Abstract
"Beak larva" is a collective term for four fossil specimens from Kachin amber, which are holometabolan larvae with a forward-projecting beak. This arrangement is very unusual, and so far it has been unclear whether these specimens are larvae of beetles or of lacewings. We present here new details of the mouthparts of one of these larvae based on synchrotron radiation-based X-ray µ-CT (SRµCT) imaging. We can identify that the main part of the beak is formed by the labrum and the labium; the mandibles insert laterally into this beak. The beak has two distinct channels to which the mandibles seem to be connected. The maxillae are tightly connected to the labium and head capsule and have an endite each (unclear if lacinia, galea, or mala), also inserting into the beak. Overall, these details reveal a mouthpart arrangement incompatible with an interpretation as a lacewing. The arrangement is most similar to that in some beetle larvae, namely those of Elateroidea. It most closely resembles that of the larvae of the species-poor sister groups Cerophytidae and Jurasaidae. It seems likely that the beak larvae are closely related to Jurasaidae, possessing an intermediate morphology. The combined data allow us to reconstruct aspects of the entire group, including the beak larvae, Cerophytidae, and Jurasaidae, indicating that they undergo hypermetamorphosis. The latter is most expressed in Jurasaidae. Combined with the paedomorphic female, this group seems to have four rather distinct morphologies and ecologies: (1) early mobile larvae, (2) physogastric, strongly wormlike larvae, (3) "normal"-appearing adult males, and (4) females with another unique morphology.