Theridion

Physical description and identification

Theridion was named by the French naturalist Charles Walckenaer in 1805 and has since become one of the biggest genera in the order, with roughly six hundred described species on every continent except Antarctica. It sits in the family Theridiidae — the cobweb or comb-footed spiders — alongside the widows, Steatoda and the common house cobweb spiders. Because the genus has been used for two centuries as a catch-all for small theridiids, its boundaries keep shifting as molecular studies move species in and out.

Adults

• Size: Theridion are small. Most species have a body length of about 2–4 mm, and even the larger ones rarely exceed 6 mm. Claims that the genus runs to 10 mm confuse it with the far bulkier Steatoda and Latrodectus; a typical Theridion is smaller than a grain of rice, and the sexes are close in size, with females only slightly larger.
• Colour: extremely variable. Species may be cream, yellow, straw, orange, tan, olive, reddish brown or near-black, usually with a darker folium — a leaf-shaped central band — down the top of the abdomen, edged by pale zigzags or chevrons. Legs are often banded or ringed with darker pigment. Within a single species, colour can vary so much between individuals that markings alone are unreliable.
• Body shape: a small, low carapace and a large, almost spherical abdomen held high. Seen from the side the spider looks like a bead on thin legs.
• Legs: long, slender, weakly spined. The leg formula is the standard theridiid one — the first pair is longest, followed by the fourth, then the second, with the third shortest — in both sexes. Older accounts claiming that males differ by having the second pair next longest do not match the family pattern.
• The comb: the diagnostic family feature is a row of curved, serrated bristles on the tarsus of the fourth leg. The spider uses it like a small rake to fling sheets of gluey silk over prey from a safe distance, so it never has to grapple with a struggling insect.
• Eyes: eight eyes in two rows of four, all roughly equal and none enlarged, which separates Theridion at a glance from jumping spiders and wolf spiders.

Eggs

Females produce one or more rounded egg sacs of white, cream or pale bluish-green silk, usually a few millimetres across, and either hang them in the web or carry them clamped beneath the body. Unlike many spiders that lay and leave, most Theridion females stay with the sac and defend it until the young emerge.

Spiderlings

The young moult once inside the sac and emerge as second-instar spiderlings. In several well-studied European species — T. sisyphium and T. impressum in particular — the mother then feeds her brood by regurgitating liquefied food, and in some cases the spiderlings finish by consuming her body, an extreme form of maternal investment known as matriphagy. This is one of the clearest examples of parental care in spiders. After a period of shared feeding the young disperse and begin building their own webs, reaching maturity after several further moults.

The web

Theridion build irregular, three-dimensional tangle webs with no radial or spiral order. A typical web is a loose scaffold of dry silk with a few sticky gumfoot lines running down to a surface below; when a walking insect touches one, the line snaps free and hoists the prey off its feet. Many species add a retreat at the top — a curled leaf, a piece of plant debris or a small silk tent — where the spider sits out the day with the egg sac. T. sisyphium builds an especially neat bell-shaped retreat of chewed prey remains and debris. See our guide to spider webs for how this differs from the engineered orb of an orb weaver.

Where Theridion spiders live

The genus is genuinely cosmopolitan. Theridion species occur throughout Europe, Asia, Africa, North and South America and Australasia, and they are conspicuously successful on islands: the Hawaiian archipelago alone holds a radiation of endemic species including the celebrated happy face spider, Theridion grallator, found only in the native rainforests of Hawaii. Describing the genus as limited to parts of Africa, America and Asia badly understates it; common species are everywhere from British hedgerows to Canadian conifer forests.

Habitat is the outer layer of vegetation. Theridion webs are built in the tips of shrub and tree branches, in heather and gorse, among grass heads, under the rims of leaves, on fences, walls, window frames, garden furniture and the eaves of sheds. A handful of species have become synanthropic and are found mostly around buildings, but unlike the true house spiders the genus as a whole belongs outdoors. They tolerate a wide temperature range and are among the first spiders to recolonise cut or burned vegetation, arriving by ballooning on strands of silk.

Are Theridion spiders venomous?

