Development of Raphidioptera Was More Gradual in the Past as Revealed by Quantitative Morphological Analysis

Snakeflies (Raphidioptera) are generally assumed to have the most gradual (and plesiomorphic) type of holometabolous metamorphosis, often including saproxylic larvae. Herein we investigate the diversity of snakeflies over time. We explore morphological details that have rarely been in focus of scientific studies such as the clavate organs of the legs. In total, 165 new immature snakefly specimens, mostly from 100 million-year-old (Cretaceous) Kachin amber, are reported. Combined with data from the literature, we assembled a dataset of 550 specimens, including immatures and adults from Cretaceous (over 200 immatures) and Eocene amber and from the extant fauna. From these, we extracted shape data of different body regions—ten subsets in total with over 2500 analysed shapes. Our analysis supports earlier observations (based on relative lengths) that snakefly larvae were much more diverse in their morphology in the past compared to their modern representatives. Furthermore, we recognise a strong morphological separation of modern larvae and adults (with pupae being intermediate), while in fossils the overlap of representatives of both life phases is quite strong. This supports earlier qualitative observations that the ontogeny of Cretaceous snakeflies was even more gradual (and likely plesiomorphic for Raphidioptera and presumably Holometabola) than in extant snakeflies. The analyses revealed that some Cretaceous and Eocene snakeflies had a slender head and prothorax morphology that is absent nowadays. This supports a difference between the modern and Eocene fauna. Additionally, a gap analysis was performed for the best-sampled subsets to explore morphological constraints in snakefly morphology.