Possible Fossil Larvae of Staphylinidae from Kachin Amber and a Quantitative Morphological Comparison Indicate That Rove Beetle Larvae Partly Replaced Lacewing Larvae

The highly diverse group of rove beetles, Staphylinidae, displays a great morphological variety across both adult and larval stages. However, due to the often cryptic habitats of their larvae, comprehensive descriptions of larval morphologies across extant ingroups remain incomplete. Also, the fossil record of rove beetle larvae appears to be sparse to date. We report here 35 newly identified fossil larvae of Staphylinidae from eight pieces of Myanmar Kachin amber, dating to the Cretaceous. Notably, one amber piece preserves three larval syninclusions, while another contains nine adults alongside twenty-six larvae, providing rare evidence of larval–adult co-occurrence in ancient environments. Given the predominantly predatory life cycle of extant rove beetles, the morphological similarities of these fossils to modern groups suggest comparable ecological roles in the Cretaceous, likely involving specialised forward-protruding mandibles adapted for a predatory lifestyle. A morphometric analysis of larvae of Staphylinidae alongside other extant and fossil predatory insect larvae still revealed significant differences between extant and fossil rove beetle larvae. Furthermore, Cretaceous rove beetle larvae differ from Cretaceous lacewing larvae in their head and mandible shape. In the modern fauna, the diversity of lacewing larvae is lower, and some modern rove beetle larvae now occupy areas formerly occupied by lacewing larvae. This result indicates that rove beetle larvae diversified after the Cretaceous, taking over certain ecological functions of lacewing larvae, likely representing a case of ecological substitution.