# The About 100 Million Years Old Enigmatic “Beak Larva” Is a Weird Click Beetle Relative

## TL;DR

A 100-million-year-old fossil larva with a beak-like mouth is likely a relative of click beetles, showing unusual developmental stages.

## Contribution

New synchrotron imaging reveals the beak larva's mouthparts are most similar to beetle larvae, not lacewings, and suggests a close relationship with Cerophytidae and Jurasaidae.

## Key findings

- The beak larva's mouthparts resemble those of beetle larvae in the Elateroidea group.
- The larva likely belongs to Cerophytidae or Jurasaidae, with an intermediate morphology.
- The group exhibits hypermetamorphosis, with four distinct larval and adult forms in Jurasaidae.

## Abstract

“Beak larva” is a term used for four fossil larvae with specific mouthparts resembling a beak. These mouthparts are rather untypical, and it is unclear whether the larvae are lacewings or beetles. We present new details of one beak larva, scanned with synchrotron radiation. We can identify the composition of the beak from the position of the different structures of the mouthparts. The arrangement of the mouthparts opposes the interpretation of the larva as a lacewing and is most similar to mouthpart arrangements in the beetle group Elateroidea. Within the latter, the closest similarity is with the species-poor sister groups Cerophytidae and Jurasaidae. It seems most likely that the beak larvae are part of the group Cerophytidae + Jurasaidae, with an intermediate morphology between Cerophytidae and Jurasaidae. With the accessible data, we are able to reconstruct aspects of the whole group, also indicating that they undergo a special developmental pattern described as hypermetamorphosis. In Jurasaidae, this pattern is most expressed, with four distinct morphologies and ecologies. These are an early mobile larva, a wormlike larva with an inflated trunk, “normal”-appearing adult males, and adult females with another unique morphology.

“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.