Saturday, March 08, 2008

Identifying Tegenaria

We rearranged the potted plants in the back patio yesterday, and I took the opportunity to clean out the corners. In one of them, against the house wall and behind a stored board, where it would be warm and dry, I found a pair of spiders.

The large one, probably more experienced, disappeared down a hole within seconds. The tiny one tried to pretend she* wasn't there. Even though I brought the camera to within an inch of her, she didn't twitch. She stayed put while I swept down the wall, detouring around her, and didn't leave until I had gone to put away the broom.

So I got a few photos.

With flash.

tegenaria spider
With a desk lamp; softer light, and allowing for a closer approach.

It's a Tegenaria. Which may or not mean trouble. Tegenaria agrestis, the "hobo spider", has been long accused of aggressive behaviour and dangerous bites, which can turn tissue necrotic or cause allergic reactions. But T. domestica or T. duellica (gigantea) may be harmless.

Which is she? It is impossible to be sure without a microscope. BugGuide says,
The actual spider (not a photo) needs to be examined by an expert for a definite identification.
From a link on that page, though, I found a useful article; "How to identify (or misidentify) the hobo spider". How to tell if your spider is NOT a hobo. Just what I needed!

If the spider has any of the following, it is NOT T. agrestis. And probably not dangerous:
  1. Spots on the sternum.

    I couldn't check this, since I didn't capture the spider.

  2. Distinct stripes on the cephalothorax.

    Let me see... The cephalothorax (head-chest) is the front section. Very definite stripes down the back. (photo 1.) Not a hobo.

  3. Dark rings around the legs.

    Black and brown striped legs. Not a hobo.

  4. Shiny dark orange legs with no hair.

    Photo 2. Dull legs, with some hair. Not a hobo.

  5. Long, pointy palps (Two "grabbers" in front of the eyes).

    I didn't get a look at the palps.
So that's a relief. By the size (not enormous), I would guess these are probably T. domestica.

Now this one, on the other hand:

Broken marks on cephalothorax, not definite stripes. (See photo below) No spots on legs. Hairy, though. Blunt palps, I think.

Compare her with this one, positively identified as a hobo, on BugGuide:

I found her in about the same spot, last June. And it seems possible that she was a hobo. Next time, I'll capture any I find, carefully, flip it over, and examine the sternum, too.

For now, I shake out my back door shoes every time I put them on, and keep gardening gloves handy. I don't want hobo bites on my toes or fingers.

* She, because the palps, what I could see of them, seem to be of uniform thickness. Male spiders carry heavy, bulbous palps.


  1. We had a large Tegenaria living in our garage last year. It may be T. duellica (it sure seemed big), but reading the descriptions I'm leaning toward T. domestica. If I see another large one I'll measure it. I added a photo to my Flickr collection.

    It's good to know what isn't a Hobo. Thanks for the information.

  2. And thanks to the good people at BugGuide.

  3. How neat! If somewhat creepy. :) I don't know much about spiders, except that I don't like the big ones that crawl up from your drainpipes. Definitely something I should learn more about.

    Oh, and - tag, you're it! ...but only if you want to play (assuming you haven't yet!). :)

  4. Anonymous5:13 am

    For anyone worried about being bitten by spiders, I recommend reading this article:

    Short summary: people have an unfortunate tendency to attribute any "necrotic lesion" to spider bites, even though they are far and away more likely to be due to other causes. Spiders hardly ever bite anyone, particularly in Canada.

    My own experience with spiders bears this out: I've messed with spiders in an "interested hobbyist" sort of way ever since I was a kid, and none of them have ever even *offered* to bite me. It's still a good idea to clean out shoes and gloves routinely before putting them on, but it will save you from way more rocks/splinters/other mechanical hazards than from spider bites.

  5. Anonymous5:20 am

    Hm, looks like the link is getting truncated, how about if I try it again:

    Paper on misdiagnosis of spider bites (pdf file)

  6. Very interesting, Tim.

    Thanks for the link. It makes me feel better about that corner where the spiders hang out.

  7. Anonymous5:39 am

    The bottom one, and possibly the top one both look like hobos.

  8. Anonymous, Hi!

    The last one has been definitely id'd as a hobo. The first one is the same spider as the second, so not a hobo.


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