Heydenius araneus n.sp (Nematoda: Mermithidae), a parasite of a fossil spider, with an examination of helminths from extant spiders (Arachnida: Araneae)

A mermithid, a parasite of a spider (Araneae: Thomisidae) in Baltic amber (40 mya), is described as Heydenius araneus n.sp. (Nematoda: Mermithidae) and represents the first fossil record of a nematode parasite of an arachnid. After a critical examination of reports of naturally occurring helminths of extant spiders, I conclude that although mermithid parasitism is well established in this host group, previous reports of hairworm parasites of spiders are "nomina dubia," putative records, or refer to mermithid nematodes.