Chelicerates as Parasites

Among Chelicerata, larval instars of sea spiders (Pycnogonida) can be parasitic. The oldest putative sea spider from the Cambrian ‘Orsten’ is immature and resembles comparable instars of modern species with a parasitic phase to their life cycle. All other parasitic chelicerates are mites, with several examples in both the Acariformes and Parasitiformes clades. Fossils revealing parasitic behaviour, or belonging to purely parasitic clades, come from various amber sources from the mid-Cretaceous onwards. From Acariformes there are records of Parasitengona, Myobiidae, Pterygosomatoidea, Resinacaridae, Acarophenacidae, Pyemotidae and Apotomelidae. Parasitiformes is represented by several ticks (Ixodida) and potentially Laelapidae from the Mesostigmata. Parasitism appears to have evolved independently within mites on several occasions. Possible transitions to this lifestyle via nest associations and/or phoresy are discussed. Arachnids as victims of parasites include amber records of nematode worms (Mermithidae), erythraeid mites (Erythraeidae), mantid flies (Neuroptera: Mantispidae), ichneumon wasps (Hymenoptera: Ichneumonidae) and spider flies (Diptera: Acroceridae).