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Large blue butterflies belong to the genus Maculinea
and are members of the large butterfly family
Lycaenidae. Many butterflies in this family have some sort
of association with ants. Large blue butterflies
have an unusual life cycle in which most of their larval life
is spent as parasites inside ant nests.
All large blue butterflies are rare because of this interaction
with ants. There are several species of
Large blue butterflies in Europe and Northern Asia.
The species of large blue butterfly we study at the Universities
of Copenhagen and Aarhus is the
Alcon blue, Maculinea alcon. The life-cycle of the
Alcon blue is shown below. Click on the small
pictures or the text to see larger pictures and to find out
more during the life of the Alcon Blue.
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In the early summer the caterpillar turns into a pupa (also called
a chrysalis), still inside the ant nest. The adult butterfly will
emerge from the pupa about a month later. The adult has to get out
of the ant nest quickly to prevent the ants killing it and leaves
before its wings are expanded.
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The adult butterflies are on
the wing throughout July. |
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The females lay their
eggs on the flowers of the Marsh Gentian, Gentiana pneumonanthe.
The caterpillar hatches through the base of the egg into the
flower, where it spends two to three weeks eating the flower
tissue and the developing seeds. The caterpillar moults three
times during this period, but stays very small (3-4 mm long,
and weighing about 1-3 mg). |
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After its third moult, the caterpillar
chews a hole in the flower through which it exits. It then
lets itself down from the gentian flower to the ground on a
silken thread. Once on the ground it waits. |
| D. |
If the caterpillar is found by a red ant (Myrmica
species), the ant will pick the caterpillar up
and take it back to its nest. |
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Once inside the ant
nest, the caterpillar will be fed by the worker ants and will
probably eat a few of the ant larvae and pupae. The caterpillars
will stay in the ant nest all through the autumn, winter and
spring. They grow a lot during this time, reaching about 12
mm in length and weighing up to 100 mg. (10gr). |
| F. |
In the early summer the caterpillar turns into
a pupa (also called a chrysalis), still inside the ant
nest. The adult butterfly will emerge from the pupa about
a month later. The adult has to get out of the ant nest quickly
to prevent the ants killing it and leaves before its wings
are expanded.
In July the butterfly takes to its wings, and it starts all
over again. A
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Alcon blue, Maculinea alcon
The Alcon blue butterfly, Maculinea alcon,
can be seen flying in mid to late summer.
In Denmark it is on the wing during the month of July.
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The Alcon blue is a member of the family of butterflies
called
the Lycaenidae. This family includes the blues,
coppers and
hairstreaks.
The lycaenidae is one of the largest butterfly families.
One reason for the success of this family of butterflies
is that
their caterpillars and pupae often have some sort
of association
with ants.
Most of the lycaenidae that associate with ants have
a mutualistic
relationship with them. However the Alcon blue and
other large
blue butterflies (butterflies in the genus Maculinea)
are parasites
of ant colonies
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The
picture on the right shows a male Alcon blue sunning
himself on a cloudy day in Denmark.
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The female Alcon blue (left) generally has more prominent
black spots
on, and a broader black border to her wings, but this
is quite variable.
Little is known of how Alcon blues find and choose
their mates.
Male Alcon blues emerge slightly earlier in the year
than females,
and males can often be seen flying backward and forward
over
areas with ant nests from which females may emerge.
It is quite common to see mating pairs where the
female has not
finished expanding her wings, so it is likely that
females are often
mated soon after they emerge, and have little opportunity
to assess
and choose particular mates
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Both sexes have underwings with a distinctive pattern
of black spots.
The Alcon blue, like all the large blue butterflies,
is rare.
It is classified as endangered or vulnerable to extinction
on most
countries in western Europe.
Luckily the populations in Denmark are healthier
than most of
those in the rest of Europe.
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Egg
laying
Females of the Alcon blue lay their eggs
on the flowers of the Marsh Gentian, Gentiana pneumonanthe.
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The Marsh Gentian is now quite rare throughout Europe,
and is considered endangered in several countries.
It grows on wet heathlands.
Large blue butterflies are unusual in that their
caterpillars hatch through the base of the egg and
burrow straight into the flower head.
Inside the flower head the caterpillar will develop
through the first three instars in about two weeks.
