Sunday, April 15, 2012

Spider Sunday: Triangulate Cobweb Weaver

One of the spiders most frequently encountered in basements, garages, and cellars over much of North America is the Triangulate Cobweb Spider, Steatoda triangulosa. It is a member of the cobweb weaver family Theridiidae, and therefore related to black widows. Not surprisingly, it is sometimes mistaken for a widow.

This species is widely distributed in the U.S., from Massachusetts to Nebraska, Colorado, Utah, Oregon, and California, and all points south (though nearly absent in the southwest and in Florida). It is also recorded from southern Ontario, Canada. It is also known from central and southern Europe, the Mediterranean region, and what was formerly southern Russia. Indeed, it is suspected that S. triangulosa was introduced to North America from overseas.

Adult females are only 3.6-5.9 millimeters in body length, males even smaller at 3.5-4.7 millimeters. Both genders usually sport the same distinctive pattern on the abdomen: a background color of dirty white or beige with a pair of bold, wavy, rusty or purplish brown lines down the back. The cephalothorax is normally dark reddish brown, and the legs pale brown or yellowish with dark bands at the joints. Males have a more slender, leggy appearance than females.

The Triangulate Cobweb Weaver spins an irregular, tangled web with a more-or-less sheet-like central platform. It seems to prefer dark recesses where it can dash into a crevice if disturbed. Look for them in your basement, cellar, garage, water meter box, or under bridges and in culverts, where the entire web may be protected from the elements. The spider hangs upside down in its web.

Mating occurs from late spring through early autumn. Females deposit about thirty eggs at a time, wrapping them in an opaque, white, spherical sac. The eggs may be seen through the sheer fabric enclosing them. A female may produce six or more egg sacs in her lifetime.

This species preys on a variety of insects, including the Red Imported Fire Ant (“RIFA”) in the southern U.S. Unfortunately, it is an unlikely candidate as a biological control agent of that pest (MacKay and Vinson, 1989). This is apparently a spider that “plays well with others,” as it may build its web in relatively close proximity to the webs of other cobweb weavers, cellar spiders, and even brown recluse spiders.

Don’t be alarmed by the presence of this species. It is not recognized as being dangerously venomous to people or pets. Indeed, it is such a small spider that its bite is unlikely to puncture human skin anyway

Sources: Howell, W. Mike and Ronald L. Jenkins. Spiders of the Eastern United States: A Photographic Guide. Boston: Pearson Education, Inc. 361 pp.
Jackman, John A. 1997. A Field Guide to Spiders & Scorpions of Texas. Houston: Gulf Publishing Company. 201 pp.
Jones, Dick. 1983. The Larousse Guide to Spiders. New York: Larousse & Company, Inc. 320 pp.
Levi, Herbert W. 1957. “The Spider Genera Crustulina and Steatoda in North America, Central America, and the West Indies (Araneae: Theridiidae),” Bull Mus Comp Zool 117(3): 367-424.
MacKay, W.P. and S.B. Vinson. 1989. “Evaluation of the spider Steatoda triangulosa (Araneae: Theridiidae) as a predator of the red imported fire ant (Hymenoptera: Formicidae),” J NY Entomol Soc 97: 232-233.
Moulder, Bennett. 1992. A Guide to the Common Spiders of Illinois. Springfield: Illinois State Museum Popular Science Series, vol. X. 125 pp.

30 comments:

  1. Thanks for posting this! We just spotted one of these spiders in a corner of a shelf, and were trying to research it. Great information, description and photos. Most of all, glad to know it is harmless. Looks like we will let this guest stay.

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  2. Thank you for the kind compliments! Please consider following my blog.

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  3. I love these spiders. I have a bunch in my house. They're so beautiful, like tiny golden orbs. Thanks for posting this information.

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  4. Could anyone tell me how big these spiders get in comparison to maybe a coin

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    1. Apologies for the late reply, I was offline for several days....The legspan of one of these spiders would not even cover a dime or a penny. They are quite small.

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    2. Females I would say would only reach 1/3 to 1/2 the size of a dime as a rough estimate, and males seem to be even smaller than that.

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  5. these spiders are funny. Ihave three that I am keeping as sort of pets. They are really neat to watch. I recently afound a yellkw sac of whom I added and watched the dynamics change in their environment.

    However out of the three triangulate cobweb spiders one is super super avgressive. why would that be?

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    1. Thank you for making the observations that you are. Unless all three spiders are in the same container, I do not know why one of them would be aggressive in its behavior.

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  6. As an electrician in Alabama I have encountered many of these spiders. When their web is disturbed, they tend to drop out of it. So wiggling around in a crawl space under a house it was not uncommon for me to end up with one of these little guys dropping down my shirt. I have been bitten a few times from them, but the pain is mild, and ends quickly after they stop biting.

