Soldier beetle larvae are much more common in the fossil record than previously anticipated

Solider beetles (Cantharidae) are relatively well known in the fossil record. Yet, not many fossil larvae of this group have been reported, despite the importance of this developmental phase. The larval phase of solider beetles is different from that of many other beetles, as in some species the early larvae (“pre-larvae”) differ from the later larvae. Therefore, the development of Cantharidae is possibly associated to hypermetamorphosis, a developmental pattern with early larvae being different in morphology and fulfilling different ecological functions than the later larvae. This type of differentiation between life stages has been thought to be a driving force in the evolution of beetles and other holometabolans (wasps, butterflies, etc.). Details of the evolution of this developmental pattern should therefore be informative for improving our understanding of evolutionary response to ecological selective pressures, and fossils could provide details for a well-resolved reconstruction. Unfortunately, so far only three fossil larvae of Cantharidae have been reported. Here we add 44 new specimens from Baltic and Kachin (Myanmar/Burma) amber and one new adult specimen in Baltic amber. Together with fossil and extant specimens from the literature we assembled a dataset of over 300 specimens, immatures and adults. Shape analyses and measurements were performed to investigate the change over time (evolution) of the change over time (ontogeny) on a quantitative level. Our results indicate a higher diversity of extant forms of Cantharidae, but also fossil morphologies could be recognised that are absent in the modern-day fauna, indicating certain losses of diversity.