Spiders will just break your heart

Last Wednesday, I was so optimistic. I’d paired up Vera with a first-generation son of Gwyneth (GI-♂), they’d gone at it hot and heavy, and had produced an egg sac — an egg sac with a peculiarity I’ll get to in a moment. My plan was to go in today and open up the sac and see what embryos I’d find.

The peculiarity was that although I’d classified all my spiders as Parasteatoda tepidariorum, there was still a little uncertainty. The patterns of pigment on their cuticles were a bit ambiguous. Here’s part of the description of P. tepidariorum from Common Spiders of North America.

This is a medium-sized to large cobweb weaver. The abdomen appears teardrop shaped as the spider hangs updide down in its web, spinnerets uppermost. The cephalothorax is tan or brown. The abdomen color is extremely variable, usually shades of light brown with a mottled tan or brown. Some individuals are nearly black, others unmarked and pale. The legs are darker at the joints.

The egg cases of this spider are tan and tardrop-shaped, point uppermost.

This describes my spiders perfectly, given that there is variability in the patterning of the abdomen. They look different, which had me worried that I may have gathered a couple of different species, but then the description of the egg case settled it for me: Gwyneth had produced lovely tan, teardrop-shaped egg cases. Case closed, right? Gwyneth and her progeny were P. tepidariorum.

Then Vera produced an egg sac. Vera herself also fits that description of P. tepidariorum to a T.

But the egg sac she produced was a cottony, pure white ball. What? I flipped through my books looking for some indication of what this might mean, and found a description of Steatoda triangulosa, which is also common in this part of the country.

This is a medium-sized cobweb weaver. The cephalothorax is reddish brown. The legs are light brown with dark brown bands. The apdomen is light brown with two rows of angular spots or bands of dark reddish brown separated by white areas with a mottled appearance. In some individuals this pattern looks checkered.

The egg case is a fluffy white sphere.

Uh-oh. Are Vera and Gwyneth from two different genera? There’s nothing in Vera’s abdominal pattern that fits S. triangulosa, but maybe that’s highly variable in this species, too, and the egg case definitely fits this description. But she’d bred with P. tepidariorum! Were the new eggs hybrids? Was I just a terrible ignorant klutz playing the taxonomy game poorly? I went into the lab to take a few steps towards finding out.

And…disappointment. The “fluffy white sphere” was gone — it had been torn apart. The eggs within were dried up lumps. Uncertainty reigns.

I put the two lousy parents into a petri dish and made a video. You look and tell me: Parasteatoda tepidariorum or Steatoda triangulosa? Looking at just the adult morphology, I’m saying P. tepidariorum, but what do I know. Video below the fold, with an agitated pair of spiders scurrying about.

All I can do is move on. Vera is in a new vial all by her lonesome. I’ve bunked GI-♂ with a GI-♀ (that’s right, he’s with his sister committing incest, and yes he was inseminating her within minutes). But at least it’s not miscegenation!

I’ve also paired up another GI-♂ with a daughter of Xena, XI-♀. So I’ve got two pairs breeding right now. The females are young and on the small side, about half the size of Vera, so we’ll have to see if they’re old enough to reproduce.

Cross your fingers, or whatever superstition brings you a sense of satisfaction.


  1. davidc1 says

    Don’t know what to say ,apart from ,why not play with an easily identified model of spider ?

  2. nomdeplume says

    Taxonomy is often seen as the Cinderella of biological sciences. Difficult to get funding for, perceived as being done by very very old zoologists locked away in the basement of museums, making a fuss about minor morphological differences. Old fashioned nineteenth century zoology, not modern up to date shiny stuff with DNA analysis, ecology, etc. But taxonomy is, always was, the bedrock of zoology – you have to know what species you are working on before you can work on it. I know PZ knows this, and I wish him luck with this taxonomic puzzle!

  3. says

    PZ – have you considered starting a thread on Arachnoboards.com? It’s a fairly friendly community of Spider lovers – mostly focusing on Tarantulas as pets – but husbandry and breeding are very much a part of it, and they also have threads dedicated to ‘true’ spiders.
    On that note, I am far from an expert, but it seems to me that you have a Parasteatoda tepidariorum. S. triangulosa’s markings are generally relatively obvious (generally…).
    Spiders are known to abandon and/or destroy their egg sacs when there’s something wrong. I’ve known tarantulas to go about making an egg sac that they know to be futile, but do so sloppily. It’s possible the ‘fluffy white’ sac was just a rushed job: loose threading looks fluffy and white.

