Jalmenus evagoras (Donovan, 1805)
Imperial Hairstreak
(one synonym : Polyommatus coelestis Drapier, 1819)
Don Herbison-Evans
Stella Crossley
Will Douglas (Burra Creek, NSW)

Initially the Caterpillars of this species are orange, and they wander around looking for an ant trail. They then follow this hopefully to find a cluster of older caterpillars.

Jalmenus evagoras
(Photo: courtesy of Merlin Crossley, Melbourne, Victoria)

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 bipinnate Wattles (MIMOSACEAE) such as:

  • Sally ( Acacia falcata )
  • Mudgee Wattle ( Acacia spectabilis ).

    The host is often a juvenile bush under 2 metres tall. The caterpillars always attended by swarms of black ants from the subfamily DOLICHODERINAE , including :

  • Iridomyrmex anceps, or
  • Iridomyrmex rufoniger.

    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.

    Jalmenus evagoras
    (Photo: courtesy of Helen Schwencke, from Create More Butterflies)

    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.

    Jalmenus evagoras
    (Picture: courtesy of CSIRO Ecosystem Sciences)

    The male adult is metallic greenish-blue in colour with black wing margins. A small black bar occurs at the end of the cell on each forewing. Each hindwing is extended into a black tail, and is decorated with two orange spots and some thin white lines. The female is similar except for whitish central areas on the wings.

    Jalmenus evagoras
    (Photo: courtesy of Martin Purvis, Ingleburn, New South Wales)

    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.

    Jalmenus evagoras
    (Specimen: courtesy of the The Australian Museum)

    The under-surfaces 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 wingspan is about 4 cms.

    Jalmenus evagoras
    (Photo: courtesy of Martin Purvis, Ingleburn, New South Wales)

    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.

    Jalmenus evagoras
    eggs, magnified
    (Photo: courtesy of Ken Walker, Melbourne, Victoria)

    The eggs of this species are pale grey or green, and ridged with a white lattice with little spikes. The eggs have a diameter of about 0.6 mm. The eggs are laid in clumps on the stems of a foodplant.

    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.

    The species have been found in south-eastern Australia in :

  • New South Wales, and
  • Victoria.

    Further reading :

    Imperial Blue,
    Australian Geographic,
    Issue 24, Oct-Dec 1991, pp. 36-42.

    Michael F. Braby,
    Butterflies of Australia,
    CSIRO Publishing, Melbourne 2000, vol. 2, pp. 724-726.

    Edward Donovan,
    General Illustration of Entomology,
    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,
    Volume 1 (1805), p. 137, and also Plate p. 136..

    Frank Jordan & Helen Schwencke,
    Create More Butterflies : a guide to 48 butterflies and their host-plants
    Earthling Enterprises, Brisbane, 2005, pp. 22, 62.

    Linda Rogan,
    The Imperial Blue Butterfly,
    DVD, Friends of Warrandyte State Park, 2009.

    Australian Butterflies
    Australian Moths

    (updated 2 November 2009,18 June 2019, 3 August 2020, 6 September 2021)