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Entomology (Study of Bugs)/Black spider with big red fangs


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Hello Bug Eric,
I'm just back from argentina, from visiting my family over there.
It turns out, for three days in a row, every morning we found one or two of these spiders, dead in the swimming pool.

I'm guessing they came out at night for a drink, because they were definitely not present during the day.
We're talking about Rosario, Argentina, near the Parana river.
This was two weeks ago, summer over there, and the river was at a historic high level.

Here are some photos:
They were about half an inch large (but their sized varied).

Any chance you can tell me what they are, and how poisonous they are?


Thank you for sharing the fantastic images of this spider, and for prompting a little research.  I have never seen one of these before, but I did figure out what it is....

This is an adult male trapdoor spider in the family Actinopidae, probably genus Actinopus .  The red things are not fangs at all, but the "pedipalps," leg-like mouthparts that in male spiders are modified into sex organs.  The spider transfers sperm from his genital opening to those pedipalps, and inseminates the female through the hook-like appendages at the tip of the palps.

Male spiders of nearly all species, even those normally confined to webs or burrows, wander in search of mates at some point in their lives.  That is why you found this fellow lumbering about.

Very cool!  I do hope you will share your pictures and observational notes with a site like Project Noah or i-Naturalist so other people can see it.  Also, our collective scientific understanding of most spiders is very small, and all observations are valuable.


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Eric R. Eaton


I answer insect and spider identification questions ONLY. Attach images if possible. No "what bit me?", "what do I feed this bug in captivity?", or science fair project questions please. NO TECHNICAL QUESTIONS ABOUT INSECT PHYSIOLOGY.


Principal author, Kaufman Field Guide to Insects of North America. Professional entomologist employed previously at University of Massachusetts, Chase Studio, Inc., and Cincinnati Zoo; contract work for West Virginia Department of Natural Resources, Smithsonian Institution, and Portland (Oregon) State University.

Author, Kaufman Field Guide to Insects of North America, Missouri Conservationist magazine, Ranger Rick, Birds & Blooms, Timeline (journal of the Ohio Historical Society). I have contributed to several books as well.

Oregon State University, undergraduate major in entomology, did not receive degree.

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One of the top 50 experts in all categories for, 2009.

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Principal author of the Kaufman Field Guide to Insects of North America, Smithsonian Institution (contract), Cincinnati Zoo (employer), Portland State University (contract), Chase Studio, Inc (employer), Arkansas Museum of Discovery (guest speaker). Currently seeking speaking engagements, leadership roles at nature festivals, workshops, and ecotours.

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