A cretaceous fly trap? remarkable abdominal modification in a fossil wasp

Abstract

Background: Carnivorous insects have evolved a range of prey and host capture mechanisms. However, insect predation strategies in the fossil record remain poorly understood. Results: Here, we describe †Sirenobethylus charybdis n. gen. & sp., based on sixteen adult female wasps in Kachin amber from the mid-Cretaceous, 99 Mya (million years ago), and place it in Chrysidoidea: †Sirenobethylidae n. fam. The fossils display unique morphological modifications on the tip of the abdomen consisting of three flaps from the modified abdominal sternum 6 and tergum and sternum 7; the lower flap formed from sternum 6 is preserved in different positions relative to the other flaps in different specimens, indicating that they form some sort of grasping apparatus. Nothing similar is known from any other insect; the rounded abdominal apparatus, combined with the setae along the edges, is reminiscent of a Venus flytrap. Phylogenetic analysis suggests that the new family is a separate lineage close to the base of Chrysidoidea. Conclusions: †Sirenobethylus probably was a koinobiont parasitoid wasp; the abdominal grasping apparatus may have been used to temporarily immobilize the host during oviposition. The new fossils suggest that Chrysidoidea displayed a wider range of parasitoid strategies in the mid-Cretaceous than they do today.