Mirror Spider (Thwaitesia argentiopunctata)
Barely four millimetres long, the Australian mirror spider looks as though someone has glued fragments of a broken mirror to its back. The silver patches are crystals of guanine held under the skin, and they appear to grow and shrink as the spider's mood changes — one of the strangest optical tricks in the whole spider world, and one almost nobody notices because the animal is the size of a peppercorn.
By Dr. James WatuwaPublished June 16, 2019Updated September 13, 2026✓ Reviewed by Dr Richard J. Pearce (PhD)
In this guide
Physical description and identification
The mirror spider is unmistakable once you have found one, which is the hard part. It is tiny, and the reflective patches on the abdomen do exactly what mirrors do — they show whatever is around the spider, so against a sunlit leaf the animal reads as a scatter of light rather than as a body.
Adults
- Size: females are about 3–4 mm in body length and males slightly smaller at around 3 mm. Including the legs the whole spider still sits comfortably on a fingernail.
- Colour: the carapace and the long, fine legs are translucent amber to yellow-brown, sometimes with dark leg joints. The abdomen is the show: a base of cream, yellow, pale green, orange or red overlaid with irregular plates of silver or pale gold, each outlined in dark pigment so the surface looks like cracked mirror glass or a scatter of sequins.
- Body shape: the abdomen is high, rounded and taller than it is long, ending in two low humps at the rear — the origin of the name "twin-peaked Thwaitesia".
- Eyes: eight small eyes in two rows of four, as in all cobweb spiders, with no enlarged pair. See our guide to spider eyes for how the arrangement differs between families.
How the mirrors work
The silver is not pigment and not a coating. It is guanine, the nitrogen-rich waste product spiders normally excrete, stored instead as flat crystals inside specialised cells just beneath the transparent cuticle. Stacked crystal layers reflect light the way fish scales do. What makes this species remarkable is that the patches are not fixed: when the spider is calm the reflective areas are broad and nearly continuous, and when it is disturbed they contract to small separate dots, the surrounding pigmented tissue apparently expanding over them. The change takes seconds and is easy to trigger simply by moving a camera closer.
Eggs and spiderlings
Reproduction in this species is poorly documented — a reminder that a spider can be famous in photographs and still barely studied. Like other members of the family it builds a small silk egg sac and keeps it in the web, and the spiderlings are miniature versions of the adults that gain their reflective plates as they grow. They disperse early in life, as young juveniles rather than on reaching maturity, which is the normal pattern for cobweb spiders.
The web
The web is a small, flimsy, irregular tangle of fine threads slung under a leaf or between twigs in shaded foliage, quite unlike the engineered wheel of an orb weaver. The spider hangs in it upside down and waits. It is easily missed even when the spider itself is spotted, because a few dozen threads in deep shade are almost invisible; see our guide to spider webs for how tangle webs catch prey without a symmetrical frame.
Where the mirror spider lives
Thwaitesia argentiopunctata is endemic to Australia and has been recorded widely across the mainland states and in Tasmania. It is a spider of vegetation rather than buildings: wet and dry sclerophyll forest, rainforest edges, woodland, coastal scrub and shaded suburban gardens, where it works the undersides of leaves and the interiors of shrubs.
The wider genus Thwaitesia is pantropical, with species in Asia, Africa, Madagascar, the Pacific islands and tropical America, and several of them share the same reflective abdomen. The Australian species is simply the best photographed. Because it never enters houses and is barely visible at arm's length, most people who live inside its range have never knowingly seen one.
Are mirror spiders venomous?
Like almost every spider, the mirror spider has venom glands and uses them on small insects. It is of no consequence to people. The venom has no known medically significant components, the fangs of a 3 mm spider are too short and fine to penetrate human skin in any meaningful way, and there are no documented cases of medical harm. It is not related to the widow spiders of the genus Latrodectus despite sharing the family Theridiidae with them — the family contains both the most dangerous cobweb spiders and a great many completely harmless ones like this.
Can mirror spiders bite?
In practice, no. A mirror spider's defence is to drop out of the web on a silk line or to shrink its reflective patches and sit motionless; biting a creature hundreds of thousands of times its size is not part of its behaviour, and the fangs are unlikely to break skin even if it tried. No bite requiring treatment has been recorded.
If a tiny spider does nip while being handled, the sensible response is the same as for any minor skin break: wash with soap and water, apply a cold pack if it stings, and watch the site for the unusual swelling or spreading redness that would suggest an allergic reaction or infection rather than venom. The better approach is not to handle it at all — at this size the spider is far more likely to be injured by the encounter than you are.
Size is the reassurance: any spider small enough to sit inside the letter O on this page cannot deliver a medically meaningful bite. The dangerous members of this family, such as the widows, are ten to twenty times heavier and have fangs to match.
Ecological importance and behavior of the mirror spider
The mirror spider is a small sit-and-wait predator of the foliage layer, taking flies, midges, tiny beetles, aphids, leafhoppers and other insects that blunder into its threads. At this scale a spider is an important check on exactly the insects too small for larger predators to bother with, and a shrub may support several of them in different leaf clusters.
What the mirrors are for: the leading explanation is camouflage by reflection. A broken field of mirrors takes on the colours of the surrounding leaves and sky and destroys the outline of a body, which matters enormously to an animal hunted by birds that search visually for small dark shapes. The ability to shrink the patches when threatened suggests a second layer: a startled spider that suddenly stops glinting is harder to relocate once a predator has looked away. The idea that the spider produces a "dazzling display" to frighten predators is the opposite of what the change actually does.
