Woodlice – slow-moving terrestrial isopods also known as `pill bugs'– would appear to be a juicy treat for any predator. They are the main component of many ground-dwelling fauna and can be found in large numbers. But it appears that, in general, these isopods have few invertebrate predators:only a few animals prey upon them occasionally, including harvestmen,centipedes and ants. Yet one genus of tropical ant, Leptogenys,includes species that are exclusively oniscophagous: they eat nothing but woodlice and are equipped with specially adapted mouthparts to kill and carry their prey. Some spiders also dine occasionally on woodlice. For example,non-web-building Dysdera spiders, found around the Mediterranean and in Europe, are also thought to feed on woodlice with their unusually long,curved mouthparts, known as chelicerae. Milan Rezác, Stano Pekár and Yael Lubin from Israel and the Czech Republic wondered whether the size of a spider's chelicerae is a good indicator of their dining preference and style.

Selecting five species of Dysdera, each with different shaped chelicerae, the team starved the spiders for 2 weeks before presenting them with a single live lunch, ranging from woodlice and centipedes to springtails,bugs and flies. As soon as the spiders caught their prey, the victim was removed, to ensure that the arachnids remained hungry enough to strike again. The team found that species with unmodified, slightly curved chelicerae, never went for woodlice, while the species with long, hook-like appendages all captured woodlice, and in the case of D. abdominalis, which has very elongated chelicerae, this was the only food they would take.

To investigate how these different cheliceral designs might procure each species an advantage when faced with the behaviour and morphology of woodlice,Rezác and coworkers offered each spider a choice of dining – a rolling woodlouse, a `clinging' non-rolling woodlouse and a fly. They found that Dysdera spiders used three grasping tactics – each specific to their cheliceral shape.

Species with elongated chelicerae used a `pincer' tactic, capturing the woodlouse with their extended fangs, one on the isopod's back, the other on its belly. If this was done quickly enough, the hapless woodlouse did not have time to roll up. One species with concave chelicerae used the `fork' tactic,spearing the woodlouse through its belly before it rolled up. Finally, one species with long, flattened chelicerae used the `key' tactic, in which the fangs were slid like a stiletto between the plates of the isopod's armour,succeeding even if the woodlouse had rolled up.

Particularly interesting was the finding that D. abdominalisspiders, with their elongated chelicerae, were unable to transport prey with their mouthparts, and had to use their legs and pedipalps to hold the victim,making it tricky to walk. This may explain why not all Dysderaspecies have adopted an onicophagous strategy – in an environment where there are many predators, stumbling along with your prey may be disadvantageous.

But why haven't other spiders developed large chelicerae to exploit abundant woodlice supplies? The team point out that in many arachnids male chelicerae are the focus of intense sexual selection pressure from females. This does not appear to be the case in Dysdera, but in other species chelicerae are used in mating not eating; this could keep woodlice off the menu for most species, leaving the isopods to carry on pottering, unperturbed by arachnid predation.

Řezáč, M., Pekár, S. and Lubin,Y. (
). How oniscophagous spiders overcome woodlouse armour.
Journal of Zoology