The Woodlouse Spider
The discovery of a spider making a home in your isopod enclosure is a frightening thought for all isopod hobbyists. But did you know there is a species of spider that preys primarily upon woodlice? Yes, that’s right, the isopod nemesis is known as the woodlouse spider (Dysdera crocata and Dysdera erythrina). The spider is also known as the woodlouse hunter, sowbug hunter, sowbug killer, pillbug hunter and slater spider. All known names sound terrifying!
The woodlouse spider species is known for their impressive long, curved fangs which stand out against its relatively small size which gives the spider a formidable appearance. It is a tawny orange to dark-red colouring (a cuter nickname for this spider is the baked bean spider) with a shined exo-skeleton and tubular abdomens. Both males and females are generally similar in appearance with the front pair of legs longer in length in adult males in both species. The two species can be differentiated by the short, stout spines on their legs. A crocata male spider measures 9-10 mm and females 11-15 mm. Whereas, the erthrina male spider measures 7-8mm and females 9-10mm.
Most spider species have eight eyes, but the Woodlouse spiders have six, arranged in a semi-circle. They are slow-moving, short sighted and largely nocturnal hunters. Interestingly, they don’t use webs to capture prey. During the day, they are usually fund inside an oval white silk cell attached to surfaces under stones or wood. Within these silk cells, they moult and build their egg sacs.
Both species are found where woodlice are in abundance such as under logs, rocks and leaf litter. Typically, the Dysdera crocata is often found in or near building, including in cellars and kitchens as well as along coastal habitats, gardens and waste grounds. The Dysdera erythrina is more commonly found in permanent grasslands and woodlands.
You may think there is nothing to worry about that this menacing sounding spider will live in another country and our isopod colonies are safe…if you think that, you would be wrong. Both species of the Dysdera woodlouse spider is quite widely distributed in Britain, but absent from much of Scotland and the uplands of Wales.
There are very few spider species that would make a meal on a woodlouse due to most spiders finding them distasteful and often struggle to penetrate through the woodlouse’s armoured plates. Woodlice spiders are adapted to exploit thriving on woodlice as a food source. Not only do they find woodlice palatable, but they are also able to hold the woodlice in a vice like grip and pierce through the woodlice exoskeleton to the softer underside. A woodlice defence mechanism of rolling into a tight, armoured ball is no deterrent or true defence to a woodlouse spider.
The length and strength of a woodlouse spider’s fangs puts them among the very few British spider species that can pierce through human skin. If you are ever faced with handling one of these spiders, please do so very carefully. Although their bite is not dangerous it will be briefly painful.
Other invertebrates are not safe from the Dysdera spider, they also prey on silverfish, earwigs, millipedes, burying beetle, centipedes and crickets. Other spider species are also not safe, the woodlouse spider frequently dominates other spides with its large fangs and has been known to kill other spiders.
Not surprisingly, the courtship of these spiders is typically aggressive with the risk of injury to each other being high. Both sexes use their formidable jaws during a complex courtship display. After successfully mating, the female lays a cluster of up to 60 yellow eggs within a strong silk cell. The eggs hatch after 3-4 weeks which are looked after by the female. A month later the female will tear open the egg sac to release the tiny spiderlings. A spiderling will take around 18 months to become an adult and they can live another 2-3 years. Adults of both Woodlouse spider species are present throughout the year but the numbers of Dysdera crocata peak in May and June. Dysdera erythrina adults also show an early summer peak but have a second, larger, one in early autumn.
This formidable spider is truly the stuff of nightmares to all isopods!