Hellgrammites and their relatives in the deep past – the fossil record of megalopteran larvae (Insecta)

Dobsonflies, fishflies, and alderflies are all representatives of the group Megaloptera. Today, the group Megaloptera comprises around 400 formally described species, making it one of the smaller ingroups of Holometabola, the group of “flying insects” characterised by a strongly expressed metamorphosis. Like most holometabolans, megalopterans spend the majority of their lives as immatures, also known as larvae. These larvae are aquatic and predatory; some of these are very large and known as “hellgrammites”. The fossil record of megalopteran larvae is relatively poor, reaching back into the Triassic. On a qualitative level, the fossil larvae strongly resemble their modern counterparts. We report new finds such as Triassic sedimentary fossils and fossils in amber from the Cretaceous and Eocene. Together with the already known specimens, we compared the fossils to their modern counterparts, using quantitative morphology of different body regions. For further comparison, we also included larvae of other holometabolan lineages with similar life habits. Especially larvae of Gyrinidae, the group of whirligig beetles, are similar to megalopteran larvae in many aspects: the body is rather elongate, the mouthparts are prominent and forward-protruding, the trunk region has a pair of elongate gills on most segments. The analysis revealed that at least some of the fossil megalopteran larvae had morphologies not present in the modern fauna, indicating a certain loss of diversity over time.