Taxonomic investigations of the Delias mysis (Fabricius, 1775) complex from northern Australia indicate two additional species in the Australian fauna: Delias aestiva Butler, 1897 stat. rev. and Delias lara (Boisduval, 1836). The latter species, which is illustrated from Australia for the first time, was until recently known under the name Delias mysis onca Fruhstorfer, 1910. Evidence from adult morphology (male genitalia), colour pattern of the adult and immature stages, behaviour, and ecology indicates substantial phenotypic divergence between D. aestiva and D. mysis. Within Australian limits, all three taxa are allopatric: D aestiva is endemic to the Top End, Northen Territory, D. mysis mysis is restricted to northern and north-eastern Queensland, whereas Delias lara lara is known only from three specimens from the Torres Strait islands, Queensland. Delias aestiva is perhaps the most remarkable member of the complex and indeed the genus, breeding in tropical mangrove habitats in coastal estuarine areas where the larvae specialize on mature foliage of the tree Excoecaria ovalis Endl. (Euphorbiaceae). This host preference is novel given the general tendency of Delias to feed on hemiparasitic plants in the order Santalales (Loranthaceae, Santalaceae and Viscaceae). Under laboratory conditions, however, larvae successfully completed development on the mistletoe genera Amyema, Dendrophthoe and Decaisnina (all Loranthaceae) with no significant reduction in larval survival. These findings, together with phylogenetic hypotheses of the Aporiina and Delias, indicate a recent evolutionary host shift from Loranthaceae to Euphorbiaceae. The foliage of Excoecaria produces toxic latex, which is composed of a variety of secondary plant compounds, including diterpenoids, triterpenoids, alkaloids and phorbol esters. The mechanism of detoxification has not been established, although the larvae of D. aestiva are gregarious, regurgitate fluid as part of their chemical defence, and the adults are highly aposematic. Adults are seasonal, being chiefly on the wing during the cooler dry season; during the wet season, the larval food plant is seasonally deciduous and it is suspected that the butterfly undergoes pupal diapause. The cryptically coloured green pupa and tendency to pupate singly in concealed situations of D. aestiva are highly unusual traits among Delias and are hypothesized to be adaptive responses associated with pupal diapause during the wet season. The unique habitat association, novel food plant specialization, and restricted distribution of D. aestiva emphasizess the biogeographical peculiarities of northern Australia, especially patterns of historical (vicariant) differentiation between the Top End and Cape York Peninsula within the Australian Monsoon Tropics. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107, 697–720.