Sunday, August 28, 2011

Spider Sunday: Crevice Weavers

Perhaps no spiders are more often misidentified than the “crevice weavers” in the genus Kukulcania, family Filistatidae. They are quite common throughout the southern United States, where they are frequently confused with everything from brown recluse spiders to tarantulas.

There are currently five species and one subspecies of Kukulcania recognized in the U.S. Here in Arizona we have both K. arizonica and K. hibernalis (the “Southern House Spider”). Indeed, houses are often where you find them. Look for their sprawling, lacy webs issuing from the exterior of window frames, under the eaves, and similar situations.

Filistatids in general belong to a larger group of arachnids called “cribellate” spiders. These spiders possess an extra silk-spinning organ called a cribellum, and a comb on the outside of the metatarsal segment of each fourth leg. This row of short, stout, curved hairs is called a calamistrum and is used to “fluff” the silk that issues from the plate-like cribellum. This “carding” of silk is accomplished by very rapid vibration of the fourth leg as it rests on the third leg. This is in contrast to most other cribellate spiders that employ a slow rocking motion. The web that results is therefore not sticky, but the threads so random that they easily entangle potential prey.

The odd web is just the beginning of weird for Kukulcania crevice weavers. The genders differ so dramatically in appearance that one can be forgiven for assuming they are different species. Mature females are a lovely, velvety black or dark gray in color, with a body shape and lumpy eye arrangement reminiscent of a tarantula. Mature males are beige or pale brown in color, with small bodies and extremely long legs. They are mistaken for brown recluse spiders much of the time, but note the differences. Male Kukulcania have eight eyes, grouped atop the crown of the head as they are in the female. Recluse spiders have only six eyes, grouped in three pairs across the front edge of the carapace. Also notice the extremely long, elbowed pedipalps of the male crevice weaver. No recluse spider is that….well,…well-endowed.

We’re not finished, it gets stranger. All filistatids known thusfar have their jaws (chelicerae) fused. That means that each one of the pair is incapable of moving independently, like in most other spiders. They are also built to lose their legs if necessary to escape a predator. Actually, many kinds of spiders possess the ability to break off parts of their legs without losing much blood in the process. This phenomenon is known as “autospasy,” and in the case of crevice weavers the weak joint is at the juncture of the tibia and patella segments.

Interestingly, female Kukulcania continue to molt after they become adults. Mygalomorph spiders (tarantulas) are the only other spiders known to do this. It seems to be related to longevity: both spiders take a long time to reach sexual maturity, and are capable of living several years. Like the cellar spiders I addressed in an earlier blog entry, filistatids are “haplogynes,” meaning that the females have largely unmodified, if not non-existent external genitalia. The paired openings to her reproductive tract are under what is more-or-less a slot-like orifice.

The best time to observe Kukulcania species is after dark. Females will venture to the lip of their retreat in anticipation of prey. Males will be wandering in search of love and romance. See if you can’t find one or both of the sexes on the exterior of your own home, garage, shed, or barn.

13 comments:

  1. Eric, your timing is perfect! My brother just this morning sent me a photo of a block wall he built that's now looking like a Kukulcania condo. He lives in Perry Hall, Maryland, north of Baltimore, in northern Maryland. Everything I can find says they're only found in "southern states." BugGuide reports no images for Maryland.

    Are you aware of any specimens that far north? Any ideas what else they could possibly be? The nest openings look identical to Kukulcania. They appear to be at least 1/2" wide, with the lacy webs extending for 2 to 5 inches around the openings.

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  2. Joe: It would not surprise me to learn that the range of these spiders extends up the Atlantic coast. Many species exhibit that distribution pattern because the coastal climate is milder than it is inland at that latitude. Thanks for sharing this!

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  3. Thanks Eric. A few of the people I referred to this article remarked on how interesting and informative it was. I have to admit that I only know half of this stuff about this spider. Thanks for doing the research!

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  4. Loved the article, Eric!

    Joe, there are also other types of spider that can create similar looking webs... in Maryland, I'd look into amaurobiids, like Callobius & Amaurobius. Could even be an Ariadna of Segestriidae... they build similar webs that issue from a hole. I would check into those before Kukulcania, that's my two cents at least. Maybe your brother can spy on the wall at night and snap a shot of the spider while it's out.

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  5. Thank you Lady A! I'll look into those, see if I can get him to do some night photography. Now I'm really curious.

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  6. We left that ever-growing net/retreat alone, now for more than a year, and when we have guests we tell them that we have a friend who cares so much for the spiders that we have to respect them...but the one in the last pic has lately moved to the inside of my studio window, grabbing the occasional escapee from my beetle photo set-up -- that goes too far!

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  7. My brother sent me nighttime photos of the spiders. They're just Agelenids. They're making gauzy, filamentous funnels in the rock wall like Filistatids.

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  8. oooh i have one of these as a pet. pretty little girl. i also just caught a camel spider, or sun spiders as they are called here.

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  9. It should be a long-lived pet, Adrienne :-) Good luck with both of your two arachnids!

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  10. I very large one just scurried across my floor and under my bed. There are tons here.

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    1. Awesome, Amber! It *was* awesome, right? Hope so, anyway :-)

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  11. Replies
    1. Virtually all spiders are *venomous*, but only a few are dangerously venomous to healthy humans. This species is *not* one of them. The difference between "poisonous" and "venomous" is explained in my post "Is it Poisonous?" : http://bugeric.blogspot.com/2013/10/is-it-poisonous.html

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