Phengaris (Maculinea) arion
Switzerland, July 2008
Female, Switzerland, July 2006
July 2006, Switzerland
Female, France, July 1982 - my only picture of the lowland form
Aberrant, Switzerland, July 2004
This is the most widespread and generally the commonest of the four
European species formerly classified in the iconic genus Maculinea - now Phengaris.
All four share the peculiar - and amazing - characteristic that the
larvae must be adopted by ants and taken into the heart of an ants'
nest to survive. This happens in the third instar, when the caterpillar
stops feeding on thyme (in the case of the present species) and begins
secreting a honeydew-like substance attractive to ants. After a certain
period of attention by ants, the caterpillar is sufficiently perfumed
with ant pheromones to be picked up as one of theirs and dragged into
the nest. There, it is fed on ant larvae by its doting hosts and
continues to grow until eventually it pupates underground. The
following summer it emerges from the chrysalis, climbs up into the
light and flies. This singular life cycle means the butterfly is not
only dependent on the physical and floral aspects of a habitat but also
on the presence of the correct species of ant. It is probable that the
extinction of the large blue in the UK was hastened by the loss of
rabbits to myxomatosis, leading to deepening of the grass sward with
consequent cooling of the ground and loss of ants. The butterfly has
been successfully reintroduced in this country and the new colonies are
now stable thanks to careful habitat management and protection.
All except one of the above pictures were taken in mountains, where the large blue is typically rather melanic. Some males may be completely black. In the lowlands the upperside is much brighter, with broad but clearly demarcated dark borders and sky blue basal areas. There is an arc of discal spots on the forewings, the spots on the female being noticeably larger. In mountain forms these spots may be obscured by the melanic suffusion. The underside of both sexes follows the same pattern as that of all the Phengaris species but is much better marked and more contrasting than the others. In particular, there is a well defined double submarginal series. In other species there is either no submarginal series (dusky large blue) or essentially a single series with a shadow (scarce large blue and alcon blue). In Switzerland, the large blue often shares habitat with dusky large blue and scarce large blue, and as the upperside is heavily melanic in this country a view of the underside is often important to confirm identity.
The large blue is found above all on hot banks where thyme is
abundant. Females spend a lot of time around the thyme, nectaring on it
as well as checking it out for laying. The flight season is rather
short, beginning in late May or early June and running into July.