Ordgarius

Physical description and identification

Ordgarius is a genus of stout, knobbly spiders in the orb-weaver family Araneidae, erected by the German arachnologist Eugen von Keyserling in 1886. Somewhere between a dozen and fifteen species are currently recognised. They belong to the araneid lineage of moth specialists — the bolas spiders — alongside Mastophora in the Americas and Cladomelea in Africa, and like their relatives they build no capture web of any kind as adults.

Two corrections are worth making at the outset. Older accounts claim only one or two members of the genus hunt with a bolas; as far as anyone has observed, the swinging droplet is the normal adult method throughout Ordgarius. They also claim the genus has no distinguishing features, when the hard, warty, hump-backed abdomen and very short, thick front legs identify it at a glance. The best known species is the magnificent spider (Ordgarius magnificus) of eastern Australia; O. hobsoni, O. monstrosus and O. sexspinosus are the Asian species most often photographed.

Adults

• Size: adult females of most species measure 8–15 mm in body length, with O. magnificus at the large end. Males are dwarfs of roughly 2 mm — about 0.08 in, not the 0.07 in that a slipped decimal point has spread across the internet.
• Colour: variable by species and almost always cryptic. O. magnificus is creamy white flushed with yellow and pink; O. hobsoni is brown and grey; others are mottled buff, olive or chalky. The common theme is a broken, glistening surface that reads as bird droppings rather than as an animal.
• Eyes: eight small eyes in two rows, the lateral pairs set well apart on raised mounds. None is enlarged, which rules out the big-eyed hunters of the same foliage.
• Distinctive features: a heavily sclerotised abdomen carrying paired tubercles, horns or spines that differ between species and are used in identification, on a broad lumpy carapace with short, powerful legs.

Eggs and egg sacs

Females wrap their eggs in tough, brown, spindle- or pear-shaped sacs hung from silk stalks among leaves and twigs, often several together in a cluster. The leathery wall protects the brood from parasitic wasps and from the weather. A female may build a series of sacs in a season, each holding hundreds of eggs.

Spiderlings

Spiderlings emerge pale and translucent, darken over their first moults, and disperse by ballooning. In the closely studied American genus Mastophora, juveniles and the tiny males are far too small to manage a bolas and catch small flies on short sticky threads instead; the same two-stage life is assumed for Ordgarius, whose juveniles have never been seen swinging a bolas.

How they hunt

Shortly after dusk the female spins a short horizontal trapeze line between twigs, hangs beneath it, and produces a single vertical thread tipped with a droplet of exceptionally sticky, slow-drying glue — the bolas. She then releases volatile compounds that imitate the sex pheromone of particular female moths. Male moths follow the scent plume upwind; when one comes within reach she swings the bolas at it in a short arc, the droplet grips a scaly wing or the body, and she hauls the line in. A fresh bolas is spun for each hunting bout. This chemical trickery is why bolas spiders catch almost nothing but male moths, and why they need no orb — a striking departure from the ordinary spider webs of their family.

Where Ordgarius spiders live

The genus has a broad but patchy Indo-Australian distribution. In Australia it is concentrated along the eastern seaboard, in Queensland and New South Wales, with records reaching into Victoria. Asian species occur in India, Sri Lanka, Myanmar, Vietnam, southern China, Japan and Indonesia, and the genus also reaches New Guinea. No Ordgarius is native to the Americas, Africa or Europe; the bolas spiders of those regions belong to other genera.

Habitat is consistent wherever they occur: wooded country with a dense shrub layer — open forest, rainforest margins, heath, regrowth scrub and the shrubby edges of gardens. The spiders keep to foliage a metre or two above the ground, and a hunting female needs an open pocket of air beneath her perch. Because they are small, nocturnal and dressed as rubbish, they are almost certainly more common than the sparse records suggest.

Are Ordgarius spiders venomous?

Yes, in the ordinary sense that nearly all spiders are venomous: Ordgarius uses venom to subdue the moths it catches. No species in the genus is medically significant to people or pets. The venom is tuned to insects, no antivenom exists or is needed, and no serious envenomation has been documented. These are not relatives of the Australian funnel-webs or the widow spiders.

Can Ordgarius spiders bite?

In principle yes, but a bite is close to a non-event. An adult female is small, slow, has short fangs, and responds to disturbance by drawing her legs in and dropping on a dragline rather than defending herself. Recorded bites are essentially absent, and the few anecdotal reports from bolas spiders amount to a brief pinprick and a little redness that fades within hours. Wash the site with soap and water, apply a cold pack if it is sore, and seek medical advice for any bite that swells dramatically or spreads.

Ecological importance and behavior of Ordgarius spiders

Ordgarius spiders are specialist predators of night-flying moths, and their diet is unusually narrow for an orb weaver. Because the lure imitates particular moth pheromones, an individual spider effectively targets a small set of noctuid and related species, most of them male. Several of the moths taken by bolas spiders elsewhere are crop and garden pests, so a population in an orchard edge does quiet, highly selective pest control. One correction to the older literature: these spiders take very few butterflies, which fly by day and are not drawn to a moth sex pheromone.

Natural predators: birds are the main threat and the main reason for the disguise, since a foraging bird that reads the spider as excrement passes it by. Spider-hunting wasps, parasitic wasps that target egg sacs, ants, lizards and larger spiders take their share, and tiny wandering males are the most vulnerable stage.

Prey–predator dynamics: the bolas strategy is a textbook case of aggressive chemical mimicry, and a two-way arms race. Because the spider exploits the moths' own communication channel, selection acts on moth pheromone blends, which in turn favours spiders that can adjust the blend they emit.

Seasonal behaviour: adults are found mainly in the warmer months, with egg sacs appearing from late summer and spiderlings dispersing in spring. Most individuals live about a year, growing through repeated moults.

Relationship with humans: entirely benign. Ordgarius neither enters houses nor builds a web across a doorway. If you find one, leave it in place and look again after dark, when the bolas comes out.

Species in the genus and similar spiders

Quick facts

Did you know?

• Eugen von Keyserling erected Ordgarius in 1886, working on Australian material sent to European museums.
• The glue on a bolas droplet stays workable far longer than ordinary orb-web gum, letting a spider hold one ready for an hour or more.
• A hunting female does not throw the bolas like a lasso. She swings it in a short arc timed to the moth's approach, and most nights end with only one or two captures.
• Because the lure is chemical rather than visual, a bolas spider catches the same moths a pheromone trap in that hedge would, which is how entomologists worked out what they were doing.

In summary, Ordgarius is an orb weaver that threw the orb away. A dozen or so small, warty, bird-dropping-coloured species across Australia and Asia hunt by counterfeiting a moth's own mating signal and swinging a single drop of glue at whatever answers it — and they are completely harmless to people.