The caterpillar feeds on the flower tissue and developing
seeds during this time.
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The picture to the left shows a third-instar caterpillar.
The caterpillar grows very little during its time
inside the Gentian plant. When it moults for the third time
and becomes a fourth-instar caterpillar it is
only 3-4 mm long, and weighs only 2-3 mg.
At this stage the caterpillar will chew a hole in the plant
from the inside and crawl out from
the flower head.
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Caterpillar
emergence
The fourth instar caterpillar emerges from the gentian flower
shortly after it has moulted
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The newly-moulted caterpillar chews a hole in the
flower-head from the inside out and crawls outside.
As the picture on the right shows, several caterpillars
can develop in and emerge from the same gentian flower.
The caterpillar then crawls onto the bracts of the
flower, or onto the leaves of the gentian, and lowers
itself to the ground on a silken thread.
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The picture above is a scanning electron micrograph
of a fourth-instar caterpillar that has just emerged
from
a gentian. The caterpillar is quite difference in
appearance from the third-instar caterpillar. It has
almost no
hairs on the upper surface and has a few special glands
here that the third instar caterpillar lacks.
The caterpillar will wait on the ground until it
is discovered by a worker ant. The caterpillars usually
leave
the plant during the morning or evening, when the
chances of drying out are quite low, but the chance
of
being discovered by an ant are high.
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Adoption
If the caterpillar is discovered by the right sort of ant
it will be adopted into the ant nest.
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If the caterpillar is discovered by a red ant (an ant
in the genus Myrmica),
then it will go through a process known as adoption.
If any other type
of ant discovers the caterpillar, however, it will be
treated like any other
caterpillar, which usually means being eaten.
During the adoption procedure, a Myrmica ant will touch
the caterpillar
all over with her antennae. During this time the caterpillar
may secrete a
droplet of a sweet secretion from a special gland on
its back, which the
ant drinks. After a while the caterpillar will flatten
the rear or middle of
its body, and the ant will pick the caterpillar up and
carry it away.
The ant will carry the caterpillar back to the ant
nest and place it among
the ant larvae.
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Growth
and development in the ant nest
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Once inside the ant nest, the caterpillars are fed
by the ants much as if they were their own larvae. Worker
ants
regurgitate liquid food that the caterpillars drink.
The ants seem to prefer feeding caterpillars to feeding
their own
larvae, so that fewer ant larvae can develop. In the
laboratory the caterpillars will also eat ant larvae
directly, and
this probably also happens in nature (some other large
blue butterflies live entirely by eating ant brood when
they
are inside the ant nest).
Although caterpillars will be adopted by any Myrmica
ant species that finds them, they are only seem able
to
survive in the nests of one or two species of ant. The
Alcon blue is unusual among the large blue butterflies
in
that the species of ant in the nests of which it can
survive is different in different parts of Europe.
At the moment we are carrying out research at the Universities
of Copenhagen and Aarhus into why this
should be.
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If caterpillars are adopted into a nest
of the right species of ant they will grow rapidly, increasing
in weight about 100 times in their first month in the
nest. They will stay in the nest throughout the autumn,
winter and spring, and will turn into pupae in the early
summer.
Compare the size of the caterpillars with
the ants on the picture above! |
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Pupation
and emergence
The caterpillars pupate inside the ant nest, and the ants
continue to look after the pupae.
The pupae are initially very light coloured, but become
darker as the time for the adults
to emerge approaches. Both light and dark pupae can be seen
in the picture above.
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Just before the adult emerges the wings
of the butterfly inside the pupal case
detach from the case, and the pupa
becomes silvery (see above). |
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The adult butterfly emerges from the pupa after 3-4
weeks, still
inside the ant nest. The butterfly must crawl out
of the ant nest
before it can expand its wings. At this stage the
ants will
sometimes attack the adult, so it is covered in very
loose hair-like
scales. If an ant tries to bite the butterfly it will
only get a mouthful
of scales.
Once outside the nest, the butterfly will expand
its wings fully.
end of story
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Much of the research at the Universities
of Copenhagen and Aarhus on large blue butterflies is done
in collaboration with other research groups, particularly
the other members of the EU-TMR network on social evolution.
Click here to learn more about these collaborations.
All pictures are ©
David Nash |
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