Butterflies of
Britain & Europe
Large Blue
Maculinea arion
LINNAEUS, 1758
Family - LYCAENIDAE
subfamily -
POLYOMMATINAE
introduction
|
habitats |
lifecycle |
adult behaviour
|
 |
|
Large Blue
Maculinea arion, female, Collard
Hill, Somerset |
Introduction
The
Large Blue is widely distributed across Europe, but absent from southern Spain,
Portugal, northern Scandinavia, and most Mediterranean islands. It's range also
includes Turkey, Russia, western Siberia, Mongolia, temperate China and Japan.
Since the discovery of the first British colony in 1795, the butterfly has
always been considered a rarity. In the 19th century, collectors searched far
and wide, successfully locating colonies at sites including the cliffs of Dover,
Marlborough Downs, Winchester, Somerset, the Cotswold hills, south Devon, Buckinghamshire, and at Barnwell Wold in Northamptonshire.
Butterfly collecting was an obsession among the middle
classes during the Victorian period, and the Large Blue was regarded as a major
trophy which could be sold for substantial profit. As a direct result most
colonies were exterminated by collectors. By 1950 only 30 sites remained. The
spread of myxomatosis then caused the collapse of rabbit populations, and as a
consequence the sites quickly became under-grazed, and the microclimate became
too cool to support the ant Myrmica
sabuleti, with which
the butterfly has a symbiotic relationship. By
1970 only 3 or 4 colonies survived, all on nature reserves. Populations however
continued to decline dramatically, and the
Large Blue
became extinct in Britain in 1979.
In the 1980's, following extensive research into the butterfly's ecology by Dr Jeremy Thomas, plans were laid to reintroduce the butterfly using livestock
imported from Sweden. These reintroductions, mostly on private nature reserves,
met with varying degrees of success.
At the time of writing ( 2009 ) the
Large Blue has become successfully re-established at several sites in Devon and
Somerset, and it's future in Britain, at least in the short term seems assured.
|
 |
|
Large Blue ovipositing ( note egg already laid on lower
right of thyme flower ), "site H", Somerset |
Habitats
In Europe the butterfly
can be found in a wide variety of habitats including coastal cliffs, limestone
gorges, stony plateaux, and alpine pastures. In France for example I have found
colonies as high as 2000m in Vanoise National Park, at 1500m on Mont Dore in the
Massif Central, and at roadside verges and woodland glades at Causse de Gramat.
In Britain the butterfly
is confined to a small number of dry calcareous grassland hills in Devon and
Somerset, where thyme grows in profusion, and where the microclimate is warm
enough to allow the ant
Myrmica sabuleti
to flourish. Maintaining these conditions normally requires a program of cattle
and / or sheep grazing. The butterfly also benefits from the presence of a
limited amount of scrub, typically gorse or hawthorn, and thrives best where sheltered
areas of long grass are available for roosting.
Colonies everywhere tend to be small, typically less than 20 adults, but I know
of well managed sites where up to 70 butterflies can be seen in a day,
distributed over an area of several hectares. Population densities normally vary
between 2 - 10 butterflies per hectare, but can be as high as 50 per hectare
where conditions are ideal.
|
 |
|
Large Blue
Maculinea arion, male, "site H", Somerset |
Lifecycle
The butterfly emerges in June, or
sometimes in early July. The entire flight period in Britain
spans about 40 days, with individual butterflies living for up
to 2 weeks, although the average lifespan is only about 5 days.
The pale bluish-white eggs are
laid singly on the flower buds of various plants in the family
Labiatae - favouring marjoram in lowland areas of France, but
usually selecting thyme
Thymus pulegioides
in the Alps and in Britain. The plants chosen by the females
usually have several open florets.
|
 |
|
Large Blue
Maculinea arion, female, "site H",
Somerset |
It is
possible to find several eggs on the same flower-head, but these
will have been laid by different females, or on return visits by
a single female.
