What is this orange backed spider?

The Question: I live in North Texas. When I grabbed a pair of pants out of my closet, a spider dropped out. It was black everywhere but the top half of the abdomen, which was bright orange. It was fairly slender and the legs weren’t all that long. Also, it wasn’t a hairy spider like most of the ones we have around here. Any idea what this might be?

Submitted by: Robert, Texas, USA

The Short Answer: Our trusty spider expert Mandy Howe at spiders.us read the above description and suggested a species in the genus Castianeira, and based on the location and orange abdomen, probably one of these three: Castianeira crocata, Castianeira descripta or Castianeira occidens. When I sent Robert pictures of these spiders, his response was, “That’s it!”

C. crocata, C. descripta and C. occidens look roughly similar and all three are characterized by a highly visible red/orange abdomen. Without a photo or the specimen, it’s impossible to say which of these species was hanging out in Robert’s closet, but it was almost certainly one of them.

More Information: Many of the spiders in the Castianeira genus are “ant mimics,” meaning their bodies, and sometimes their behaviors, seem to imitate the shape and coloration of ants (and some wasps). Why would spiders imitate ants? There are at least two reasons: to be able to approach ants so as to prey on them, or to put off predators by imitating ants that don’t taste good or that have a painful bite or sting. The imitation of an unpalatable species by a palatable species is called Batesian Mimicry. It’s easy to see why any predator that doesn’t like the taste of weaver ants, shown in the photo below (left), or is wary of their bite, would avoid the ant mimic spider shown beside it (Myrmarachne plataleoides). Batesian mimicry turns out to be fairly common among insects and other arthropods, and imitating ants is a common ploy. For example, in addition to spiders that imitate ants, there are beetles that imitate ants, flies that imitate ants, and even praying mantises that imitate ants.

So what are the orange-backed Castianeira spiders imitating? Most Castianeira spiders imitate ants, but it appears that the orange-backed species are imitating “velvet ants,” which are not actually ants at all.  Velvet ants are wasps. Female velvet ants are wingless and since wasps and ants have similar body shapes, it’s not surprising that people are fooled. And if you look at this picture of the velvet ant, you can see that it has an orange abdomen, like Castianeira occidens and the other Castianeira species mentioned above. Velvet ants have a very nasty sting (remember, they are really wasps), and because of their store of sting toxin, apparently, they taste terrible to birds. Castianeira spiders probably escape predation by imitating them.

The Puzzle – Imperfect Mimicry: But a closer comparison of the spider and its model shows that beyond a vaguely orange abdomen, the mimicry isn’t at the same level as that of the ant mimic copying the weaver ant in the photo above. Biologists call the Castianeira/Velvet Ant system a case of “imperfect mimicry,” and it presents an evolutionary puzzle. It’s fairly straightforward to imagine how natural selection can hone mimicry to make it more and more exact. But why does that process seem to have been truncated with Castianeira imitating velvet ants?

In recent decades several hypotheses to explain imperfect mimicry have been suggested:

  1. Maybe the mimics only look imperfect to us. Experiments have shown that the vision of birds is similar to ours in that when we think a mimic is very good, or not so good, birds almost always agree with us. But it’s very difficult for us to imagine what a spider with eight eyes sees or what the compound eye visual system of a praying mantis is like. Maybe to a praying mantis predator, the Castianeira spider and the velvet ant look as alike as the weaver ant and its “perfect” mimic.
  2. Maybe the evolution of mimicry is still in progress. In the evolution of the weaver ant mimic, there was presumably some point in the past when the spider looked less like the ant than it does today. Perhaps an imperfect mimic like the Castianeira spiders only began evolving velvet ant mimicry a short time ago (in evolutionary terms) and hasn’t had enough time to reach perfect mimicry yet.
  3. Maybe the mimic is simultaneously mimicking more than one model species. Often, there are a number of velvet ant species found in the same area. It’s possible that a mimic that looks sort of like several model species might gain even more protection than a perfect mimic of a single model species.
  4. Maybe the mimic has to simultaneously avoid two or more types of predators. It appears, for example, that by being imperfect mimics, some spiders gain at least some protection from spider predators by looking like ants, and some protection from ant predators by looking like spiders. If they mimicked their ant models too well, they’d suffer more predation from the ant predators.
  5. As a mimic evolves to become closer to the model, the “force” of natural selection may become weaker. It’s possible that there is little advantage to be gained by going from 80% mimicry to 95% mimicry. This may be especially true if the model species is particularly noxious or dangerous or if the mimic is rare compared to the model.

