Wednesday, July 31, 2013

The Giant House Spider Invasion

Ok, it wasn't really an invasion, but it did turn out to be an interesting night.

Sam, Scott, and I were watching TV earlier this evening when Sam suddenly spotted a rather large spider on the carpet in the middle of the living room.  While cowering on the couches, Sam and I demanded that Scott take care of it...but first, take a picture of it.


This is the photo he took of the spider just before stunning it with a whack from a flip flop and then squishing it with a napkin.  He then deposited it into the garbage and uploaded the photo to Facebook.

Well, the Facebook discussion was lively. Usually when you find a large spider in the house in this area, especially if it is brown, people think they are hobo spiders.  While hobo spiders do live here, they are impossible to identify without a microscope.  There are quite a few spiders that are mistaken for hobo spiders.  In fact, WSU has a ten-page PDF document entitled "How to Identify (or Misidentify) the Hobo Spider."

Well, I got interested enough to overcome the heeby jeebies I get around spiders and very carefully, touching only the very edge, removed the napkin from the kitchen garbage.  Then using a pencil and a pair of dirty tongs I found on the counter, I opened the  napkin and found that the spider was intact and not squished beyond recognition.  Gaining a little courage, I used the pencil to move it around, making sure it didn't move on its own (or I'd scream and run).  I pulled out our handy-dandy flash magnifier (if you don't have one, you need one), and looked at it more closely...literally...because you have to get really close with a flash magnifier to see clearly.  Yes, I had all kinds of shivers running through me.

This is what I saw:


See that white line?  It is pointing to three little circles on the underside of the cephalothorax.  There are three more on the other side, too.  According to the WSU document linked above, that's enough to identify the spider as NOT a hobo spider.  It is either a giant house spider or a barn funnel weaving spider.

Ok then....next step.  Look for the female epigynum, or opening of the reproductive organs.  None.  Ok, this is a male spider.

Next step.  Using a exacto knife, I cut off the left palp, or male reproductive structure.  I then put it on a slide and put it on the microscope.  Using a flashlight to add contrasting light from the side, I examined that thing from all directions and could only find one prong.  Hobo spiders have two.  Funnel weavers have a small prong.  This was definitely a large one.

Next step.  I looked at the RTA, or retrolateral tibial apophysis on the palp.  It definitely had the saddle-shaped top of a giant house spider.

So, with all of those observations, it is confirmed to be a giant house spider.

And that folks, is the only way to know, for sure, if the brown spider you found is a hobo spider or not.  As giant house spiders outnumber hobo spiders three to one, it is more likely a giant house spider.  However, barn funnel weaver spiders also look similar.  People also mistake several other species of spiders for hobo spiders, including the callobius spider, which I have also caught in my home (and had people tell me was a hobo spider).

Being a giant house spider, he was a beneficial spider.  In fact, having a colony of giant house spiders in the basement is almost guaranteed to prevent hobo spiders in your home.  However, given how closely they resemble hobo spiders and my dislike for large spiders running across my carpet, I wasn't going to stop to exam it alive first. 

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