Thwaitesia

Physical description and identification

Thwaitesia is a small genus of comb-footed spiders in the family Theridiidae, erected by the English arachnologist Octavius Pickard-Cambridge in 1881 on material from Sri Lanka. Roughly twenty-two species are currently recognised, scattered through the tropics and warm subtropics of Asia, Africa, Australia and the Americas, with one species reaching North Africa. The genus is famous for a single feature: patches of brilliant, mirror-like silver on the upper surface of the abdomen, which is why the best known member, Thwaitesia argentiopunctata, is called the mirror or sequined spider.

One point worth correcting straight away is how that silver is produced. The plates are not metallic scales and they are not pigment. They are deposits of guanine — a nitrogen-rich waste product that many spiders store in reflective cells — held in a layer just beneath a thin, transparent cuticle. The same chemistry gives the silvery sheen to many orb weavers, but in Thwaitesia it is broken into discrete tiles rather than spread as a wash.

Adults

• Size: these are genuinely tiny spiders. In the species that have been measured, such as T. bracteata and T. affinis, females reach roughly 4.5 mm (0.17 in) in body length and males about 2.7 mm (0.10 in). Most of the genus sits between 2.5 and 5 mm, which places it among the smallest spiders people are ever likely to photograph.
• Colour: colour varies sharply between species. T. argentiopunctata combines a silver-plated abdomen with green, cream, red and yellow, while others are amber, straw-coloured or translucent olive with only a few silver flecks. The carapace is usually pale brown to orange and unmarked.
• Body shape: the abdomen is high, rounded at the front and often drawn up into a pair of low humps or a single blunt point at the rear, which is where the alternative name "twin-peaked spiders" comes from. The carapace is small in proportion to the abdomen, a normal theridiid build.
• Legs: long, fine and sparsely spined, with the first pair longest. Like all Theridiidae they carry a comb of curved, serrated bristles on the hind tarsus, used to fling bands of gluey silk over struggling prey.
• Eyes: eight small eyes in two rows of four, none enlarged. At this body size the eye pattern is only visible under magnification; see our guide to spider eyes for how the arrangement differs between families.

The mirror effect

Photographers regularly report that the silver tiles look larger in a calm spider and shrink to narrow slivers when the animal is alarmed or handled. The favoured explanation is mechanical rather than optical: the reflective layer sits in flexible tissue, and changes in the spider's blood pressure and abdominal volume spread or contract the plates while the surrounding pigmented tissue expands into the gaps. The function is still unproven, but the most likely benefit is disruptive camouflage — a broken mosaic of light on a wet, glossy leaf is far harder for a bird or a wasp to resolve as a spider than a solid silhouette.

Eggs

The female wraps her clutch in a single silken sac that she suspends in the web or fastens to the underside of a leaf. Counts of about thirty eggs per sac have been reported, which is modest even by cobweb-spider standards and consistent with the animal's small body.

Spiderlings

Very little has been published on the juveniles. As in other theridiids, the spiderlings moult once inside the sac, emerge as second-instar young, remain clustered near the mother's web for a short period and then scatter — sometimes on foot, sometimes on a thread of silk carried by the wind. The silver deposits build up gradually with successive moults, so young Thwaitesia are far duller than adults. Our page on the spider life cycle covers the stages in more detail.

The web

Thwaitesia spiders build small, irregular three-dimensional tangle webs — the classic cobweb architecture of the family — rather than flat, geometric orbs. Webs are typically slung under a broad leaf, between twigs or in the outer foliage of shrubs, often no larger than a few centimetres across, and the spider hangs upside down at the hub. Because the silk is fine and the spider is tiny, these webs are extremely easy to walk past without noticing. For the differences between tangle webs, sheet webs and orbs, see our overview of spider webs.

Where Thwaitesia spiders live

The genus is tropical and subtropical rather than strictly tropical. T. argentiopunctata is Australian and is photographed most often in the wet forests of the east coast; other species occur in Sri Lanka, India, Southeast Asia, Malaysia, New Guinea, the Pacific islands, tropical Africa including Kenya and South Africa, and Central and South America. T. algerica pushes the range into North Africa and the drier Mediterranean fringe, which is why describing the genus as purely tropical undersells it.

