Our Beautiful World

Alcon blue, Ants and Marsh Gentian



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.



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.

A. The adult butterflies are on the wing throughout July.
B. 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).
C. 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.
E. 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



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.

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



The picture on the right shows a male Alcon blue sunning himself on a cloudy day in Denmark.


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



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.

 



Egg laying

Females of the Alcon blue lay their eggs on the flowers of the Marsh Gentian, Gentiana pneumonanthe.



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.

 


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.



Caterpillar emergence

The fourth instar caterpillar emerges from the gentian flower shortly after it has moulted

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.



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.



Adoption

If the caterpillar is discovered by the right sort of ant it will be adopted into the ant nest.

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.


Growth and development in the ant nest

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.

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!



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.

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).

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

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

 Links
 Västergötland: Alkonblåvinge (Sveriges Entomologiska Förening) swedish
 Klockgentiana Gentiana pneumonanthe L. - swedish


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