Frozen mid-flight: Parasitism of dipteran hosts by parasitengonan mite larvae in amber.

A syninclusion is composed of more than one specimen, fossilised in the same amber piece. In rare cases, clear signs of interactions between organisms before their entrapment in the tree resin can be observed. This is known as “frozen behaviour”. “Frozen behaviour” in amber is valuable not only due to its scarcity, but also because it offers information to reconstruct the network of interactions between organisms of past ecosystems. One of the most common forms of interaction in amber is parasitism, and among these, the most commonly reported parasites are mites. Mites of the group Parasitengona have a complex life cycle in which their larvae are parasitic, while during active post-larval stages (deutonymph and adult) they are predatory. Larvae feed on their hosts, but also use them to disperse. Most lineages of terrestrial parasitengonans parasitise other euarthropodans during their larval stage, feeding on their haemolymph, with the exception of some groups associated with vertebrates and very few non-parasitic ones. Due to their strong attachment devices that keep their mouthparts still connected to their host during the process of amber formation, they are among the most frequently reported fossil parasites. Here we present and describe the parasitic relationship of fossil terrestrial parasitengonan larvae with dipteran hosts. We show around 50 new records of such associations, more than doubling the previously reported cases. Moreover, we further discuss the relevance of dipterans as hosts of terrestrial parasitengonan larvae both in the fossil record and for modern lineages.