A brief overview of the hymenopteran megaradiation – one of evolution’s greatest success stories

Hymenopteran diversity has evolved over 250+ Ma and currently comprises approximately 150,000 recognised species; the total number is likely to be many times higher, given the status of knowledge on many potentially hyperdiverse parasitoid wasp taxa. The common ancestor of all Hymenoptera was probably a herbivorous sawfly, but the majority of the order is carnivorous. The first transition from the herbivorous to the carnivorous lifestyle took place early in the evolutionary history of Hymenoptera, probably via a woodwasp intermediate stage with larvae feeding internally in wood with the help of symbiotic fungi. The first parasitoid wasp was most likely an idiobiont ectoparasite feeding on insect larvae inside wood. The parasitoid wasps are the most diverse ecological group of hymenopterans, having specialised in a wide range of primarily insect but also other terrestrial arthropod hosts. Secondarily herbivorous lifestyles have evolved many times independently from among parasitoid or predaceous wasps: gall-makers (Cynipidae), seed parasitoids/predators (Megastigmidae), fig pollinators (Agaonidae), pollen collectors (Anthophila) and fungal gardeners (Attini). These forms of herbivory are highly specialised compared to the original primary herbivory allegedly ancestral to Hymenoptera. Within the stinging wasps (Aculeata), several groups are provisioning predators or pollen collectors rather than parasitoids. These provisioning nest builders have in turn given rise to eusocial Hymenoptera several times independently, in ants, bees and vespoid wasps. Some eusocial taxa are the most abundant Hymenoptera in their ecosystem and have a correspondingly high ecological impact. In spite of being mostly beneficial to humanity, hymenopteran diversity is currently under threat due to human impact on the biosphere.