Decision letter: Extreme adaptations for aquatic ectoparasitism in a Jurassic fly larva

Abstract

Parasites have been exploiting other organisms for millions of years. However, little is known about ancient parasitic insects, as it is rare to find fossils that are preserved well enough for them to be identified as parasites. This is particularly true for ectoparasitic insects, which live on the skin of their hosts. As a result, the only widely accepted ectoparasitic insect from the Mesozoic era is the giant flea, which infested dinosaurs, pterosaurs or mammals. Now, Chen, Wang, Engel et al. have discovered a new genus and species of ancient aquatic fly, which may be the earliest currently known aquatic ectoparasitic insect. Named Qiyia jurassica—after the Chinese word for ‘bizarre’ and the Jurassic period when it lived—its larva has a combination of features that mark it out as a parasitic ancestor of modern water snipe flies. In addition, the well-preserved fossilised larvae used to identify Q. jurassica have some more unusual features. The mouth of Q. jurassica had a structure commonly found in ectoparasites, designed to pierce skin and suck blood. The larva also had several features that were particularly well-adapted for gripping the host animal while underwater. The prolegs—stumpy fleshy structures found on the abdomen—were covered in bristles that pointed upwards, anchoring the larva in place. Q. jurassica also had an unusual sucker on its thorax that would have provided a firm grip that held its head still during feeding. Although many modern aquatic ectoparasites—like leeches—have suckers, the Q. jurassica sucker may be unique amongst insect larvae, as it has six large ridges and is covered in spines. Both features may have provided extra grip. Chen, Wang, Engel et al. suggest that Q. jurassica feasted on the blood of salamanders, as many salamander fossils have been found in the same region. The larvae could have attached to unexposed areas of the salamander—behind the gills, for example—where feeding would also have been easier due to the rich supply of blood vessels, and the thinner, more easily pierced skin. The wide range of features found on Q. jurassica suggests that Mesozoic ectoparasitic insects were more diverse than previously thought.