A few weeks ago I wrote about a “fashionable urban jumping spider” – one that is found inside and outside of your homes. Today I wanted to focus on two other synanthropic spider species that tend to be very common inside our homes, and more specifically, on the ceiling, and with a particular fondness for crown moulding. You probably already know exactly what I am talking about – small (< 1 cm in length), cream-coloured spiders that build silken retreats at the junction between walls and ceilings, and that often scurry up and down walls and around your house. These are most likely one of two species within the family Miturgidae, in the genus Cheiracanthum. Here’s a lovely photo of C. inclusum taken by Spider Joe (thanks for the permission to use this photo! You can view more of his work here)
In North America, the two species of Cheiracanthum that you may find in your home are C. inclusum and C. mildei. Searches on the Internet (and sometimes in the scientific literature) reveal that these are commonly referred to as “yellow sac spiders“. This common name is largely a hold-over from when Cheiracanthum was previously within in the family Clubionidae, which are all commonly called “sac spiders”. The yellow part obviously refers to their cream-like colouring. Personally, I think that “ceiling spider” is a better common name, because that’s where they are very commonly found.
Almost every house that I go into has Cheiracanthum tucked away up in the ceiling, and I think most Arachnologists would support this claim. This means there you are most likely living in very close association with these spiders every day, and that they are likely distributed across most of (populated) North America.
The million dollar question: do they bite humans? Any of you that have been following my blog will know my opinions about spiders biting humans. I argued previously that spiders bites are exceedingly rare, and attention should be paid to more likely causes.
By the way, that previous post has received a lot of attention, and I was rightfully accused of using a provocative title. It’s true – I did use that extreme title on purpose, in part because I feel it important to contrast the incredible volume of misinformation out on the Internet about “deadly” spiders.
There have been seven reported bites by Cheiracanthum in the literature for continental North America, including one case in Canada (Alberta, reported on by Leech & Brown in 1994) and they are summarized in this article by Rick Vetter and colleagues (2006). The symptoms presented include pain, redness and swollen area around the bite mark, mild pricking sensation, itching and some nausea. As Vetter et al. point out, “Effects of Cheiracanthum envenomation should be referred to as mild or moderate…”, and “…almost all Cheiracanthum bites hurt similar to bee stings..”. So, yes, they can bite, and it hurts, and the symptoms disappear within a few days, at most. What is most important to point out is that bites from Cheiracanthum are extremely rare – especially since these spiders are living in homes all across North America. This blog post should, therefore, cause you to not worry about these spiders in your house. If they really liked to bite humans, there would be hundreds of verified bites, and they would be occurring all the time. This just doesn’t happen. I do not consider it to be under-reporting, given the general hysteria about spiders. This quote by Vetter et al. sums up this sentiment very well:
“Unfortunately, circumstantial evidence presented in the spider bite literature often evolves into convention (and incorrect) medical wisdom. it is disconcerting to repeatedly see an almost predictable misconception where a medically inclined audience read reports that clearly state “suspected”, “probable”, or “reported” in alleged bite incidents, yet will transform this non-definitive data into conclusive proof of spider involvement”
Next time you are watching that lovely Cheiracanthum wandering around on your ceiling, just remember that it’s not a real threat to you, and it is fun and fascinating to watch. It is catching annoying flies for you, and providing some natural biological control in your own home. Keep them around! Cheiracanthum have a lot of helpers, also: I have documented a about ten different species inside my house over the past few years and I’m sure you have a similar diversity of house spiders, if you look hard enough. I’ll have to write about a few more of those species sometime in the future. Stay tuned!
Leech, R. & T.M. Brown. 1994. The first Alberta record for Cheiracanthum inclusum (Hentz) (Araneida, Clubionidae), with observations on a human bite reaction. Can. Entomol. 126: 187. http://pubs.esc-sec.ca/doi/abs/10.4039/Ent126187-1?journalCode=ent
Vetter, R.S., G.K. Isbister, S.P. Bush & L.J. Boutin (2006) Verified bites by yellow sac spiders (genus Cheiracanthum) in the United States and Australia: where is the necrosis? Am. J. Trop. Med. Hyg. 74(6) 1043-1048. http://www.ajtmh.org/content/74/6/1043.full