Common Imperial Blue
(one synonym : Polyommatus coelestis Drapier, 1819)
ZESIINI , THECLINAE , LYCAENIDAE , PAPILIONOIDEA
Will Douglas (Burra Creek, NSW)
The Caterpillars are flattened and have fleshy spiky dorsal tubercles on most segmemts. They are coloured dark green with an orange dorsal line and other lighter markings. They feed communally in a web on the foliage on various Wattles ( Acacia, MIMOSACEAE ) such as:
The host is invariably a juvenile bush under 2 metres tall. The Caterpillars always attended by swarms of black ants from the subfamily DOLICHODERINAE , including :
These ants have been observed defending the caterpillars from predators such as mantids and spiders. Other species of ants attack and kill the Caterpillars. A good way of locating the caterpillars is to follow ants as they run along the branches of potential foodplants. The Caterpillars grow to a length of about 2 cms.
They pupate communally in their web, amongst the foliage of the foodplant. The individual pupae are black and shiny, with orange between segments. Naomi Pierce at Harvard has found that the caterpillars and pupae make chirping noises used to communicate to each other and to their attendant ants. The pupae have a length of about 1.3 cms.
The adult is metallic blue in colour with black wing margins. A small black bar occurs at the end of the cell on each fore wing. Each hind wing is extended into a black tail, and is decorated with two orange spots and some thin white lines. The undersurfaces of the wings are creamy fawn, marked with black lines of various lengths. The undersides have black termens with pale orange-brown subterminal bands, and bright orange patches to mark each tornus.
The coloured tails of the butterfly look like white-tipped antennae on bright red/orange and black colourings which, with wings folded (the habitual posture) makes the back end of the butterfly look like the front end (the actual head and antennae being quite bland). This is perhaps a decoy perhaps in case of bird strike.
The wingspan is about 4 cms. The adults are seldom seen more than 20 metres from a foodplant, and are inclined to congregate around the foodplant, so are easily detected this way.
In Melbourne in December and January, we have found eggs and Caterpillars and pupae and adults on the same bushes at the same time. There appear to be two generations per year in Melbourne, with the later generation overwintering as eggs, and hatching the following spring. The reproduction of the species has been studied by Diane Wagner at Harvard and Lesley Hughes at Macquarie University.
Two subspecies have been recognised in south-eastern Australia :
Further reading :
Issue 24, Oct-Dec 1991, pp. 36-42.
Michael F. Braby,
Butterflies of Australia,
CSIRO Publishing, Melbourne 2000, vol. 2, pp. 724-726.
An Epitome of the Natural History of the Insects of New Holland, New Zealand, New Guinea, Otaheite and other Islands in the Indian, Southern and Pacific Oceans.
The Imperial Blue Butterfly,
DVD, Friends of Warrandyte State Park, 2009.
Imperial Hairstreak butterfly (Jalmenus evagoras) (also known as Common Imperial Blue Butterfly,
Issue 57 (June 2010), pp. 13-15,
Butterflies and Other Invertebrates Club.
Successful butterfly colony transfer?,
Butterflies and Other Invertebrates Club Newsletter, Issue 24 (March 2002).
Further to Jalmenus evagoras translocation,
Butterflies and Other Invertebrates Club Newsletter, Issue 32 (March 2004), pp. 14-16.
(updated 2 November 2009)