Cobweb Spider Devoured by Wasp Grub

2012 October 31

Here’s one that I thought would be particularly appropriate for Halloween, as in some ways it’s even creepier than the doomed caterpillar in the previous post: it’s got spiders, and poison, along with the traditional drawn-out gruesome death!
To start with, I happened to spot this little drama right beside our front door on June 9, 2012. A wasp was attacking a hapless cobweb-weaver spider.

The spider seemed paralyzed, and the wasp acted as if she were stinging it repeatedly. This kept on for some time.

In this next picture, I think the little spider in the upper left was a male, who was courting the female spider when this happened. If I was going to anthropomorphize (and I am!), I’d say he was looking on in horror.

Eventually, I left, expecting that the wasp was going to drag the paralyzed spider back to her nest. Surprisingly, when I came back a few hours later, the spider was still there (although she seemed a bit groggy). I suspected that there was more to come, so I caught her in a little container. She seemed OK at that point, and ate the flies that I fed her.

But after a couple of weeks, we started noticing this little grub growing around her waist. It was tucked in right where she couldn’t reach it, so even if she was aware of its presence there wasn’t much she could do about it[1]

Over time it got bigger, getting so large after about another week that it was nearly as big as the spider. The spider was practically immobile by this point, other than weaving a mesh of webbing on the side of the container where she was standing.

Finally, the grub killed and finished eating her, dropping her dried husk to the ground. You can see the shed skin of the grub stuck to the side of her withered, pathetic remains.

The grub then spun a loose cocoon right on the spot the spider had prepared earlier by laying down webbing, and made a pupa that could be dimly seen through the cocoon.

And then, finally, a wasp came out (a female, judging from the ovipositor), ready to fly off and attack more spiders. Unfortunately I don’t have a picture, because the instant I opened the container to get a better look, she flew off. I’m sure no one will be surprised to hear that she looked pretty much like the original wasp that attacked the spider, though. She may never have gotten out of the house, but we have a number of spiders around in secluded corners, so it is possible that these parasitic wasps have now joined our house fauna[3].

The wasp appears to be one of the ichneumons in the Polysphincta genus group. This is mainly based on the choice of host. There are hundreds of different ichneumon wasp species. Each species of ichneumon tends to specialize on a particular host species, so there are just about as many ichneumon species as there are species of caterpillar, spider, beetle, or other small arthropod that are big enough and common enough to make suitable hosts. In general, if you find an ichneumon without knowing what its host is, there is a good chance it will be practically unidentifiable.

[1] The position of the poor spider is more or less like this: Imagine you are in your house, minding your own business[2], when suddenly some maniac bursts through your door, knocks you down, and injects you with something that makes you pass out! After a while you wake up, and feel OK except for a nagging sore spot in the middle of your back where you can’t reach. And man, you’re hungry! So you eat and eat. And you’re aware of a growing weight on your back that you can’t quite see or touch. It’s getting bigger, and meanwhile you are still soooo hungry, but also growing weak. Finally, you can barely move. The thing on your back has gotten so large that you can now see parts of it, and even touch its squirming, repellent skin. But you are too weak to do anything more, so you just lay there as it drains the life from your body. And then, finally, it eats you.

Nice, eh?

[2] Although, I suppose if the business you are minding is the business of ambushing passers-by, murdering them, and drinking their blood, you might not get a lot of sympathy when something this horrible then happens to you.

[3] Like other wasps, ants, and bees, they can still lay eggs even if they don’t have a mate. It’s just that the unfertilized eggs will become males, while the fertilized eggs will become females (Haplodiploidy). This is actually kind of an inspired solution to sex selection: if you have plenty of males, then the eggs will all get fertilized and the next generation will have a higher proportion of females. If there are not enough males, then a lot of eggs go unfertilized, and the next generation has a bunch more males to work with. So even if this female was the only wasp of her species in the house, she would be able to at least have sons, if not daughters. Which buys time either for another female to make her way into the house, or for her sons to ultimately escape to the great outdoors. This isn’t quite as much of a guarantee of being able to reproduce as being fully parthenogenic (capable of reproducing more females without any participation from males) would be, but it does neatly avoid the problem that a number of species that are capable of parthenogenesis have run into. Sometimes, a species that can reproduce parthenogenically gets into a situation where the males die out. There is then no way to re-create new males, and the species turns into a few lines of genetically identical clones. They then get wiped out over time by diseases and parasites that they are incapable of adapting to without the ability to swap genes sexually.

9 Responses
  1. October 31, 2012

    I wonder what would happen if you did a grubectomy. Would the spider recover, or die anyway? Is she poisoned, in other words, or just parasitized?

  2. October 31, 2012

    And you just stood by and let this happen? Ugh. No treats for you tonight.

  3. October 31, 2012

    She acted normal enough until the grub got large, so she probably wasn’t poisoned. But, I really don’t know if a “grubectomy” would have been possible without also mangling the spider. Like most of our spiders, she was not particularly large – her body was about the diameter of the tip of a pencil eraser. In the earlier stages, where removing the grub could have potentially done her some good, I couldn’t even reliably see the grub with the naked eye. It was only in the blown-up macrophotos that I could see it clearly.

    If I tried going in with pointed tweezers and a magnifying glass to operate on a tiny, rapidly scurrying spider, it would almost certainly result in the spider losing a bunch of legs and probably being accidentally either crushed or cut in half. Spiders are alarmingly fragile, their legs buckle or come off at the slightest provocation and their bodies have almost no crushing resistance. I try to avoid directly handling them because of this.

  4. October 31, 2012

    Very seasonally appropriate! And very yukky…but fascinating.

  5. October 31, 2012

    True. (Spiders are fragile.) And I don’t know how to sedate a spider without killing it. But I am insanely curious.

  6. Carole permalink
    October 31, 2012

    What an interesting experience you had, thanks for sharing.

  7. November 1, 2012

    How long did it all take?

  8. November 2, 2012

    dfg: It was pretty fast. From egg-laying to adult was 25 days.

  9. November 3, 2012

    Pretty fantastic series of pictures. We have a similar ichneumon here (and possibly the same spider – is that Steatoda borealis?), but I’ve only seen the grub once – on a small male spider last spring. I wondered if the female puts male eggs on small spiders and female eggs on larger spiders. In any case: Excellent!

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