by Joseph DeSisto
In my last post, I talked about centipedes with unusually long, narrow fangs, and how in the woodlouse spiders of the genus Dysdera, such strange devices are an adaptation for hunting woodlice. These spiders, however, deserve more than a mention in an article about another animal, so let’s talk about woodlouse spiders, and why they are so cool.
Woodlouse spiders are weird. They look weird, they act weird, and naturally I think they’re great. Here in North America we only have one species of Dysdera, and that’s D. crocata. This species is actually introduced from Eurasia, where it lives with around 200 other members of the genus. These spiders are relatively large, mostly hairless, and do not spin webs. Dysdera are also quite handsome as far as spiders go, cloaked in red, orange, and pale brown.
The biggest reason these spiders look so strange has to do with their fangs. A spider’s fangs are called chelicerae, and are modified mouthparts, in contrast to a centipedes “fangs” which are modified legs. Fangs can come in all shapes and sizes, but the woodlouse spider’s are probably some of the biggest, relative to its body size, of any spider. The fangs are enormous, and indicate a specialized hunting strategy: woodlouse spiders, as their name suggests, are specialist predators of woodlice or pill bugs.
Or are they? The question, as it turns out, is more complex than you might expect. Pollard et al. (1995) experimented by offering D. crocata spiders different sorts of prey: two types of woodlouse, but also flies, beetle larvae, crickets, and other many-legged morsels. The result: the spiders did not prefer woodlice. So what’s going on here?
More than a decade later, Řezác and Pekár (2007) did pretty much the same experiment, and again saw that the spiders didn’t prefer woodlice over the alternative, in this case fruit flies. In fact, their spiders actually ate more flies than woodlice! But they also conducted a second experiment in which they raised young Dysdera on diets of either woodlice, flies, or a mix of the two.
When the spiders weren’t allowed to choose their food, those that ate woodlice developed significantly faster than those that ate only flies. It seems that, although woodlouse spiders are adapted to be woodlouse specialists, certain circumstances, including captivity, can cause them to change their preferences. Perhaps in the captive setting, flies are just easier prey than woodlice.
It is worth pointing out that Řezác and Pekár conducted their experiment with a different but closely related species of spider (Dysdera hungarica) than Pollard et al. This may seem trivial, but in fact there is a lot of variation in feeding strategies among the woodlouse spiders. Although all appear to be specialized to some degree, the way in which their enormous fangs help them dispatch their armored prey varies quite a lot between species.
For example, Řezác et al. (2008) found that Dysdera spiders with concave chelicerae use them to stab woodlice from beneath, whereas others have flattened fangs they can slide between the prey’s plates of armor. Not only do different species have different methods of killing woodlice, some species with relatively “normal-looking” fangs refuse to attack woodlice in captivity.
So, not to worry if you were hoping to study woodlouse spiders: there is plenty of work still to be done.
Pollard, S.D., R.R. Jackson, A. Van Olphen, and M.W. Robertson. 1995. Does Dysdera crocata (Araneae Dysderidae) prefer woodlice as prey? Ethology Ecology & Evolution 7(3): 271-275.
Řezác, M. and S. Pekár. 2007. Evidence for woodlice-specialization in Dysdera spiders: behavioral versus developmental approaches. Physiological Entomology 32: 367-371.
Řezác, M., S. Pekár, and Y. Lubin. 2008. How oniscophagous spiders overcome woodlouse armour. Journal of Zoology 275: 64-71.