    If the model is very noxious, predators aren’t going to take a chance that they will make a mistake. So they’ll avoid anything that looks even remotely like the model. Maybe birds are so reluctant to eat a velvet ant by mistake they’ll avoid any small crawling creature with an orange splash. A Castianeira spider that has a more accurately orange abdomen gains no advantage and therefore natural selection doesn’t favor more accurate mimicry. If the model is not that noxious, on the other hand, predators may be willing to test anything that doesn’t look exactly like the model.

    If a mimic is very rare compared to the model, it doesn’t need to be that accurate. Imperfect mimics can lose out when a predator tries to eat a mimic and discovers it doesn’t taste noxious. It may then break the code and figure out which are the mimics and which are the models. But if there are very few mimics compared to models, nearly every time a predator takes a chance, it’s likely to bite into a noxious model. It might never come across enough mimics to learn the difference. If mimics are common compared to models, on the other hand, there is a very strong incentive for predators to figure out which is which, and mimics have to be very good so predators can’t reliably tell them apart from models.

  6. There may be a cost to mimicry. If a species has to greatly modify its appearance or body shape to mimic its model, that may reduce its ability to function. For example, while ants have three distinct body sections, spiders have only two. But some ant mimic spiders have developed a constriction in their abdomen to better imitate the separation between the thorax and abdomen of ants (see the picture of the weaver ant mimic above). According to Dr. Malcolm Edmunds, naturalist and professor emeritus at the University of Central Lancashire, this greatly reduces the mimic’s ability to produce eggs in large quantities. An imperfect ant mimic like the Castianeira spiders doesn’t face this handicap to reproduction.In the case of coloration, there may also be a cost. The bright coloration of noxious species like velvet ants tends to make them obvious. It’s to their advantage to give clear signals to predators, “Stay away from me!” This works for noxious species. But for the non-noxious mimics, drawing attention when you aren’t the real thing may have a downside if some predator species are fooled and others are not.

As you can see, there are quite a few theoretical possibilities that might explain imperfect mimicry. One recent study suggests that the most likely factors are the weakening of selection as mimics become more perfect and the balancing of the costs of perfect mimicry. It may also be that multiple factors are at play in different cases of Batesian mimicry. In any event, the Castianeira spider that fell out of Robert’s closet apparently didn’t fool him. He correctly identified it as a spider and not a velvet ant.

But how many of us would look at the creature to the left and correctly identify it as a spider (look closely at the “antennae” and you’ll see that they are actually the front two legs of the spider)?  That’s because this is another close-to-perfect Myrmarachne ant mimic spider like the yellow one shown above.

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Thanks to imperfect mimicry experts Tom Sherratt and Chris Hassall at Careleton University in Ottowa for their help.

Penney, H D, Hassall, C, Skevington, J H, et al. (2012). A comparative analysis of the evolution of imperfect mimicry. Nature, 483(7390), 461-U110.

Sherratt, TN. (2002). The evolution of imperfect mimicry. Behavioral Ecology, 13(6), 821-826.

Speed, M P, & Ruxton, G D. (2010). Imperfect batesian mimicry and the conspicuousness costs of mimetic resemblance. American Naturalist, The, 176(1), E1-E14.

Edmunds, M. (2000). Why are there good and poor mimics?. Biological journal of the Linnean Society, 70(3), 459-466.76(1), E1-E14.

Cite this article as: Pelletier, TC. (July 14, 2012). What is this orange backed spider? Retrieved from http://askanaturalist.com/what-is-this-orange-backed-spider/ on March 22, 2017.

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