There is no evidence that any of the species have been introduced artificially. Indeed, the absence of such forms as the Spider Heteropoda venatoria, the Scorpion Isometrus eruopaus (=maculatus, De Geer.) and the centipede Scolopendra moriscans, which accompany man in all his wanderings and establish themselves wherever the conditions of existence are favourable, seems to indicate that the species or their ancestors now found in the island crossed the sea that separates it from Malaysia by purely natural means - the Scorpions and the Myriopods and possibly some of the Spiders in connection, in all probability, with floating tree trunks or other vegetation…
C. W. Andrews, 'A monologue of Christmas Island (Indian Ocean): Physical Features and Geology', 1900

A note on Swell Moths on Christmas Island, an anamoly.

Swell Moths (Brana calopasa) feed exclusively on the tree Berrya cordifolia. Both species are found only in Sri Lanka, India and Christmas Island. It is hard to say if the tree and the moths are native or introduced to Christmas Island, but both species are well-established.

Yellow Crazy Ant, Anoplolepis gracilipes
Invasive

A diminutive Malaysian wasp is set to be imported to Australia in order to wage war against a plague of destructive crazy ants on Christmas Island.

The tiny wasp, which is just 2mm long, doesn’t sting or build nests but, it is hoped, will severely dent crazy ant populations.

It will do this, scientists believe, by preying upon an insect that produces a sugary substance called honeydew that crazy ants consume.

“By reducing the ants’ food supply, we hope to interrupt their breeding, and potentially stop them from building their devastating super colonies,” said Dr Peter Green, a La Trobe University researcher who is leading the project.

Park Australia, a federal government agency that oversees Australia’s leading national parks, is backing a move to import the micro wasps to Christmas Island this year.

Crazy ants were thought to have originally been brought to the island by south-east Asian traders in sea cargo. A horde of the pests is blamed for killing tens of millions of Christmas Island’s red crabs over the past 20 years. The loss of these crabs has had a negative knock-on impact upon the island’s entire ecosystem.

M. Oliver, ‘Australia to import micro wasp to wage war against plague of crazy ants’, The Guardian, 2015