Interest in these pictures seems to be more widespread than usual, judging from both the view count for this topic and the number of emails that I have received about it.
Regarding the ID, consensus seems to be that this parasite is much more likely a "mermithid" worm (Nematoda:Mermithidae) than a "nematomorph" or "horsehair worm" (Nematomorpha).
Dr. Andreas Schmidt-Rhaesa of the Zoological Museum in Hamburg, Germany, wrote (and graciously granted permission to post) that:
Those are very fine images. Nevertheless, they are not sufficient to identify the worm with safety. Mermithids and nematomorphs are very similar, but I can identify no character that would point to one of the two. I suspect it to be a mermithid, but can not be very sure. The most reliable would be to send the worm to me and let me have a look directly. Sorry - this is all I can say at the moment.
There are some practical and legal issues with shipping biological material, so I inquired whether there were distinguishing characters that I might photograph. The answer to that question was:
Dear all,
There are some hints to distinguish nematomorphs from mermithids:
- when the posterior end is divided into two lobes it is surely a nematomorph
- when both ends are pointed, it is likely to be a mermithid
- when the anterior end is coloured by a white tip, followed by a brown or black ring, it is a nematomorph
- when the body is transparent and large eggs are visible, it is a mermithid
Juvenile nematomorphs may look a lot like mermithids, but probably these features help already. Dessication is usually not a problem, because the worms are quite robust.
Best wishes,
Andreas Schmidt-Rhaesa
Google search on
mermithid spider turned up lots of interesting stuff, including this November 2000
article in the journal Invertebrate Biology:
Title: Heydenius araneus n.sp. (Nematoda: Mermithidae), a parasite of a fossil spider, with an examination of helminths from extant spiders (Arachnida: Araneae)
Abstract. 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.
I was curious how these various groups relate to each other, so I did some searching. What I found is that Nematoda and Nematomorpha are sister groups, both being phyla within
Ecdysozoa, a large group that comprises all the animals that shed an exoskeleton. Ecdysozoa includes such other well known and quite diverse groups as the Arthropoda (insects, spiders, etc.) and Tardigrada (water bears).
So it seems that despite their very similar appearance, the mermithid and nematomorph worms are not very closely related. Fascinating.
--Rik
PS. The article linked above by Sue Alden (
this one) provides a wealth of information about these parasites. Thanks, Sue!
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). This one actually takes the cake........ with cream. 