Habitat is consistent across that huge range: humid, shaded vegetation. Thwaitesia are foliage spiders of rainforest understorey, gallery forest, plantation edges, gardens and damp scrub, almost always on the underside of living leaves, where humidity is highest and the spider is hidden from above. They are not house spiders, they are not found in leaf litter, and they do not build the sheet webs of ground-level cobweb spiders. Populations are easiest to find after rain in the warm season.

Are Thwaitesia spiders venomous?

Yes, in the sense that applies to nearly every spider: Thwaitesia uses venom to subdue the small flies, gnats, aphids and other soft-bodied insects that stumble into its web. There is no evidence that the venom of any species in the genus is medically significant to people or pets, and no confirmed envenomation has been published.

Being in the same family as the widow spiders means nothing on its own. Theridiidae contains around 2,500 species, and only the genus Latrodectus — plus, to a far lesser degree, some Steatoda — produces venom that matters to humans. Thwaitesia is a leaf-dwelling micro-predator with no such chemistry and no capacity to deliver it.

Can Thwaitesia spiders bite?

In practice, no. An adult Thwaitesia has a body two to five millimetres long, and its fangs are a fraction of a millimetre — comparable to the thickness of a human hair. They are almost certainly incapable of penetrating human skin except perhaps on the thinnest, softest areas, and no verified bite has been documented. The spider's response to a hand is to drop from its web on a dragline and freeze, which is the standard escape behaviour of small theridiids.

If you handle one for a photograph, the risk you are running is to the spider, not to yourself: at this size they are easily crushed and they dehydrate quickly out of humid air. Let it walk onto a leaf rather than onto skin.

Ecological importance and behavior of Thwaitesia spiders

Thwaitesia are sit-and-wait predators of the leaf surface. They hang inverted in a small tangle of silk, detect the vibration of an insect blundering into a thread, and use the comb on the fourth leg to throw gluey silk over the prey before biting it. Their prey is correspondingly small: fungus gnats, midges, tiny flies, springtails, aphids and parasitic wasps. Individually insignificant, but in the dense understorey of a tropical forest a population of these spiders removes an enormous number of small insects from the leaf layer every night.

Natural predators: the main threats are insectivorous birds, lizards, mantises, ants, predatory bugs and — most dangerous of all for small web spiders — spider-hunting wasps and kleptoparasitic spiders that raid the web. The broken silver pattern is thought to help against visually hunting predators by destroying the spider's outline against a wet leaf.

Prey–predator dynamics: because Thwaitesia webs sit in the outer foliage, they intercept a different insect community from the ground-level tangles of Theridion or the aerial orbs of larger web builders. The genus occupies a narrow band of the forest — the underside of leaves in humid shade — and shares it with very few competitors.

Growth and seasonality: like all spiders they grow by moulting, shedding the entire cuticle including the transparent window over the guanine layer. In consistently warm and wet regions breeding is more or less continuous; in seasonal climates such as eastern Australia, adults peak in the warm, wet months and the population overwinters as juveniles. Lifespan is short — about a year at most.

Relationship with humans: entirely benign. These spiders cause no damage, enter no houses and bite no one. Their real significance to people is aesthetic and scientific: the mirror spider is one of the most photographed arachnids in the world, and interest in how its reflective layer works has drawn attention to guanine optics across the whole order.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The silver plates are a waste product. Spiders excrete nitrogen mainly as guanine, and storing it in reflective cells beneath the skin turns metabolic rubbish into camouflage.
• Within Theridiidae, Thwaitesia sits close to genera such as Spintharus and Episinus — small, oddly shaped foliage spiders that abandoned the ground-level cobweb lifestyle of their relatives.
• Despite being one of the most photographed spiders on the internet, the genus has almost no published behavioural research: web architecture, courtship and even the exact mechanism of the mirror effect are described mainly from field photographs.
• The genus name honours George Henry Kendrick Thwaites, the nineteenth-century naturalist whose collections in Ceylon, now Sri Lanka, produced the first described species.

In summary, Thwaitesia is a genus of harmless, millimetre-scale cobweb spiders that turned a metabolic waste product into one of the most striking optical displays in the animal kingdom. If you find one, admire it and photograph it — there is nothing here that can hurt you, and plenty that science still cannot explain.