An ecological survey of tree trunk invertebrates in northern New South Wales was used as a model to demonstrate both the scale of arthropod diversity and the limits of our knowledge. Sticky traps were an effective way of systematically sampling trunk-utilising invertebrates, particularly Diptera, and over 103,000 arthropods in 215 families were caught. This paper discusses sticky-trap methodology, the taxonomic impediment in studying invertebrates beneath family level, seasonal differences in composition and abundance, and rarity. The category “tourist” in arboreal arthropod trophic guilds is regarded as highly misleading, as such taxa are usually local, but simply have their major trophic interaction as larvae. Approximately 75% of the invertebrates caught were soil detritivores during their larval stage. Because invertebrates are so diverse, and our knowledge of their ecological functioning is sketchy, conservation of forest invertebrates depends on maintaining overall forest integrity. Establishing biotic integrity indices that reflect essential ecosystem functioning, such as soil detritivore activity, may provide a useful tool for monitoring forest health.