Abstract Aim To evaluate the arthropod community of Motmot in relation to primary colonization of young volcanic surfaces. Location Motmot, an island in Lake Wisdom which occupies the caldera of Long Island, Papua New Guinea. Methods Arthropod sampling by means of pitfall, water and tube traps, fallout collectors, and hand collecting. Results At least thirty-five species of arthropod were collected in 6 days between 23 June and 3 July 1999. Lycosid spiders and ants dominated in all areas. The predator-scavenger arthropod population is largely or entirely dependent on allochthonous input of aquatic insects from the surrounding lake. Main conclusions Major changes in the arthropod fauna since the pioneer surveys of Ball and his colleagues in the 1970s are the loss of a strand flotsam community as the island has eroded to form a predominantly cliffed coastline. Ant and spider diversity has increased. The current colonists include a number of widespread ‘tramp' species sensu Diamond.