1991 | Namib Desert

Corolla spider (Ariadna sp.) stone circles

Despite our fragmented technological age, we still rely on physical metaphors for communication. We ‘make contact’ with someone, or ‘reach out’, even if that contact is by phone, or we’re reaching out through an email. However, animals typically don’t deal in metaphors, and that includes the subject of today’s post: a creature that treats ‘staying in touch’ very literally.

Meet the corolla spider. It’s part of a widespread arachnid family known as the Segestriidae, or tube-dwelling spiders, members of which live on every continent except Antarctica. If you’re not sure that you’re looking at one of the Segestriidae, just check out the legs—members of this family typically rest with three pairs of legs facing forward and only one facing backwards (versus the two-pairs-forward, two-pairs-backward seen in other spider groups).

Our particular corolla spider lives in the Namib Desert, a hyperacid coastal desert in southwest Africa. If you can picture wavy orange sand dunes, blowing winds, occasional scrub or lichen, and put all that next to the ocean: that’s the Namib.

The actual spider species involved hasn’t yet been fully described—new members of the Ariadna genus are found relatively often, given its worldwide distribution—but its behaviour has attracted scientific attention for the past three decades. In particular, our knowledge of these spiders relies on the dedicated work during that time of Giovanni Costa and Erminia Conti of the University of Catania in Sicily.

Stone age connections

Like many other tube-dwelling spiders, the corolla is an ambush hunter that operates out of a burrow. It digs 5-10cm into the sandy soil, lines the cylindrical hole with silk, and then crouches just inside, waiting for prey passing in the night. The quirk of having more forward-facing legs may in fact be an adaptation to this lifestyle, giving the spider a leg-up when it pounces on a nearby beetle or ant.

So far, this is fairly standard phobia-inducing behaviour. In order to assist prey detection, however, some Ariadna species also lay out a series of web strands surrounding the burrow, a kind of pressure-sensitive floor that alerts the spider to nearby movement (we can think of it as a low-tech version of the floor avoided by Tom Cruise in the first Mission Impossible film).

In the Namib, laying out a web system is a usually waste of time. The shifting sands and winds rapidly render it useless. So the corolla spider follows the lead of many groups of ancient humans who wanedt a permanent structure, and it builds a stone circle.

Specifically, the spider carefully selects quartz pebbles from the surrounding landscape (most often seven stones, with 90% of burrows having 6-9 stones). On average each stone is more than twice the spider’s own weight. It then rolls and pushes these stones into a circle around the burrow entrance, forming the distinct pattern you can see in the image at the top of this post. The spider is so reliable in choosing similar stones that fit closely around the burrow that Costa nicknamed it the ‘mathematical’ spider.

When Costa first encountered these stone circles, in 1991, he wasn’t sure what purpose they had. Were they windbreaks, protecting the burrow from encroaching sand? Did they add some structural strength to the fragile burrow edge? Did they hide the spider somehow, or help regulate burrow temperature and humidity? The value of the circles was clear from the commitment a spider showed: broken circles were quickly repaired with a hard night’s work.

Costa and his colleagues tested some of these theories, building little artificial barriers and catching sand in artificial burrows. But the windbreak hypothesis didn’t hold up—the same amount of sand got blown into the artificial burrows with or without a protective ring.

Remote sensing

One remaining hypothesis was more promising. With the circle in place, the corolla spider attaches individual threads of silk to each stone. It then crouches at the entrance, with those six forward legs resting on the strands. In effect, the stones could expand the radius of prey detection, while solving the problem of silk threads scattering in the wind.

Further testing by ecologist Jon Henschel supports that idea. The spiders overwhelmingly use quartz grains for their circles, at a rate far higher than that material is found naturally in the landscape. Almost two thirds of the stone circles were made purely of quartz.

Quartz is unusually good at transmitting vibrations, and it also produces a small electrical current when squeezed. Either of those features could help a spider detect the faintest of touches from a passing ant, or even help distinguish prey movement from contact with random wind-blown dust. The spiders are much more sensitive to touches on the stones than on the nearby ground, as Henchel found when he used a human hair to mimic ant footsteps around a burrow.

Corolla spider investment in home improvements make sense—they can spend several years in the same burrow, extracting what they need from the tiny patch of surrounding land. As they grow from spiderlings to adults, they slowly enlarge the burrow width at a rate of around a millimetre per year.

It’s still an open question just what the spiders sense when a beetle or ant brushes by their little fortress. The intriguing notion of a tiny electrical current generated by the quartz remains a possibility, although one that is diminished by the fact that more recent work found corolla spiders in the same area using lichen along with stones in their circle.

The decision-making, construction and maintenance behaviour of these spiders is similar in many respects to that of nest-making species such as birds or great apes. However, the fact that the arachnids use moveable objects and remain in (extended) contact with the stones qualifies this as true stone tool use. Given the emphasis some archaeologists place on stone tools in the evolution of human hunting, it’s a humbling reminder of nature’s creativity that there’s also a tiny spider in southern Africa, planning and executing complex hunts using stone tools.

Further viewing: for the curious, here’s a—somewhat dramatic—video of corolla spider stone circle hunting, courtesy of Animal Planet:

Sources: Costa, G. et al. (1993) Seven stone spiders on the gravel plains of the Namib Desert. Bollettino dell'Accademia Gioenia di Scienze Naturali 26: 77-83. || Costa, G. et al. (1995) A 'mathematical' spider living on gravel plains of the Namib Desert. Journal of Arid Environments 29:485-494 || Henschel, J. (1995) Tool use by spiders: stone selection and placement by corolla spiders Ariadna (Segestriidae) of the Namib Desert. Ethology 101: 187–199. || Conti, E. et al. (2019) How soil granulometry, temperature, and water predict genetic differentiation in Namibian spiders (Ariadna: Segestriidae) and explain their behavior. Ecology and Evolution 9: 4382–4391. || Conti, E. et al. (2018) Ariadna spiders as bioindicator of heavy elements contamination in the Central Namib Desert. Ecological Indicators 95: 663–672. || Costa, G. et al. (2013) Opening and closing of burrows by the Namibian spider Ariadna sp. (Araneae: Segestriidae) in a year of heavy rainfall. The Journal of Arachnology 41: 215–218.

Main image credit: Conti et al. (2019) Ecology and Evolution Fig. 2 || Spider image credit: Conti et al. (2018) Fig.2