Miocene caddisflies from Australia: iron-rich sediments preserve internal organs, tracheoles, and corneal nanocoating of larvae and pupae

Abstract

Trichoptera (caddisflies) are insects with terrestrial adults and aquatic larvae. Although caddisflies spend most of their lives as larvae and pupate in the water, fossils of these stages are rarely found. However, ~100 pupae and uncased larvae were discovered at McGraths Flat, an Australian Miocene Lagerstätte. Many of the fossils are extremely well preserved. In addition to external features that include a spinneret among the mouthparts and claws on the abdominal prolegs, some larvae show silk glands, parts of the gastrointestinal tract, and tracheoles. The pupae are all at the pharate stage; in some, large compound eyes can be seen, and some exhibit male genitalia. Scanning electron microscopy revealed ommatidial structures, such as rhabdoms, corneas, and most remarkably the corneal nanocoating. Two caddisfly morphotypes are preserved at McGraths Flat. The larger morphotype belongs to the suborder Annulipalpia and can be assigned, most probably, to the family Dipseudopsidae. The smaller morphotype closely resembles extant members of the family Hydroptilidae (suborder Spicipalpia). No adult caddisflies or individual wings, no immature pupae, and no larval or pupal cases have been found in the deposit. This unusual fossil record suggests an unstable palaeoenvironment characterized by abrupt change.