[1] Today P. arion can be found in Europe, the Caucasus, Armenia, western Siberia, Altai, north-western Kazakhstan and Sichuan.

The large blue butterfly is well known in behavioural ecology as it is a brood parasite of a single species of red ant, Myrmica sabuleti.

Large blue caterpillars grow to about half an inch (13 millimetres) in length, and spend up to 9 months before they undergo metamorphosis to a chrysalis to become a butterfly.

At once recognized by the large number of ocelli on the underside, especially on the hindwing, and by the bright blue dusting of the base beneath.

jasilkowskii Hornuz the ocelli are absent beneath in the cell as in euphemus, from which this aberration is at once distinguished by its blue-green basal scaling on the underside.

— Quite a number of local forms have been separated Northern specimens, which are feebly spotted, are named alconides by Aurivillius.

— In the South two aberrant forms have been found, namely ligurica Wagn., at the Eiviera between San Remo and Bordighera, with a conspicuous row of white marginal ocelli on the upperside of the hindwing, and aldrovandus S. L., from the Vesuvius, the underside darkened with brown.

[ now species Phengaris cyanecula (Eversmann, 1848)] (83 d) is an Asiatic form, from the Caucasus to Mongolia, with the metallic blue green dusting of the hindwing beneath being abundant, bright, and extending almost to the distal edge.

Larva adult pale ochreous, with a pale lilac tinge at the sides; head ochreous, marked with black anteriorly; prothoracic plate black; feeds until the autumn on Thyme, then disappears and is found full grown the next June in the nests of ants.

The species requires a combination of abundant amounts of its larval food plant, Thymus drucei and the presence of Myrmica sabuleti ants in order to survive.

A constant landscape synchronizes many biological activities including flowering of host plants, adult emergence dates, or larval pressures on the ant colonies.

[8] If important biological functions take place at the same times, the population becomes much more susceptible to random unfortunate events such as environmental disasters.

[9] Thus traditional farming acts to desynchronize the biological system, and allows for re-colonization of patches that are temporarily untouched.

[10] In the late 1900s, Phengaris populations began decreasing drastically throughout Europe with the large blue butterfly being particularly affected.

[11] Leading hypotheses targeted collectors, insecticides, and air pollution as factors that led to the butterfly extinction.

The succession of extinctions and decreases in population has been characterized as a result of unsuccessful conservation efforts that stemmed from a lack of understanding of the behaviour of the butterfly.

[16] Recent findings have also shown that there is a positive correlation between large blue butterfly conservation success and that of other endangered species.

[8] Since large blue butterfly predation of ant larvae can lead to desertion of the nest and B. euphrosyne tend to prefer violets growing on deserted ant nests, the fitness of B. euphrosyne appears to be indirectly affected by the presence of the large blue.

Like many members of the genus Phengaris, large blues are brood parasites, relying on another insect to raise their young.

[22] Due to differing reports and the difficulties involved in these types of studies, the nature of the host-parasite relationship is still inconclusive.

Since the parasitic-host relationship between the large blue and the Myrmica is essential for the caterpillar survival, female butterflies must lay eggs in areas where the larvae can be found by ant workers of the correct species.

[24] It was also thought that certain species of Phengaris were capable of avoiding overcrowding on food plants by detecting high egg loads.

Several caterpillar species of Phengaris, such as P. rebeli and P. alcon, will secrete pheromones that are specific to their respective hosts.

[26] Today it has been determined that it still secretes semiochemicals as a form of chemical mimicry to gain acceptance into the host ant nest.

[26] Even once Phengaris butterflies have infiltrated the host nest, they continue to hide their identity as caterpillars and will go further in their act of deception.

There have been many studies documenting the use of acoustic communication in ants, and it has been found that members of the genus Phengaris exploit this behaviour.

Different Myrmica species utilize distinct semiochemicals to distinguish themselves, but they use very similar acoustic commands once in the nest.

[32] Scientists remain unsure why there are multiple strategies within the host nest, but studies have been conducted to determine the effectiveness of each.

Since the cuckoo caterpillars remain in close vicinity of the ants, they must secrete chemicals that are almost identical to the host species in order to survive.