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!
Appearance and Identification
The woodlouse spider is known for its impressive long, curved fangs which stand out against its relatively small size, giving 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 shiny exoskeleton and tubular abdomen.
Both males and females are generally similar in appearance, with the front pair of legs longer in adult males of both species. The two species can be differentiated by the short, stout spines on their legs:
- Dysdera crocata: males 9-10 mm, females 11-15 mm
- Dysdera erythrina: males 7-8 mm, females 9-10 mm
Most spider species have eight eyes, but the woodlouse spiders have only 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 found inside an oval white silk cell attached to surfaces under stones or wood. Within these silk cells, they moult and build their egg sacs.
Where They Live
Both species are found where woodlice are in abundance — such as under logs, rocks, and leaf litter. Typically, Dysdera crocata is often found in or near buildings, including in cellars and kitchens, as well as along coastal habitats, gardens, and waste grounds. 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 are quite widely distributed in Britain, but absent from much of Scotland and the uplands of Wales.
Dysdera crocata has also been introduced to Australia and the eastern United States through human activity.
Why Woodlice Specifically?
There are very few spider species that would make a meal of a woodlouse. Most spiders find them distasteful and often struggle to penetrate through the woodlouse's armoured plates. Such predator-prey interactions are rare. But woodlouse spiders are properly adapted to exploit thriving on woodlice as a food source.
Not only do they find woodlice palatable, but they are also able to hold them in a vice-like grip and pierce through the woodlice exoskeleton to the softer underside. A woodlouse defence mechanism of rolling into a tight, armoured ball is no deterrent or true defence to a woodlouse spider.
The Bite (If You Encounter One)
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 medically dangerous, it will be briefly painful and can cause localised inflammation.
Beyond Woodlice: Their Wider Diet
Other invertebrates are not safe from the Dysdera spider. They also prey on silverfish, earwigs, millipedes, burying beetles, centipedes, and crickets. While woodlice remain their primary food source, they will properly take advantage of other small arthropod prey when available.
Reproduction and Life Cycle
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 and 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 adults 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.
What to Do If You Find One in Your Enclosure
For isopod keepers, the practical considerations:
- Identify before acting — many small UK spiders are properly harmless to isopods. Make sure it's actually Dysdera before assuming the worst
- Catch and relocate — properly to a place far from your isopod enclosures. They have ecological value outside captivity
- Don't squash needlessly — they're properly part of British biodiversity
- Check enclosure entry points — if one got in, others might. Inspect ventilation, lids, gaps
- Inspect substrate sources — if you forage your own leaf litter or wood, freezing or baking before use kills any hitchhikers
- Inspect new substrate accessories — purchased cork bark, lotus pods, and similar can occasionally have egg sacs or small spiders
For broader context on enclosure predator prevention see our predators of isopods in captivity article. For the wild context see our wild predators article.
Sterilising Foraged Materials
For keepers who forage their own leaf litter, wood, or substrate components, properly sterilisation prevents bringing predators (including Dysdera) into your setups:
- Freezing: -18°C for at least 48-72 hours in sealed bags. Kills spiders, mites, and most invertebrate hitchhikers
- Baking: 90-110°C for 30-45 minutes. Properly more thorough
- Visual inspection: as additional precaution, especially for larger items like cork bark
For more on this see our leaves feeding article covering sterilisation methodology.
Their Ecological Role
While properly a threat to captive isopod colonies if they get in, Dysdera spiders are an interesting and ecologically valuable part of British wildlife. They occupy a specialist niche that few other predators fill — and their existence properly demonstrates how evolution produces specialists adapted to seemingly difficult prey.
For isopod keepers, Dysdera spiders are properly a reminder that proper enclosure design and sterilised foraged materials matter. For naturalists, they're properly a fascinating example of British biodiversity worth appreciating.
This formidable spider is truly the stuff of nightmares to all isopods!
For setup essentials browse our accessories collection. For current stock browse our isopods collection. For comprehensive feeding and habitat guidance see our leaves article and other useful articles.
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