An unusual 100-million-year old holometabolan larva with a piercing mouth cone.

Joachim T Haug,Viktor A Baranov,Mario Schädel,Carolin Haug

doi:10.7717/peerj.8661

Abstract

Holometabola is a hyperdiverse group characterised by a strong morphological differentiation between early post-embryonic stages (= larvae) and adults. Adult forms of Holometabola, such as wasps, bees, beetles, butterflies, mosquitoes or flies, are strongly differentiated concerning their mouth parts. The larvae most often seem to retain rather plesiomorphic-appearing cutting-grinding mouth parts. Here we report a new unusual larva preserved in Burmese amber. Its mouth parts appear beak-like, forming a distinct piercing mouth cone. Such a morphology is extremely rare among larval forms, restricted to those of some beetles and lacewings. The mouth parts of the new fossil are forward oriented (prognathous). Additionally, the larva has distinct subdivisions of tergites and sternites into several sclerites. Also, the abdomen segments bear prominent protrusions. We discuss this unusual combination of characters in comparison to the many different types of holometabolan larvae. The here reported larva is a new addition to the ‘unusual zoo’ of the Cretaceous fauna including numerous, very unusual appearing forms that have gone extinct at the Cretaceous–Palaeogene boundary.

Highlights

The myriad representatives of Insecta (Hexapoda in Anglo-American tradition) are very successful in evolutionary terms concerning species richness, biomass, number of individuals, number of ecological roles or whatever measure we suggest. This success has been attributed to numerous different factors—two will be considered with some more detail here: (1) the very diverse modifications of their feeding apparatus (Grimaldi & Engel, 2005; Engel, 2018) and (2) the differentiation of the resources exploited by larval and adult representatives of Holometabola (Grimaldi & Engel, 2005), a major and especially species-rich ingroup of Insecta
This is related to the mandible morphology in Insecta; already in the ground pattern of Insecta (≈ character set of the direct ancestor) the distal region of the mandibles appears to have been reduced, in early stages, as evident from the lack of the dll patterning gene expression in the mandibles (Gotoh et al, 2017)
In this study we present a fossil holometabolan larva with a very unusual combination of characters, including piercing mouth parts, from early Late Cretaceous Burmese amber

Summary

INTRODUCTION

The myriad representatives of Insecta (Hexapoda in Anglo-American tradition) are very successful in evolutionary terms concerning species richness, biomass, number of individuals, number of ecological roles or whatever measure we suggest. A common differentiation of immature (mostly larval; see Haug, 2020 for the discussion) and adult mouth parts in various crustacean lineages involves a post-embryonic reduction of the distal parts, especially in the mandible (Haug & Haug, 2015) This is related to the mandible morphology in Insecta; already in the ground pattern of Insecta (≈ character set of the direct ancestor) the distal region of the mandibles appears to have been reduced, in early stages, as evident from the lack of the dll patterning gene expression in the mandibles (Gotoh et al, 2017). The morphology of the fossil larva is compared to morphologies in various holometabolan groups and possible phylogenetic relationships are discussed

MATERIALS AND METHODS

Findings

DISCUSSION