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How
then do these tiny wasps that only live for a few days manage to perform
their amazing task of finding and pollinating flowers that are hidden
inside the fig? Female fig wasps leaving the fig they have bred in need to
fly off in search of another fig tree to continue the reproductive cycle,
often a long and arduous journey, which only a few individuals out of
thousands manage successfully. This remarkable feat is achieved by homing
in on host tree-specific volatiles, a chemical signal released by the fig
when it is receptive for pollination. Completion of this journey is the
first test of endurance, as once the pollinator has located a receptive
fig, she needs to circumvent the next barrier. The only link the fig
cavity has to the outside world is through a tiny bract-lined opening at
the apex of the fig, called the ostiole, and it is by means of this
passage that the pollinating fig wasp gains access to the florets.
Negotiating the ostiole is no easy task, with the female wasp having to
squeeze and labour her way between the tightly closed bracts. She is,
however, remarkably adapted to do so. Her body, in particular her head and
thorax, is extremely flattened and elongate. She also has rows upon rows
of backward pointing teeth on her mandibular appendage, situated on the
underside of her head, as well as a few strong teeth on her legs. These
teeth assist her progress through the ostiole and also prevent her
slipping backwards. Nevertheless, the process of gaining access to the fig
cavity is so difficult that her wings and antennae usually break off in
the ostiole, but this fortunately does not influence her pollinating or
egg-laying ability.
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Underside of head of Courtella
wardi showing mandibles and
mandibular appendages, bearing many rows of backward pointing teeth.
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Underside of head of Elisabethiella
bergi breviceps showing
mandibles and mandibular appendages. |
Nigeriella species.
Scale bar = 0.1mm. |
The
female wasp then proceeds to pollinate the stigmas and to lay eggs in the
ovules of some of the florets. This she does by inserting her long
ovipositor down the inside of the style. The florets that have styles
longer than the wasp’s ovipositor are pollinated, but no eggs are laid
in the ovule and hence these florets set seed. The wasp larvae feed on the
endosperm tissue in the galled ovary and larval development correlates
strongly with host fig development, encompassing anything from three to
twenty weeks. Once the wasps have reached maturity they chew their way out
from the galls and emerge into the fig cavity within a short period of
each other. The wingless males mate with the females before chewing a hole
through the fig wall to the exterior to allow the females to escape –
the male’s only two functions in life, as he dies soon afterwards! The
females either actively load up pollen from ripe anthers into special
pollen pockets, or in some species passively become covered with pollen,
before exiting the fig in search of young receptive figs to complete the cycle.
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Philocaenus rotundus
laying
eggs inside a split open fig of Ficus abutilifolia.
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Courtella wardi
laying an egg
down the style into the ovary of a floret and simulataneously
pollinating the stigmas with her forelegs inside a fig of Ficus
tremula. |
Ceratosolen capensis
female emerging
from her gall inside a fig of Ficus sur. |
Once
the female fig wasps have left the fig, it ripens, changing colour and
smell, and becomes attractive to seed or fruit eating birds, bats, monkeys
and even lizards. Fig trees are considered to be keystone species in many
tropical and subtropical ecosystems, because of the all year round
production of figs, providing food in seasons when other fruiting trees
are not. Fruit eating animals play an important part in the propagation of
fig trees, acting as the dispersal agents of the seeds.
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Elisabethiella stuckenbergi
females and males emerging
from their galls inside a split open fig of Ficus thonningii.
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 Pollen pocket, containing pollen
grains, on the thorax
of a Nigeriella female. Scale bar =
0.1mm. |
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