
This is the real wasp. A queen common wasp
Hoverfly Syrphus ribesii
Another basking fly
A queen Bombus pascuorum
2 comments:
It's worth noting that if they get the chance, in certain circumstances workers will attempt to lay eggs, but because they haven't been fertilized only males are produced. It depends on how 'fit' the queen is in controlling the nest. Some great examples documented in Sladen's classic book: 'The Humble-bee: its life-history and how to domesticate it'. First published 1892, re-printed 1989.
Thank you for your comment norwegica and for the reference. It would take a very long blog post to explain all the intricacies of the bumblebee life cycle and I am bound to have left more stuff unmentioned.
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