Woodlice (Isopoda: Oniscoidea) are well defended by a heavy armour and many species have noxious secretions. Oniscophagy (feeding on woodlice) may be an important part of the biology of at least some members of the spider genus Dysdera (Dysderidae). Yet there little is known about the diet and possible specializations for feeding on woodlice in these spiders. Dysdera has unusual variability in mouthpart morphology, which may be related to the diet. Here, we investigate five species of Dysdera and consider the relationship between mouthpart morphology, capture frequency and prey-capture behaviour. We show that species with unmodified chelicerae readily capture a variety of arthropods, but refuse woodlice as prey, whereas species with modified chelicerae feed on woodlice and reject most of the other prey. Among the oniscophagic species, the grasping methods used during the capture of woodlice varied. Species with elongate chelicerae inserted one chelicera into the soft ventral side of the woodlouse and placed the other chelicera on the dorsal side of the woodlouse. Species with dorsally concave chelicerae quickly tucked their chelicerae under the woodlouse and bit the ventral side of the woodlouse's body. Species with flattened chelicerae inserted their chelicerae between the sclerites into the armour of the woodlouse.