Sand transport and burrow construction in sparassid and lycosid spiders

1. What they did

Many ground-living spiders dig burrows and line them with silk. Digging in moist sand is easy, but how does a spider carry away completely dry, loose sand, and how does a tube in dry sand stay open? The authors studied four burrowing spiders to find out.

• The spiders. Two live in the same dry dunes of Erg Chebbi, Morocco: a huntsman spider, Cebrennus rechenbergi, and a wolf spider, Evippomma rechenbergi. For comparison, they added two wolf spiders from the Americas: Allocosa senex from coastal dunes in Uruguay and Geolycosa missouriensis from Mississippi, USA.
• Watching and filming. They observed, photographed and filmed the spiders digging in the field and in the laboratory. For G. missouriensis, which they could not observe alive, they used earlier descriptions and video by other researchers.
• Microscopes. They examined the bristles on the spiders’ mouthparts, the burrow walls, and the sand left around burrow entrances under light and scanning electron microscopes.
• A sand experiment. To understand what keeps a burrow open, they pushed a foil-covered metal rod into desert sand, with and without a thin coat of glue on the foil, and pulled it out again.

2. What they found

Three ways to carry sand

• A sand basket (C. rechenbergi and A. senex). Long, curved bristles (about 2 mm) on the pedipalps overlap to form a fine mesh that holds the sand grains (0.3 to 0.6 mm). In C. rechenbergi, more bristles on the jaws and front of the head form an inner basket and a “roof”. The spider scoops up sand, carries it to the entrance and flicks it away; the ball of sand falls apart into single grains, so no heap is left. A. senex has a less developed basket and drops its sand close to the entrance.
• Sand tied with silk (E. rechenbergi). This spider has no basket. It dabs its spinnerets onto the loose sand, grabs the sand and silk bundle with its front legs and carries it up. Small, indistinct heaps of these bundles lie around the burrow entrance; only a few thin threads hold each bundle together.
• Solid pellets (G. missouriensis). In moister soil, this spider makes solid pellets of sand grains caked together with no silk, probably held by moisture and clay. It flings them 10 to 50 cm from the burrow with a rapid kick of the front legs, or silks them into the turret around the entrance. Hundreds of discarded pellets surround the burrow; an earlier study counted over 900 pellets for a single burrow.

Building the burrow (mostly C. rechenbergi)

• Digging happens at night and mostly out of sight. The spider lays a ring of silk at the top to stop sand trickling in, carries out 5 or 6 loads, adds another ring lower down, and repeats this for about two hours until the tube is 20 to 25 cm deep. The rings show as fine horizontal stripes on the finished wall.
• An average burrow (2 cm wide, 25 cm deep, about 80 ml) needs about 800 trips with a 0.1 ml ball of sand, all in under two hours.
• The finished tube is closed with a hinged lid of silk studded with one or two layers of sand grains, to keep out blowing sand and enemies such as hunting wasps.
• The C. rechenbergi tube runs straight down with an even width. The A. senex tube (about 20 cm long in a cast of a male’s burrow) is 1 cm wide at the entrance and almost 2 cm at the bottom; G. missouriensis tubes can also bulge at the bottom.
• A C. rechenbergi burrow lasts about one month before it is replaced, and is quickly repaired or rebuilt after damage such as a sandstorm. G. missouriensis is thought to stay in one burrow almost all its life.

What holds the tube up

• The silk lining is only a few thousandths of a millimetre thick. The burrow is most solid in C. rechenbergi, frailer in E. rechenbergi and frailest in A. senex, and even the C. rechenbergi tube collapses immediately when dug out.
• Long spigots on the spinnerets reach between the sand grains and tie neighbouring grains together.
• In the experiment, plain foil collapsed when the rod was removed, but foil coated with glue stood free as a cylinder even with the surrounding sand taken away.

3. What they concluded

• The burrow’s strength does not come from the silk tube itself but from the sand grains bound together around it, like the stones in a vault. The fine inner lining probably helps the spider climb up and down.
• Different spiders, even in the same habitat and the same family, have evolved different ways of carrying sand.
• To their knowledge, this is the first report of sand baskets (psammophores) in spiders; they had been known in desert ants and wasps, and evolved independently in spiders. They seem mainly suited to dry sand.
• Many other burrowing spiders have hardly been studied. It would be interesting to know whether soil moisture decides which method a spider uses, and whether one species can switch methods.

6. Other technologies, skills or innovations

• Scanning electron microscopy (SEM) and polarised light microscopy of bristles, silk and sand.
• Field and laboratory video of digging; terraria where spiders sometimes dig along the glass.
• A bee’s wax cast of an A. senex burrow (shown from Albín et al. 2015), and a simple model of a burrow made with an aluminium rod, household foil and wallpaper glue.