Imperial Blue Butterfly

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Imperial Blue Butterfly

John Dengate came across a caterpillar covered by a swarm of ants. At first glance it seemed as though the ants were killing the caterpillar, but as John explained, these two insects have a very special relationship.

Imperial blue butterfly (Jalmenus evagoras)

The Imperial blue butterfly has a wingspan of around 35mm. It is metallic blue in colour with orange spots and black tails on the hind wings. The underside of the wings is a buff colour. Imperial blues are found along the east coast of Australia, from north of Brisbane down through Sydney and Melbourne.

Mutualistic relationship

The Imperial blue butterfly lays white or pale green spiky eggs on a food plant such as a silver wattle (Acacia dealbata), black wattle (A. mearnsii) or blackwood (A. melanoxylon). When the black caterpillars emerge from the eggs, they are tended by ants belonging to the genus Iridomyrmex. The ants protect the caterpillars and the pupae from predators and parasites. In return, the caterpillars secrete food for the ants through a number of specialised organs:

a nectar producing honey gland button-like organs which produce a secretion containing amino acids tentacular organs resembling inside-out rubber gloves. When the caterpillar is threatened these organs pop up and produce chemicals that stimulate the ants to fend off predators.

The acacia is important to the ant-caterpillar relationship, because it is a nitrogen fixing plant. The caterpillar feeds on the acacia leaves then passes on nitrogen to the ants in the form of amino acids, which are the building blocks of protein.