Sunday, March 3, 2013

Spider Sunday: Flatties

Many spiders go unseen by the casual observer because they emerge only at night. Venture outside after dark, and you are likely to be astounded by arachnids you never knew existed. Take a flashlight to the outside walls of your own home and there will likely be spiders prowling across it. If you live in Florida or the southwest U.S., you will eventually see crab-like spiders of the family Selenopidae, called “flatties.”

I might not have recognized them myself were it not for Zack Lemann, Chief Entomologist at the Audubon Insectarium in New Orleans. We occasionally meet up in southern Arizona for the Invertebrates in Conservation and Education Conference at the end of July each year. In 2010, on a night field trip to Madera Canyon, Zack identified a spider sitting motionless on the outside of a restroom building as a “flattie.” I had figured it was a running crab spider in the family Philodromidae and appreciated the correction.

Flatties can be easily confused with any number of other spiders, but are readily identified by a few distinctive characteristics:

  • Extremely flattened appearance. They look like they have already been the victim of someone’s shoe, so flat are they. Even the legs look cock-eyed, oriented almost completely horizontal to the body. This distinctive leg configuration is called “laterigrade,” and only a few other spider families have this feature.
  • Legs increasingly longer from front to back. Note that in philodromid crab spiders like the one shown below the second pair of legs is the longest.
  • Six eyes in one row. Six of the spider’s eight eyes are in one row across its face. The other two set back on either side of the face.
  • Rear edge of sternum is notched. You have to turn the spider belly-up to see this character (good luck), but this feature of the “chest plate” is diagnostic.


Philodromid crab spider (note long 2nd pair of legs)

Their thin bodies allow flatties to slip into very narrow cracks and crevices, where they hide during the day. Most references indicate that these spiders hide under stones, or beneath bark on logs or trees, and between the bases of leaves of dense plants. I have personally never discovered them on the ground or under objects. They have always appeared on vertical surfaces at night in my experience.

These are medium-sized spiders, adults measuring from 7.5-13 millimeters in body length. Their sprawling legspan makes them appear larger. The mottled gray or brown or yellowish coloring helps to camouflage them on rock surfaces and tree trunks. They likely wait in ambush for prey, though when disturbed they can sprint with startling speed.

There are five species found north of Mexico, all in the genus Selenops. They collectively range from southern California to western Texas, and also in Florida and the Caribbean islands.

Sources: Jackman, John A. 1997. A Field Guide to Spiders & Scorpions of Texas. Houston: Gulf Publishing Company. 201 pp.
Kaston, B.J. 1978. How to Know the Spiders (3rd Ed.). Dubuque, IA: Wm. C. Brown Company Publishers. 272 pp.
Ubick, D., P. Paquin, P.E. Cushing, and V. Roth (eds). 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.

14 comments:

  1. I see them often - but never got one ided beyond the genus. One should think that with only 5 spp....but no ):

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    1. I suspect one has to look at a specimen's reproductive organs to get to a species ID, and that can't be done from images alone of the top of the specimen.

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  2. I found one of these on my lake and I live in MA is it poisonous I have two kids

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    1. Dee, "flatties" do not occur in your area. What you are seeing are "running crab spiders" in the genus Philodromus, family Philodromidae. They are *not* dangerously venomous (bite), nor are they poisonous (toxic if eaten).

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  3. I know you say it only lives in the southwest, but we found one and it doesn't look anything like the above link you posted. It looks exactly like the pictures in your blog. The way our food is shipped around North America can't they find their way to other areas?

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    1. Steph: Again I would suggest it might be a philodromid crab spider (family Philodromidae), or possibly a giant crab spider (family Sparassidae), rather than a "flattie." Yes, giant crab spiders sometimes hitchhike on bananas and other foods and products from the tropics. Without seeing a specimen, or at least a clear image, I cannot speculate further.

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  4. i think i found one well, actually 2 now.... in my room.... 1st one i panicked and sprayed the life out of it. sorry.... (i prefer relocating,) then the 2nd one tonight. this one is much smaller but for sure the same type.this one got the broom but i didnt see it till it was already dead and not whole. i have pics of both, can i send them to you? i only paid attention to the eyes tonight.... really wishing for confirmation that these arent poisonous as i sleep on my floor and as interesting as they are, they make my skin crawl thinking of possibly more in here.
    thanks
    ps... am sleeping in the livingroom tonight!

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    1. It would help to know your geographic location. Their could potentially be other spiders of concern that resemble flatties, if you are outside of North America....I'll rarely reprimand someone for killing the odd spider in their home, especially if it is unfamiliar. The overwhelming majority of spiders are in no way endangered species.

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    2. i am in san diego, southern california.
      i have some good pics, spray leaves them in much better condition than squishing! any way to send one to you?

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    3. Post the images online and then send me a link to them, please.

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  5. Hey! Im Im Kenya, maasai land (1h30 from Nairobi ) for a month and I already saw 2 of them corresponding perfectly to the pictures you posted in you're blog. Their behaviour fits totally with what you said about them. I just wanna know if I should rather kill them or if there is no way the would bite me cause I'd don't mind letting them walking around if no danger ? (and by the way are they poisonous ?)
    Thx !!

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    1. Being unfamiliar with African spiders, I would hesitate to say that what you are describing *are* flatties. I would try and find a local expert to make the ID and decide whether they are dangerously venomous (nearly all spiders are venomous, but few pose any threat to people).

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  6. Oh okay, thank y ou in advance!

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    1. No, I mean *you* need to contact a local expert. I live on another continent!

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