Micrathena

Physical description and identification

Micrathena is a genus of spiny orb weavers in the family Araneidae, erected by the Swedish zoologist Carl Jakob Sundevall in 1833 and now holding around 120 described species. The name joins the Greek mikros, small, to Athena, the goddess of weaving and warfare — an apt pairing for a tiny spider dressed in armour.

The genus is easy to recognise and hard to split. Every female has a hard, often glossy abdomen bearing paired spines, and the group as a whole shows some of the most extreme sexual dimorphism among orb weavers: the male is a small, soft, spineless animal that looks so unlike his mate that the two sexes of several species were originally described separately.

Adults

• Size: females of most species measure 8–10 mm in body length; males only 4–5 mm. Some tropical species are smaller again, and none is large.
• Colour: highly variable between species and the main clue to which one you have. The spined micrathena is black and white, the arrow-shaped micrathena yellow with a red-brown head and legs, and M. mitrata is glossy white. Tropical species add reds, oranges, metallic sheens and near-black forms.
• Spines: the genus character. Females carry between two and ten conical spines in pairs around the rear and sides of the abdomen; their number, length and direction are the quickest way to tell one species from another. The male has low bumps at most.
• Other features: eight small eyes in two rows, none enlarged; short legs held close to the body; a stiff, seed-like feel to the abdomen, which is fused and much harder than that of a typical orb weaver.

Eggs and egg sacs

Females lay small, round, pale eggs in a loose silk sac fastened to a leaf or to vegetation at the edge of the web rather than in the orb itself. In temperate species the sac overwinters and hatches in spring; in the tropics breeding continues through the year.

Spiderlings

Hatchlings have longer, narrower abdomens than adults and only rudimentary spines. The armoured outline develops progressively with each moult, so juveniles are often difficult to identify to species.

The web

All Micrathena species build the same kind of web: a small vertical orb, roughly 10–20 cm across in the familiar species, with an open hub and a capture spiral far finer and more closely spaced than the web's size would suggest. Most hang head-down in the hub through the day, and many species are diurnal, which is unusual in a family of night workers. Webs sit low in the forest understory, strung between saplings, shrubs and low branches, and are rebuilt frequently. For how an orb is constructed, see our guide to spider webs.

Where Micrathena spiders live

Micrathena is exclusively American. The centre of diversity is the Neotropics — Brazil, Peru, Ecuador, Colombia and Costa Rica together hold the great majority of species — and the genus extends north into Mexico and the United States and south to Argentina. Nothing in the genus occurs naturally outside the Americas.

Only three species reach the eastern United States: M. gracilis, M. sagittata and M. mitrata. They are common from southern Ontario and New England to Florida and west to the Great Plains, which is why the genus feels far more North American than it really is.

The habitat is consistent: humid forest and its edges. Micrathenas occupy the understory of deciduous, mixed and tropical woodland, along ravines, stream margins, clearings and trails, usually within two metres of the ground. They do not enter houses and are rarely found in open country far from trees.

Are Micrathena spiders venomous?

Yes, as almost all spiders are: Micrathena species use venom to subdue the small insects caught in their webs. No species in the genus is of medical significance to humans or pets anywhere in its range, no antivenom exists because none is needed, and there is no record of a serious reaction. The genus is unrelated to the widow spiders and to recluses and shares none of their chemistry. Size also limits what these spiders could do: the fangs are short and fine, and frequently cannot pierce human skin at all.

Can Micrathena spiders bite?

They can, but only defensively and only when pressed against skin — trapped in a hand, or caught between clothing and body. Their first response to a disturbance is to drop out of the web on a dragline or to shake the orb into a blur, and a micrathena brushed onto a sleeve will run rather than turn.

The bite itself registers as a brief pinch, occasionally with a small pink spot that fades within an hour or two. There is no swelling, no lasting pain and no aftercare beyond soap and water. Any mark that spreads, blisters or worsens over several days points to a secondary infection or another cause and should be seen by a doctor.

Ecological importance and behavior of Micrathena spiders

Micrathenas are sit-and-wait predators whose fine-meshed orbs filter the small insect traffic of the forest understory. The spider hangs at the hub, detects a struggle through the radii within a fraction of a second, then runs down, bites and wraps the prey in silk. The diet is made up of gnats, midges, small flies, mosquitoes, leafhoppers, parasitic wasps and small moths. The claim, repeated in several older summaries, that micrathenas eat birds is false: a spider of 8–10 mm takes nothing larger than a small insect.

Natural predators: mud dauber and spider-hunting wasps are the specialists, paralysing orb weavers to provision their nests. Birds, mantises, jumping spiders and web-raiding spiders take the rest, and parasitoid wasps attack eggs and juveniles.

Prey–predator dynamics: the spines and the hard, fused cuticle are widely interpreted as anti-predator armour, making the spider awkward to grip and swallow; bright colouring in some species may reinforce the message. Because many micrathenas hunt by day and low down, they exploit a shift and a layer that most orb weavers, including relatives such as Neoscona, leave unworked.

Seasonal behaviour: temperate species are strictly annual — eggs overwinter, spiderlings appear in spring, adults mature from July and die after autumn laying. Tropical species can be found year-round. Males stop building webs when they mature and spend their last weeks searching for females, signalling from the edge of an orb by plucking the silk before approaching.

Relationship with humans: benign. Micrathenas do not come indoors, and the only regular contact is a face full of web on a woodland path. Clearing a web with a raised stick costs the spider a morning's work and nothing more.

Species in the genus and similar spiders

Quick facts

Did you know?

• The name is a compound of the Greek mikros, small, and Athena — the goddess of both weaving and warfare, which suits a miniature spider in armour.
• Carl Jakob Sundevall, the Swedish zoologist who erected the genus in 1833, is better remembered for his work on birds than on spiders.
• Sexual dimorphism here is extreme even by orb weaver standards: the male is roughly half the female's length, a fraction of her bulk, and has none of her spines.
• Micrathenas are among the few orb weavers that work in daylight and rebuild in the morning, rather than spinning at dusk.
• Only three of the roughly 120 species live in the United States. The genus is really a tropical one that happens to have three famous northern representatives.

In summary, Micrathena is a purely American genus of small, spiny, hard-shelled orb weavers — visually spectacular, ecologically useful and entirely harmless. If you have walked into a small, very tightly woven web in an eastern wood in September, you have already met one.