Unusual snakefly larvae in about 100 million-year-old amber and the evolution of the larva-pupa transition

The evolutionary success of Insecta, more precisely of its ingroup Holometabola, has partly been explained by their ontogeny, with larvae and adults differing in their morphology and ecology. This differentiation occurs in large and well-known groups such as beetles, butterflies and bees, but also in the relatively species-poor group of snakeflies (Raphidioptera). Despite the rather small number of species, snakeflies are evolutionarily very significant as they were part of the early diversification of Holometabola and still exhibit several plesiomorphic traits retained from the ground pattern of the latter, for example, a mobile pupa. Furthermore, during development, some snakeflies show a mixture of larval and pupal, sometimes even of adult characters, a phenomenon called metathetely. We here report a 100 million-year-old fossil snakefly larva from Myanmar amber with possible characters reminiscent of metathetely. Different dimensions of the body were measured in the specimen and other snakefly larvae and pupae, and ratios were calculated and compared among the larvae. The new fossil shows similarities to extant pupae in the larger length of the prothorax, similarities to modern adults in the small width of the prothorax, but also similarities to other fossil snakefly larvae such as the undivided tarsus and the antenna being subdivided into only five elements. Such a mixture of characters from different developmental stages points to a less pronounced metamorphosis in fossil snakeflies than in extant ones. Similar ontogenetic patterns, with a more gradual development in earlier representatives evolving into a more pronounced metamorphosis in modern representatives, are also known in other groups of Euarthropoda and point to heterochronic events in the evolution of these lineages.