Quantifying the unquantifiable: why Hymenoptera – not Coleoptera – is the most speciose animal order

Abstract We challenge the oft-repeated claim that the beetles (Coleoptera) are the most speciesrich order of animals. Instead, we assert that another order of insects, the Hymenoptera, are more speciose, due in large part to the massively diverse but relatively poorly known parasitoid wasps. The idea that the beetles have more species than other orders is primarily based on their respective collection histories and the relative availability of taxonomic resources, which both disfavor parasitoid wasps. Since it is unreasonable to directly compare numbers of described species in each order, we present a simple logical model that shows how the specialization of many parasitic wasps on their hosts suggests few scenarios in which there would be more beetle species than parasitic wasp species. We couple this model with an accounting of what we call the “genus-specific parasitoid-host ratio” from four well-studied genera of insect hosts, a metric by which to generate extremely conservative estimates of the average number of parasitic wasp species attacking a given beetle or other insect host species. Synthesis of these two approaches suggests that the Hymenoptera may have 2.5 - 3.2x more species than the Coleoptera. “…if the micro-hymenopterists would get off their lazy asses and start describing species, there would be more micro-Hymenoptera than there are Coleoptera.”– Terry Erwin (in Rice, 2015)