Triangular Spider (Arkys lancearius)

Physical description and identification

The triangular spider is one of the most instantly recognisable small spiders in Australia. Everything about it is built around a single hunting method: no web, no chase, just a patient wait and a fast grab. It belongs to Arkyidae, a small family of araneoid spiders that was long buried inside the orb weavers and only re-established as a family of its own after molecular work in 2017 showed that these spiders had abandoned web-building entirely.

Adults

• Size: females reach about 8 mm (0.31 in) in body length, males about 5.5 mm (0.22 in). This is a spider smaller than a fingernail, and most people walk past dozens without seeing one.
• Shape: the abdomen is wide at the front and tapers to a point at the rear, producing the triangular or heart-shaped outline that gives the species its name. It is carried high and slightly forward over the cephalothorax, so from above the spider looks almost all abdomen.
• Colour: highly variable — yellow, apricot, orange or deep red, patterned with black and white marks that catch the light like enamel. Some individuals carry a pale central band, others a scatter of dark spots. Colour alone will not identify a specimen to species; the shape and the legs will.
• Legs: the diagnostic feature. The first two pairs are markedly longer and thicker than the rest, curved forward and armed with rows of stout spines that close against each other like the arms of a mantis. The third and fourth pairs are short and slender and are used only for holding on.
• Eyes: eight small eyes in two rows on a low head. Vision is modest; prey detection relies heavily on leaf vibration and on air movement picked up by leg hairs.

Eggs

The egg sac is pinkish-cream, roughly 8 mm across and wrapped in loose pale threads that are attached to a leaf or a stem near the female's hunting perch. A single sac holds in the region of 70 eggs, each about 0.7 mm in diameter.

Spiderlings

Spiderlings hatch in early summer already showing the triangular outline, which becomes proportionally longer and more sharply pointed with each moult. They are pale at first, and the black and white markings intensify as they mature. See our overview of the spider life cycle for how this timetable compares with web-building relatives.

Hunting instead of a web

The triangular spider spins no capture web. Older accounts describe it as waiting "from a hidden den", which is misleading: it does not build or occupy a den at all. It perches in plain sight on the upper surface of a leaf, on a fern frond, along a stem or on a flower head, with the spiny forelegs held open and the body pressed low. When a fly, small moth or other insect lands within reach, the forelegs snap shut around it and the fangs go in immediately. Silk is used only for the dragline, the egg sac and the moulting platform. It is a pure sit-and-wait predator in the same functional guild as the crab spiders, arrived at from a completely different ancestry.

Where the triangular spider lives

Arkys lancearius is Australian. It is recorded most often from the eastern seaboard — Queensland, including the Brisbane region, and New South Wales — and extends south and west into Victoria and South Australia where suitable vegetation exists. The genus Arkys as a whole reaches beyond the continent into New Guinea and nearby islands, but this species is best treated as an Australian endemic. Our regional guide to spiders in Australia places it among the harmless natives that share gardens with far more notorious species.

The habitat is open, sunlit foliage rather than dark cover. Typical sites are eucalypt woodland and dry sclerophyll forest, the margins of rainforest, heath, tussock grass and — regularly — suburban gardens, where the spider turns up on shrubs, low hedging, ferns and the leaves of ornamental plants.

Are triangular spiders venomous?

Yes, in the way almost every spider is: Arkys lancearius has venom glands and uses them to kill the flies and small insects it catches. The venom is tuned to insect physiology and is not medically significant to humans. No antivenom exists for any arkyid, none has ever been required, and there are no records of serious reactions, tissue damage or systemic illness from a bite by this family.

The risk is as much mechanical as chemical: a spider with an 8 mm body has to be pressed against skin to bite at all. Australians who worry about spiders in the garden are thinking of the Australian funnel-web spiders and the redback — genuinely medically significant animals that look nothing like this one.

Can triangular spiders bite?

They can, but it is an uncommon event and takes deliberate handling or accidental pressure to provoke. A disturbed triangular spider normally drops from its perch on a dragline, or simply tucks its legs in and relies on looking like a bud or a piece of debris.

Where a bite does occur, the effect is minor and local: a brief prick or pinch, possibly a small red mark, sometimes a little itching, resolving within hours. No spreading pain, no swelling of consequence and no after-effects have been described. Wash the site with soap and water and apply a cold pack if it is uncomfortable. Medical attention is warranted only in the general circumstances that apply to any small wound — signs of infection developing over the following days, or a known history of severe allergic reaction to insect and spider bites.

Ecological importance and behavior of the triangular spider

The triangular spider occupies a narrow and useful niche: it removes small flying insects from foliage at the exact height where they are densest. Flies make up the bulk of its recorded diet, along with small moths, leafhoppers, parasitic wasps, midges and the occasional ant. Prey is seized with the spined forelegs, held clear of the body and bitten, and eaten in place.

Natural predators: small insectivorous birds such as thornbills, fairy-wrens and gerygones glean spiders of this size from foliage; mud-dauber and spider-hunting wasps stock their nests with them; praying mantises, assassin bugs and larger spiders including jumping spiders take them opportunistically. Spiderlings are additionally vulnerable to ants and to parasitoid wasps that attack egg sacs.

Prey–predator dynamics: the spider's colour is not an accident. The bright yellows, oranges and reds combined with an angular, lumpy outline read from a bird's distance as a flower bud, a leaf gall or a scrap of dropped plant material — the same strategy taken further by the bird-dropping spider. The disguise works in both directions, concealing the spider from birds and from the flies it is waiting for.

Seasonal behaviour: adults are most often seen from spring through autumn, with egg sacs produced through the warmer months and spiderlings appearing in early summer. Individuals live roughly one to two years. Mature males leave their perches to search for females and approach carefully, tapping on the leaf surface.

Relationship with humans: entirely benign and quietly beneficial. A triangular spider in a garden is a free fly trap; the best thing anyone can do for it is to leave shrub and fern foliage unsprayed.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was named in 1837 by Charles Athanase Walckenaer, the French naturalist and civil servant who also erected the genus Arkys. The epithet lancearius refers to a lance or spear, a nod to the sharply pointed abdomen.
• Arkyids were classified as odd, web-less orb weavers for well over a century. A 2017 molecular study of araneoid relationships separated them out again, confirming Arkyidae as a family in its own right and making them one of the clearest cases of orb-web abandonment in the whole order.
• The claim that these spiders "cannot see well because their eyes are small" is roughly right for the wrong reason. Arkyids do rely far more on touch and vibration than on sight, but plenty of small-eyed spiders hunt well; what matters is that this lineage invests in leg mechanoreceptors instead of in eyes.

In summary, the triangular spider is a small, brightly coloured Australian ambusher that traded its ancestors' orb web for a pair of spined forelegs and a good disguise. It is harmless to people, beneficial in a garden, and one of the few spiders that can be identified from a photograph on outline alone.