The Adonis blue
(Polyommatus bellargus) is a butterfly in the family Lycaenidae. It is found in
the Palearctic ecozone (Western Europe, Central Europe, South Europe, South
Russia, Iraq, Iran, Caucasus, Transcaucasus, Turkey).
Description
The males have brilliant eye-catching
sky-blue wings, while the females are brown and far less conspicuous. Both
sexes have distinctive black lines that enter or cross the white fringes to the
wings.
The Adonis Blue and many
other species have conspicuously coloured males, but relatively plain females.
Males use "flash colouration" to confuse predators. If a bird sees
the butterfly in flight or basking on a flower, it targets the bright blue
image. The butterfly reacts to threat by settling immediately on the ground and
closing it's wings, revealing the spotted greyish underside which is an
effective camouflage when it is settled on chalky soil.
Females are surreptitious,
tending to move very little until mated. When searching for egg-laying sites
they tend to move slowly and deliberately, and for them, plainer colours are a
better means of defence. Most female Adonis Blues are dingy brown in colour,
with hardly any blue scales, so they easily escape the notice of birds.
Distribution
and habitat
The Adonis blue is found
across Europe from Eastern and Central Europe across to southeast England and
the Iberian Peninsula.
The preferred habitat of
the Adonis blue is calcareous grasslands with hot and dry conditions. This is
because the larva feeds on horseshoe vetch (Hippocrepis comosa) which is
largely restricted to these habitats. This plant flourishes on sheep-grazed
pastures and disappears after a few years when cattle graze. Sometimes
human-created landforms provide refuges for the plant and the butterfly; an
example of this is on Martin Down in Dorset, where horseshoe vetch only occurs
on ancient earthworks with thin calcareous soils.
Lifecycle
There are two generations
in Britain, the first emerging in May, and the second in late August.
The flat whitish eggs are
laid on the underside of terminal leaflets of horseshoe vetch Hippocrepis
comosa. There are no alternative foodplants. The butterflies choose plants
which are in full sunlight, in sun-baked positions at the edge of rabbit
scrapes, paths or other patches of bare ground. The eggs are laid singly, but often
several females will visit a particular favoured plant, and consequently as
many as 30 can be found in very close proximity.
The larvae feed diurnally.
When tiny they feed on the under surface of the leaves where they nibble here
and there, producing tiny holes, but leaving the upper cuticle intact. The
feeding damage can be seen from above as a peppering of microscopic shiny spots.
When older the larvae rest on the upper surface of the leaves but are very well
camouflaged and difficult to spot.
They are commonly attended
by ants Lasius niger, Lasius alienus or Myrmica sabuleti. The ants are
attracted by a sugary secretion which they milk from a gland near the
caterpillar's tail. The larvae seem unable to survive without the ants, whose
presence deters other insects that would otherwise attack and kill them. At
dusk the larvae retreat to the base of the foodplants, where they assemble in
groups of about half a dozen. They are followed by the ants which cover them in
a thin layer of soil particles, and stand guard over them during the night.
Adonis Blue larvae ( and
those of most other Blues ) appease their attendant ants by "singing"
to them, thereby avoiding being eaten themselves. It is not known how the
larvae produce their song, although there are microscopic knobs and plates
around the opening of the honey gland which may be involved.
Larvae of the second
generation enter hibernation almost immediately after hatching, and awaken to
begin feeding in March.
When fully grown they are
deep green, each segment having a prominent hump. A series of bright yellow
dashes runs along each side below the spiracles, and a pair of broken yellow
stripes run along the back.
The pupa at first is pale
olive, but changes to become straw coloured after a few days. It is formed in
crevices on the ground, but ants quickly cover it with a thin layer of soil.
Sometimes pupae have been found inside the brood chambers of ants. Immediately
prior to emerging from the pupal case, the butterfly produces a crackling song
to pacify the ants. It then rapidly breaks through the soil to crawl up a stem
where it hangs to dry it's wings.
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