At one of the British sites, in
June 2007 I observed 2 different females ovipositing on the
flower buds of self-heal
Prunella vulgaris
( see photo below ). Unfortunately, although self-heal is
closely related to thyme, larvae are unable to survive on this
plant for more than a few days
and always die.
|
 |
|
Large Blue
Maculinea arion, female ovipositing
on self heal, "site H", Somerset |
The caterpillars hatch after 7-8 days, and begin feeding on the
flower sepals, on which they are well camouflaged. They eat
holes in the sepals to enable them to reach and feed upon the
developing seeds. Each larva needs to eat every seed on the
flower-head if it is to survive to the fourth instar, and will
eat attack any competing caterpillar that it encounters, and eat
it to ensure that it has sole access to all the seeds on the
plant.
Upon reaching the fourth and final
instar it
loses interest in
feeding, becomes restless, and eventually releases it's grip on the
foodplant and falls to the ground. It wanders about until
it is located by an ant of the species Myrmica sabuleti
or M.
scabrinodis.
The ant then begins to caress the larva with it's antennae,
which stimulates the larva to secrete a honey-like fluid from a
"Newcomer's gland" on it's back. After drinking the
honey the ant wanders
off, but returns later with other ants to further "milk" the larva.
After several milking sessions the
larva becomes immobile, hunching it's back, and allows an ant
to seize it and carry it into the brood chamber of the ant nest.
Once settled in its new home the larva becomes carnivorous, feeding for the
next 6 weeks on tiny ant grubs.
In early September the larva
enters hibernation, remaining quiescent until the following
April, when it resumes feeding on the ant grubs. During this
carnivorous stage the larva is tolerated by the adult ants in
exchange for providing them with a regular supply of "honey"
from it's dorsal gland.
It is likely that
larva emits pheromones which appease the ants and ensure its
safety, but many are killed by worker ants, and others fail to
reach maturity, as only the largest ant nests produce the 1500
or so grubs that are necessary for the larva to complete its
growth.
The pupa is formed within the brood chamber, where it lies for
the next 3 weeks, unprotected by any form of cocoon. Like the
larva, it exudes "honey", and is constantly attended by the
ants.
The adult butterfly emerges within
the nest, but immediately before emergence the pupa produces a
rasping "song" which stimulates nearby ants into a frenzy. The
butterfly then breaks out of the pupa, and with it's wings still tiny and limp, crawls along
the ant tunnels until it reaches the surface, where it climbs a
stem and hangs to expand and dry it's wings. Throughout this
time the highly vulnerable butterfly is constantly surrounded by
the ants. They make no attempt to attack it - their presence in
fact acts as a very strong deterrent to any other creature that
may be considering the Large Blue as it's next meal. The
butterfly almost certainly therefore is able to appease the
ants, by releasing a chemical that either warns that it is
distasteful, or mimics the smell of something that the ants find
repellent or dangerous.
Adult behaviour
The
adults fly rapidly over short distances, settling periodically to
nectar or to rest with closed wings, on
plant stems. They are deceptively strong flyers, and if disturbed they will fly up to 100 metres before
settling. I have on several occasions observed Large Blues fly
over tall hedges, or attempt to cross open fields, but invariably
after a few seconds of investigation the butterflies return to
their habitat. In June 2009, I closely followed a particular
female at "site H" in Somerset and noted that it's "home range"
extended a distance of almost a kilometre from one end of the site
to the other, and that it oviposited on plants right across the
site. This site was in fact colonised naturally from a nearby
introduced colony, and it is apparent hat the butterfly has
excellent powers of dispersal provided that areas of its habitat
are contiguous.
In overcast but warm conditions they bask with wings
held half open, on bushes or amongst grasses. On sunny mornings
both sexes fly along the bottom of slopes, where the very brief
courtship flight takes place. The female then settles on a bush or
among grasses, followed closely by the male, who crawls alongside
her, and then curves his abdomen to make contact. In England and
France I have found copulated pairs sitting half way up thistle
stems, low down on grass blades, and sitting on bare ground or
rocks.
Most nectaring behaviour takes place in the afternoon, when the
butterflies settle for a moment here and there to feed at the
flowers of marjoram, thistles, thyme, red clover and white
clover.
The butterflies
roost overnight under bushes, or amongst tall grasses, usually
where these occur in sheltered depressions.
|
 |
|
Large Blue
Maculinea arion, "site H", Somerset |
|
