Sunday, June 19, 2011

Spider Sunday: Longlegged Sac Spiders

Some spiders seem to be much more common indoors than outside. Chief among those are the “longlegged sac spiders” in the genus Cheiracanthium. They belong to the family Eutichuridae, a recent (2014) change. They were formerly in the family Miturgidae, known collectively as “prowling spiders;” but before that they were classified in the sac spider family Clubionidae.

Indeed, telling Cheiracanthium from Clubiona is not at all easy. Cheiracanthium lacks a distinct groove in the center of its carapace (the top portion of the cephalothorax), and there is no obvious tuft of curved setae (hairs) on the front edge of the abdomen. Clubiona does possess those characters. More to the point, you are not as likely to find Clubiona indoors.

The sac spiders are named for the silken retreats they spin. They do not construct silken snares like many spiders, but hunt “on foot” for small insects they can simply overpower. Their tarsal claw tufts, composed of densely-packed hairs, help them grip even slick surfaces. They easily run across the ceiling, defying gravity and giving the average homeowner the heebie-jeebies.

Cheiracanthium are mostly nocturnal, so you are likely to find them resting during the day, inside those retreats. Look for their “sleeping bags” at the juncture of a wall and the ceiling. They seldom, if ever, reuse a retreat, so you may find more than one. Females also deposit their eggs in a sac within a retreat, and the spiderlings that hatch may return to the retreat after minor explorations. The spiderlings will disperse after their next molt, though.

Because these spiders usually stick to the higher reaches of the average house, they don’t often come into contact with the human occupants. All the same, there has been a good deal of historic controversy as to whether longlegged sac spiders are dangerously venomous. Rumors of necrotic bites similar to those sometimes caused by recluse spider bites have largely been put to rest.

Spider bites in general are very rare events, contrary to what you might think from media sensationalism and internet exaggerations. One truly serious threat that is understated or omitted in the discussion of spider bites is the possibility of secondary infection that can turn any minor puncture wound into something much, much worse. Always flush any kind of abrasion or wound thoroughly with potable water. Use antibiotic ointments at your discretion, bearing in mind that overuse of antibiotics has contributed to the rise of “superbugs” that are increasingly resistant to the drugs we throw at them.

Back to the spiders at hand. There are two species of Cheiracanthium in North America, known commonly as “yellow sac spiders” or “agrarian sac spiders.” C. inclusum, is native to the U.S. and occurs coast to coast except the northernmost states. C. mildei is native to Europe but was introduced to North America decades ago. It now has a very widespread distribution in the states, including New England and Washington state. This is the species most often encountered indoors. Both species are medium-sized, the body length of mature individuals varying from 4-10 millimeters, males on the smaller end of the spectrum.

Mature and penultimate males (specimens that have only one molt remaining before adulthood) can be recognized by a “leggier” appearance and the modified pedipalps that make the spider look like it is wearing boxing gloves (see image above).

There are some very good internet resources on this genus of spiders, including this fact sheet from Penn State University, and this one from Washington State University. Do go offline, though, and see if you can’t find one of these spiders in your own home, or possibly empty retreats that the spiders once occupied.

29 comments:

  1. We have these around our house but I try to pretend they don't exist.

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  2. I'm seeing some mis-readings of this article, so I thought I'd clarify. This spider is extraordinarily common outdoors. Remember how Mazda recalled 63,000 cars because this spider's nests were clogging an engine hose? The Yellow Sac Spider definitely is not an "indoor" spider.

    I collect spiders on a regular basis for educational and research purposes. During the summer, about half the silk sacs I found outside contain this spider. Most of the rest are jumping spiders. Most of my sweeps of meadows will produce at least one of these spiders.

    I have also seen these spiders wandering my house. They wander extensively at night -- they are fast-moving spiders that can cover a lot of distance. My take is that these spiders are only found in the house because they wander in during their nighttime excursions. I am doubtful that such high-energy spiders could sustain themselves for long on the few insects they might find in a person's house. I suspect they are accidental visitors like wolf spiders often are.

    So let's please not call them "indoor" spiders.

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    1. Definitely an indoor spider. Here in SE Michigan there are literally hordes of these spiders inhabiting many homes; both in basements and both first and second floors. I have never seen one outdoors here. In my former home, they were the only live spiders I saw - all other spider species in that house were found dead - I suspect victims of the Cheiracanthium spiders of which there were plenty.

      They can bite if they wander into your bed and you inadvertently lay on them. A raised red bump, typically penny to nickel sized discovered on waking, and "achy" for a day or two.

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  3. I posted a video of one of these building his hideaway in a fish bowl, on my blog at http://wanderinweeta.blogspot.com/2011/06/desk-top-building-project.html . I thought it was Clubiona, but now I'm doubting that; the hair on the abdomen isn't as "tufty" as yours. What do you think?

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  4. With all due respect to Joe Lapp, in *my* experience, longlegged sac spiders are much more likely to be encountered indoors. Of *course* they occur outdoors, too. The pale, crawling spiders one is most likely to find in their home, however, are these (at least C. mildei), whether they are "transient" or not.

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  5. (Blogspot errored on my previous post, so I'm trying again.)

    Agreed, these are one of the most commonly *encountered* indoor spiders. However, they are one of the most common spiders, indoor or out, which helps to explain why so many are found coming inside. I just saw the Kaufman Guide post calling them "indoor" spiders and thought people might assume that it was not also an outdoor spider. It just seems like a mistake to say call it an "indoor" spider when it is so extraordinarily common outside.