  4. says

    A little extra homework indicates that early stages of Parasteatoda tepidariorum egg sac construction look VERY much like Steatoda triangulosa complete. It is possible that Vera abandoned the eggs after she realized they were infertile.

  5. says

    You may want to start a thread on Arachnoboards.com. They have people there who do breed these house spiders (and who deal with spider husbandry in general as a hobby).

  6. says

    So if the two species cannot produce viable offspring in a cross, doesn’t that mean that they must both have ridden separately on the Ark? It just got heavier …

  7. skeptuckian says

    Looking at The Spiders of North America key to the genera, the Steatoda should have a large colulus present while the Parasteatoda should have no colulus, one that is not easily seen, or made up of only 2 setae. There are three figures in the key showing these features. I can send you a pic of the figure if you would like. Like you I am an amateur but this seems to be a way to solve your identication problem.

  8. says

    #7: Thanks, I’ll see if I can spot it.

    #4: That’s an interesting idea — next time I see a fluffy white egg case, I’ll throw it on the timelapse scope and see how it progresses.

  9. robro says

    During a walk at South Hamilton Park in Novato, CA today, the air was full of gossamer spider threads. They also draped the leaves and branches. Something was ballooning, which I assume was spiders. We saw quite a few beautiful large spider webs nestled in the bushes with their owners waiting patiently in the center. It was pretty amazing.

  10. pgmoni says

    The only way to identify spider species when in doubt is to examine the genitalia (epigyne and pedipalps). You will find pictures here
    amongst other sources.
    Note that P. tepidariorum is not present in Europe, but S. triangulosa is.
    There are alos excellent drawings in M. Roberts
    Sipders of Britain and Northern Europe
    Collins field guide
    ISBN 0-00-219981-5

  11. rcs619 says

    So you’re not just breeding a spider army any more. Now you’re cross-breeding them to make hybrid super-spiders. You need to be stopped, sir. Stopped!

    This spider-business has of yours has been a lot of fun to watch. I know I’m still rooting for Vera to find love.

  12. grahamjones says

    Just been listening to nice program on BBC radio 4:


    The Spider Orchestra
    Pursuit of Beauty
    The Berlin-based Argentinian artist, Tomás Saraceno, trained as an architect. He was struck by the beauty of spider webs, their structural intricacy and began making them into sculptural works. Then he realised that every time a spider tugs a string as it spins a web, or moves along the silken strands, this causes vibrations. Using microphones and amplifiers it is possible to hear the tiny music they make…

  13. says

    @PZ – there’s an online community archnoboards (google that… websites aren’t allowed) which might also offer some clues.
    A long time reader – rarely a commenters here – but spiders are known to abandon egg sacs (at least Tarantulas are).

  14. richb says

    Oh I sure wish it was simple. This is Rich Bradley (author of book you cited) here, your conundrum is not surprising to me. Just for starters, I’m not surprised that you didn’t have a romantic event in your petri dish. These are web-building spiders and males usually court on approach to a female in her web. The approach is usually accompanied by tactile signaling from the male, not possible in an empty petri dish. I guess the best analogy I can imagine is two random humans, one male one female being tossed into a glass chamber being watched by some overlord. Would the first thing they do is mate? not likely. Some wandering spiders will mate when placed together, but in my experience cannibalism is common instead.
    There are many issues here, not least that there are many more species of spiders in your area, including many other cobweb weavers (Theridiidae). I’m guessing Vera is possibly the related Parasteatoda tabulata (p 229, also on plate 24). Her markings look consistent with that. Sadly the only way to tell her from the sister species tepidariorum for sure is by dissection of the internal genitalia. At least males can be identified by a microscopic view of the palp (in a mature male) without dissection. BTW-Seeing the colulus usually requires a dead or immobile spider under a microscope. Another issue is the maturity of your male. I can’t see enough detail in the frantically moving male to distinguish a subadult from adult male. Ah… the egg case. The egg case of tabulata is brown too, but not as consistently teardrop shaped. So the white fluffy one (which is indeed a common option in many theridiids) doesn’t fit. I wonder if she wasn’t done constructing the case, stopped part way? interrupted? maybe Vera is an unmated female? and perhaps destroyed later? (was she in the same container as the egg case when it was damaged?). Another issue with the tabulata hypothesis is that they typically build a little shelter in their cobweb to hide under. It can include leaves or other debris. If you place Vera in a container with enough space for a web, and add some loose fragments of dry leaves in the bottom, she might drag a few up and make a retreat. If she does that she is likely tabulata. Let me know your thoughts.