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  7. I have a question. One morning I found six of these spiders (4 female 2 male) in my window. At first I was too afraid to do anything to kill them and by the time I was comfortable I had become like a voyeur. Just too invested in them lol. Not only were they seemingly good neighbors but the females almost seemed as though they would help one another or share. But maybe two months later I saw one female (by far the biggest female) was killing another. After that thing a were calm again until the last couple of days. The same female that attacked and killed earlier is now the only spider in the window that I can find. The females have clearly been killed as their bodies are all wrapped up and one is kind of discolored. The males are just nowhere to be seen. As embarrassed as I am to detail this out in a question to another human being, curiosity is stronger than the embarrassment. I have searched and searched for an answer to no avail. WHY would she suddenly do this? Is she maybe going to lay eggs or something? I just can't figure it out.

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    1. I apologize for taking months to reply. I don't have an answer, either, sorry to say.

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    2. I don't know about spider vs spider behavior, but I believe the males die after mating, so she probably didn't kill those ones. Maybe after the males spontaneously died she ate them and got a taste for her own kind? Or perhaps there was a scarcity of prey? I'm not an expert but it's a thought :)

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  8. Hello. I have quite a large spider on my front window, which I thought was a cobweb weaver however it's quite large. It's approximately 1 inch or more in the body size. Is this the right species? Thank you.

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    1. Triangulate Cobweb Weaver is quite small, so I suspect yours is Parasteatoda tepidariorum, the Common House Spider, if it is making the same kind of tangled web.

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  9. I have a steatoda triangular living in my room and I'm quite OK with that ,but what I would like to know is,how long is it's life span?

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    1. Excellent question. There may not be an answer to that for this species. Generally, spiders protected from weather extremes, predators, and parasites live longer than those individuals exposed to those mortality factors. Other cobweb weavers are known to survive for two or more years in captivity, so your spider may live well over one year, even after achieving adulthood.

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  10. I have a triangulate cobweb weaver that has been living above my door for quite some time. I’m moving soon and planned to take her with me. Well, today I discovered an egg sac!! Can I move the egg sac with her? She’s kinda like a family member now. How long is the incubation process on the eggs? She must have laid them in the last week/week and a half.

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    1. Hi, Jen. No, you cannot move the egg sac without mom. The baby spiders will disperse once they emerge. Few will survive, those that do will find an out-of-the-way location to build their own webs. No worries, all of you will be fine. :-)

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  11. Hi! If I move my triangulate cobweb spider with its mom into a jar, and tried to feed it but now the mother is dead, will these babies still survive? Let me know, thanks!

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    1. Yes, the spiderlings will disperse on their own if allowed to do so.

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  12. I have a question that may be silly, as I am new to learning about spiders. I think I have a female of this type in my bathroom, can they reproduce asexually? Or is a male needed? Thanks for the informative post!

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    1. Not a dumb question, but no, spiders cannot reproduce asexually.

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  13. Hi! Not sure if this is a good place to ask this but here goes! I found a triangulate cobweb spider last month. It was so beautiful with it's purple markings,I decided to keep it for awhile. After some googling I was able to easily tell she is a female. This was further confirmed when she laid 2 egg sacs! I was also suprised to find her, looking completely different. After laying eggs she seriously looked like a male-- smaller, slender, even a different color? After a few cricket feed sessions, she has turned plump and female looking again. However, she has now laid 3 more egg sacs and is looking totally different again. Is this normal, to change appearance so much? How does she keep laying eggs if she's been enclosed for over a month? How many can she lay? There isn't a lot of info on their reproductive habits, or how often they like to eat. She seems to eat whenever I feed her.

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    1. These are perfectly legit questions, and I admire your curiosity, powers of observation, and patience. Yes, spiders can change shape drastically after laying eggs. Not sure how many egg sacs they can produce, probably depends on how much sperm she stored during mating. Spiders are opportunistic predators, able to go surprisingly long periods without food. They do require water regularly (just misting the web is sufficient).

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  14. Greetings from Argentina! I have two questions about a triangulate cobweb that lives in muy bathroom window.
    First at all I wanted to know what is the incubation time of the eggs; and second, do you think I could take the egg whitout disturbing the spider? or minimize her disturbance? I really dont want 30 mini spiders searching for a new home inside my house, but also, I want to respect the spider and treat her the best possible way. Thank you!!

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    1. Hello! I wish I knew the answers to your questions. My advice would be to move the egg sac, but feel no guilt if that is unsuccessful. This is an abundant and successful species. Thank you for caring!

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  15. how long do they live without food?

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    1. Spiders are built to fast for long periods, but that time span varies considerably depending on the spider's age, metabolism, etc.

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  16. I believe I've got one of these who has been living in my bathroom window for nearly 9 months. I've been watching her grow, but until a week ago she was jet black and only just started developing white spots in a symmetrical pattern. One of them is a definite triangle, but the rest are still small, mostly circular spots. Is this normal for this species? Will she continue to develop the triangular pattern?

    Also, she's recently had a "boyfriend" over and while he wasn't around long, it was long enough. She produced two egg sacs which I carefully moved to a safe place outside. (No babies in the house!) I'm wondering if she is likely to mate again in the future or if it's a one time deal for these spiders.

    Thank you!

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    1. Most spider species are highly variable in color and the pattern often changes as they age. Typically, spiders get darker with fewer pale markings....I can't answer the mating question, but I believe most female spiders can store sperm, like insects can.

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