    Susannah, I watched some of your beautiful video. I'd peg your spider as a Hibana species rather than as a Cheiracanthium species.

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  6. Eric, Might the LLSS may be more of an indoorsy critter under the desert conditions where you live? Joe's observations of this being "extraordinarily common outdoors" and occurring in about half the silk sacs he finds outside may be due to the more humid conditions here in central Texas... Mike (not an arachnologist)

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  7. Thanks, Joseph. I looked up Hibana, and went through the photos on BugGuide. It looks most like H. gracilis, but that's out east. H. incursa is in California, so I wouldn't be surprised to see it here in BC. I didn't see the markings on my spider's back that H. incursa has, and the abdomen doesn't have the greenish cast, but there are always variants, aren't there?

    I'll send a couple of photos in to BugGuide.

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  8. Actually, I hardly see Cheiracanthium here in Arizona at all. Back in Massachusetts, though, they were *always* indoors. Perhaps cold climates force them to seek heated human habitations?

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  9. It's true, I can only speak for central Texas. I did a little investigation, and amazingly, there's a sense in which we are both right! Thanks Mike!

    Have a look at this "House Spiders of Kansas," Hank Guarisco, 1999. http://bit.ly/laUYYH (PDF)

    Guarisco says that C. mildei is a "true synanthrope" in Kansas, that C. inclusum is native to the northeast U.S. but that C. mildei has replaced C. inclusum in houses in that area. I have yet to see a C. mildei here in central Texas, inside or out.

    Eric, you do explicitly say that C. mildei is the one most often encountered in houses. So far I'm only finding C. inclusum inside my house in Texas. I wonder if this is a north/south thing.

    It looks like C. mildei may truly be an indoor spider, at least in the northern states, while C. inclusum is indoor/outdoor, at least in the southern states. What do you think?

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  10. I say..."Yes?" Ha! This all started out pretty innocent, but clearly there is more to this, especially how the two species are partitioning habitats, and how that varies with climate and geography. I will have to leave it at that.

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  12. Thank you for helping me ID this spider which has been SCARING THE BEJESUS OUT OF ME LATELY. Living in MA and hearing that maybe they're mostly indoor here has not made me feel better, but now I can at least properly ID the darn things...

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    1. You are most welcome! Yes, spiders encountered indoors can indeed be startling, but rarely are they truly dangerous.

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  13. In January my husband got up one morning with several bites on his upper thigh. They took weeks to heal. A few days later I got up with a bite on my upper back. Within a few days the skin surrounding the bite turned red and bumpy, itched like mad and was painful to touch. The area measured about 6 cm in diameter. It took nearly 3 months to heal. Several nights later I found and killed a long legged sac spider on my bed. Since then we found one dead in a CD case and have killed an additional four, three last night alone on my way to bed. Needless to say, we're on high alert now. Learning that they are nocturnal doesn't help us sleep well at night as we wonder how many more there are and how many egg sacs might be hidden in our home. I thought spiders only bit if they felt threatened. These are biting us as we sleep! What can we do about it??!! Thanks for any suggestions.

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    1. Gran, I seriously doubt the spiders had anything to do with the "bites" on your husband's leg. Many, many mysterious wounds are diagnosed as "spider bites" when there is actually another cause. I do hope he is well now. Excluding spiders from a home entirely is essentially impossible, but I recommend the articles on the website spiders.us for information on how best to avoid spider bites.

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  14. I *think* I caught one of these spiders on my daughter's bed today, but I'm not sure. It looks a lot like the pics, but the abdomen looked almost see-through and the cephalothorax had a ring of white dots around it and the first section of the legs (closest to the body) were black while the legs were sort of see-through, as well. The way it looks to me may be partly due to the fact I was looking at pics taken from my phone before I caught it (and killed it, but didn't spend any time looking at it after it was dead.) It was on my daughter's bed, which is black, and my phone was fairly close and has a bright flash. The pic is also from the top, so I couldn't see the eye cluster well, if at all. Does any of this sound like characteristics of the LLSS? I live in SC.
    Thanks!

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    1. Well, there are a *lot* of possibilities down in South Carolina. Might have been a spitting spider (I have a blog post on those as well), but I can't be sure without seeing at least an image, and ideally a specimen.

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  15. I killed SEVEN in my house today so far! I'm terrified of spiders especially these quick sneaky things. how do I safely get rid of them? I know I won't get rid of all of them but I have a baby and a dog & don't want to use toxic sprays if possible.. please help me find some sanity these things have been occupying my days I'm going crazy trying to find them.

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    1. Please see this article and the section on "Preventing Spider Bites" for hints on how to keep spiders out of your home: http://www.spiders.us/articles/spider-bites/

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  16. Is there a way to send you a picture of a spider for identification? Found one trapped in the kitchen sink this morning that a friend said might be a sac spider, but I am not sure.

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    1. Janine: Please contact me via AllExperts.com, Entomology category. Thank you.

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  17. okay. i found one spider from the ceiling and don't know if there is more. why do they come out of where they live. this is unusual for me.

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    1. Please re-read the article, and the comments by Joseph Lapp. Thank you.

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  19. I was casually reading this article and, no joke, I look up to see one of these exact spiders crawling on my ceiling!

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    1. Ha! That's wonderful! Thank you for sharing this. :-)

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