Harvestmen from Iguazu Falls, Argentina: Geraecormobius sylvarum

Subject: Spiders (maybe) at Iguazu Falls in Argentina
Location: Iguazu Falls, Argentina
February 17, 2014 9:24 am
Dear Bugman or Bugwoman,
I know you’re probably busier than a one-armed paper hanger, but I wonder if you can identify the critters in the attached photo.
I took the photo at Iguazu Falls–a couple hours flight NE of Buenos Aires, Argentina.
The spiders (?) were massed under the protected dark side of a rock ledge. My tour guide had no idea what they were. I prodded a couple of them but they were very lethargic and barely moved. Could this be because it was the heat of the day (30 C)?
I appreciate your help and time.
Signature: the Planckster


Dear the Planckster,
These are not spiders, but they are Arachnids.  They are Harvestmen in the order Opiliones, and unlike spiders, they do not have venom.  They are also considered scavengers that feed on dead or dying creatures as well as vegetable matter. According to Arachnology.org:  Opiliones will eat:  “‘Everything’, almost. There are many kinds of Opiliones and some seem to have distinct preferences in what they eat while most seem to be less choosy and eat almost anything, especially animal matter. Some species are predatory on small insects, snails, worms, etc., whereas others walk around nibbling on plants, fruits, and dead material that they find. In captivity, you can try feeding your Opiliones freshly chopped meal worms or other juicy insects. Oatmeal, a little piece of fruit or nut might also be eaten.”
  According to BugGuide:  “In some cases, in dry climates, they gather in large numbers during the day, probably to avoid dessication, and wander about in search of food after the sun goes down.”  We will attempt to identify the species or at least the genus.  Arachnology.org has this to say about the aggregation behavior:  “This is indeed a good question. Nobody knows for sure, but scientists are investigating this behavior. It appears different kinds of Opiliones aggregate for different reasons. Some apparently aggregate to create or maintain a desirable microhabitat (temperature, humidity, darkness, etc.). Other aggregations appear to be for purposes of hibernation or rest. It may be possible that the defensive secretions of a mass of Opiliones would be more effective than those of a single individual. Also, the aggregations tend to pulsate wildly when bothered- a huge pulsating mass could be intimidating to predators.”


Dr. Adriano Kury responds to our identification request
Interesting, but I cannot recognize the species.
They are too much entangled.
It is a Pachylinae.
Can you photo a single one, or still better to send me one male?

Thank you Dr. Kury,
The person who sent the photo visited Iguazu Falls and took the photo, but we got the impression they were tourists and will most likely not be able to return to take either a new photo or to collect specimens.

Ricardinho Pinto da Rocha provides an identification
 Foto maravilhosa, são Geraecormobius sylvarum. Eu nunca tinha visto eles agregados.
Abraço, Ricardo
Babylon Translation:  Wonderful Photo, are Geraecormobius sylvarum. I had never seen them aggregates. Hug, Ricardo

Thank you kindly for your response to my email.
I appreciate your help and time.
Now I can sleep better without that question gnawing at my innards.
Jerry (the Planckster) Planck

Thanks again for the info.
You are correct–I’m no longer at Iguazu Falls.
However, the photo I sent you was a small version of what I actually took.
With my camera, I normally take both RAW and JPG pictures. The RAW photo is a much larger, uncompressed CMOS image.
To help see much greater detail when zooming in, I’ve sent a link at the bottom of this email to my Dropbox account where you can download it. It’s in a TIF format–I hope that works for you and Dr. Kury. It’s labeled “Harvestmen.tif”.
Jerry Planck

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