Natural predators: insectivorous birds, mud-dauber and spider-hunting wasps, mantises, assassin bugs and larger spiders. Parasitoid wasps that specialise in web-building spiders are a significant threat to small theridiids generally.
Seasonal behaviour: adults are most often found in the warm months, and in the tropical north the species can be active year round. Lifespan is probably under a year, as in most small cobweb spiders.
Relationship with humans: entirely benign. The mirror spider does not enter buildings, does not bite, and eats garden pests. It is one of the species most often photographed by Australian macro photographers, which has done more for public interest in small spiders than any amount of pest-control literature.
Similar spiders and how to tell them apart
| Spider | How it differs from the mirror spider |
|---|---|
| Other Thwaitesia species | Several tropical relatives share the silver plates and are separated only on adult genitalia and geography. Any mirror-backed Thwaitesia outside Australia is one of these, not T. argentiopunctata. |
| Happy-face spider (Theridion grallator) | A Hawaiian cobweb spider of similar size with a translucent yellow body, but patterned in red and black cartoon-like markings rather than reflective plates. |
| Theridion species | Small tangle-web spiders with a similar build and comparable size, but matt and patterned, never mirrored. |
| Common house spider (Parasteatoda tepidariorum) | Much larger at 5–8 mm, dull grey-brown and mottled, and lives on buildings rather than foliage. |
| Silver orb weavers (Leucauge, Argiope) | Also carry guanine silver, but they are far bigger and build large, symmetrical orb webs. Their silver is a fixed sheen, not shifting plates. |
Quick facts
| Other names | Sequined spider, twin-peaked Thwaitesia |
|---|---|
| Distribution | Endemic to Australia, mainland states and Tasmania |
| Habitat | Undersides of leaves and the interiors of shrubs in forest, woodland, scrub and gardens |
| Diet | Flies, midges, aphids, leafhoppers and other very small insects |
| Lifespan | Probably under a year, as in most small cobweb spiders |
| Predators | Insectivorous birds, wasps, mantises, assassin bugs and larger spiders |
| IUCN status | Not evaluated |
Did you know?
- The species name argentiopunctata means "silver-spotted", and the genus honours George Henry Kendrick Thwaites, the nineteenth-century botanist who directed the Peradeniya botanic gardens in Ceylon.
- Guanine is one of the four bases of DNA. Spiders excrete their nitrogen in that form, and several groups have turned the waste into decoration by storing the crystals under the skin instead of discarding them.
- The species was described in 1916 by William Joseph Rainbow, arachnologist at the Australian Museum in Sydney and one of the most prolific describers of Australian spiders.
In summary, the mirror spider is a harmless four-millimetre cobweb spider whose reflective guanine plates make it one of the most photographed and least studied spiders in Australia. The mirrors are camouflage rather than display, they shrink rather than flash when the animal is alarmed, and the spider behind them is no threat to anyone.
Frequently asked questions about the mirror spider
Why does the mirror spider look shiny?
The silver patches are flat crystals of guanine stored in cells just under the transparent cuticle. Stacked crystal layers reflect light the way fish scales do, so the spider takes on the colours around it. The reflective areas are outlined in dark pigment, which gives the cracked-mirror appearance.
How big is a mirror spider?
Very small. Adult females measure about 3 to 4 mm in body length and males around 3 mm, so the whole spider including its legs fits easily on a fingernail. That size is the main reason people rarely notice them even in gardens where they are common.
Do the mirrors really change size?
Yes. On a calm spider the silver areas are broad and nearly continuous; when the spider is disturbed they contract into small separate dots as the surrounding pigmented tissue expands over them. The change takes only seconds and can be triggered by moving a camera close to the animal.
Are mirror spiders dangerous to humans?
No. The venom has no medically significant components and the fangs of a 3 mm spider are too short and fine to deliver a meaningful bite through human skin. There are no documented cases of harm. Despite sharing a family with the widow spiders, this species poses no risk at all.
What kind of web does a mirror spider build?
A small, flimsy, irregular tangle of fine threads slung under a leaf or between twigs in shaded foliage, with no symmetrical structure. The spider hangs upside down inside it and waits for small insects to blunder in. The web is so sparse it is usually invisible until the light catches it.
What do mirror spiders eat?
Flies, midges, aphids, leafhoppers, tiny beetles and other very small insects that become caught in the web. At this scale they are a useful check on exactly the insects that larger predators ignore, and a single shrub may support several spiders in different leaf clusters.
Where do mirror spiders live?
They are endemic to Australia and recorded across the mainland states and Tasmania, in wet and dry forest, rainforest edges, woodland, coastal scrub and shaded gardens. They live on vegetation, not buildings, and are found on the undersides of leaves and inside dense shrubs.
Are the mirrors for camouflage or for display?
Almost certainly camouflage. A broken field of reflective plates takes on the colours of the surrounding leaves and sky and destroys the outline of the body, which matters to an animal hunted by birds. Shrinking the patches when alarmed makes the spider harder to relocate, which is the opposite of a display.
Are other spiders silver in the same way?
Several are. Orb weavers such as Leucauge and Argiope also store guanine crystals and look metallic, but their silver is a fixed sheen rather than shifting plates, and they are far larger and build symmetrical orb webs. Within the mirror spider's own genus, several tropical species share the effect.
How can I find a mirror spider?
Search the undersides of leaves and the shaded interiors of shrubs in Australian forest or garden vegetation, ideally with a torch or a macro lens. Look for a glint rather than a spider shape. Because they are only a few millimetres across and sit in sparse webs, patience matters more than location.
References
- World Spider Catalog (2026). Thwaitesia argentiopunctata (Rainbow, 1916). Natural History Museum Bern. wsc.nmbe.ch/spec-data/46857/taxon