Every Theridion has venom, and it works very well on flies, aphids, small moths and ants. None of it is medically significant to humans. No species in the genus appears on any national list of medically important spiders, no antivenom exists or is needed, and no verified case of serious envenomation has been recorded.

The confusion usually comes from family association. Theridiidae does contain the widow spiders, whose alpha-latrotoxin makes them genuinely dangerous, and the false widows of Steatoda, which can cause a painful but self-limiting local reaction. Theridion has neither the venom chemistry nor the body size to matter. Treat a Theridion the way you would treat a money spider: as a harmless garden predator.

Can Theridion spiders bite?

A Theridion is physically capable of biting, but at a body length of two to four millimetres the fangs are minute, and bites on humans are essentially unrecorded. If one did break the skin you would expect nothing more than a brief pinprick and perhaps a small pink mark that fades within hours. The spider's instinctive response to a large animal is to drop out of the web on a silk line or to shake the web violently, blurring its own outline.

General first aid applies to any spider bite you are unsure about: wash with soap and water, apply a cool pack for comfort, keep the area clean and watch for spreading redness, which points to infection rather than venom. Seek medical advice if symptoms go beyond the bite site, and take a photograph of the spider if you can — accurate identification is far more useful to a clinician than a guess.

Ecological importance and behavior of Theridion spiders

Theridion are ambush predators that never chase anything. The spider hangs inverted at the top of its tangle, reads the vibrations of the silk, and when an insect blunders in it approaches carefully and throws bands of gluey silk with the comb on its hind leg until the prey is wrapped and helpless. Only then does it bite. Prey is small and abundant: flies, midges, aphids, leafhoppers, small moths, ants and springtails.

Natural predators: insectivorous birds, lizards, ants, predatory bugs and mantises take adults, while spider-hunting wasps and ichneumonid parasitoids target them in the web. Egg sacs are heavily attacked by parasitoid wasps and by scavenging mites. Larger web spiders regularly kill and eat them, and kleptoparasites of the genus Argyrodes — also theridiids — steal prey from their webs.

Prey–predator dynamics: because Theridion are so numerous and so small, they sit at a busy junction of the food web, converting a huge volume of tiny flying and crawling insects into food for birds and wasps. In agricultural and forestry research they are frequently among the most abundant spiders sampled from tree canopies and hedgerows, where they contribute measurably to the natural suppression of aphids and other pests.

Social and maternal behaviour: the genus is unusually interesting for a group so small. Beyond regurgitation feeding and matriphagy, some species tolerate aggregations of webs in a single bush, and several tropical species show early stages of social tolerance between siblings. Courtship involves the male plucking rhythmic signals on the female's web before approaching; the male usually shares the web for a time after mating.

Seasonality and lifespan: most temperate species live about a year, overwintering as juveniles in leaf litter, bark crevices or evergreen foliage, maturing in late spring and producing egg sacs through the summer. In warm climates generations overlap and breeding is continuous.

Relationship with humans: wholly positive and almost entirely invisible. Theridion cause no damage, do not infest homes and quietly remove large numbers of nuisance insects from gardens and crops.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• Some Theridion mothers feed their spiderlings mouth to mouth by regurgitating liquefied prey, and in a few species the brood ends by eating the mother herself — a behaviour called matriphagy.
• The Hawaiian happy face spider, Theridion grallator, carries a genetically controlled colour polymorphism: individuals in a single population can be plain yellow or marked with a red-and-black pattern that looks startlingly like a grinning face.
• The comb of serrated bristles on the fourth leg — the feature that gives the family the name comb-footed spiders — lets a two-millimetre spider safely subdue an insect several times its own weight.
• Theridion has long been a taxonomic waiting room: many species first described here have since been moved to Parasteatoda, Platnickina, Cryptachaea and other genera, so older field guides and modern checklists often disagree.

In summary, Theridion is the quiet backbone of the cobweb spider family: hundreds of small, harmless, globally distributed species whose scruffy tangle webs clear an enormous quantity of small insects out of gardens, hedgerows and forests. They cannot hurt you, and the most interesting thing about them is not their venom but how well they look after their young.