Spined Micrathena (Micrathena gracilis)
If you hike an eastern woodland trail in late August you will meet this spider face first. The spined micrathena strings a tight, finely woven orb across gaps at head height, sits in the middle of it all day, and carries a hard, boxy abdomen studded with ten short black spines — a small spider that is impossible to confuse with anything else once you have seen one.
By Dr Richard J. Pearce (PhD)Published June 2, 2020Updated September 13, 2026✓ Reviewed by Dr. James Watuwa
In this guide
Physical description and identification
The spined micrathena is a small, hard-bodied orb weaver with an abdomen that looks more like a piece of armour than a spider. It is wider than it is long, squared off at the back, and ringed with short conical spines. The whole animal is stiff and glossy, and it stays in its web through the middle of the day — unusual for an orb weaver, and the reason walkers meet it so often.
Adults
- Size: adult females measure about 8–11 mm in body length; males are much smaller at roughly 4–5 mm. Older accounts give a combined range of 4.2 to 10.8 mm, which covers both sexes rather than either one.
- Colour: the female's abdomen is typically black and white — a white or cream ground broken by black spots and blotches, with the spine bases marked in black. Dark, almost wholly black females are common, and some individuals carry yellow along the flanks. The carapace is amber to dark brown and the legs are glossy black.
- Spines: the defining feature. Females bear ten short, blunt cones in five pairs around the rear and sides of the abdomen. Males have a flattened, elongated abdomen with the spines reduced to low bumps or missing altogether, and they are so much plainer that many people never realise they belong to the same species.
- Other features: eight small eyes in two rows, none enlarged; short legs held folded against the body; a hard, leathery cuticle that makes the spider feel like a seed rather than an insect if it lands on you.
Eggs and egg sac
The female leaves the web in autumn to lay. The eggs are small, round and pale, bound in a loose, fluffy sac of yellowish silk that she attaches to foliage or hides among dead leaves near the web site rather than in the open orb. The sac overwinters and the female dies soon after.
Spiderlings
The spiderlings emerge in spring, with elongated abdomens and only faint traces of the adult spines, and disperse to build tiny orbs in the understory. The armoured, boxy shape develops gradually with each moult, as described in our guide to spider molting, and the spiders reach maturity from mid-summer onward.
The web
The orb is small — typically 10–20 cm across — but woven far more finely than its size suggests, with a very closely spaced capture spiral and an open hub the spider sits in. It is strung vertically or at a slight tilt across gaps in the forest understory, and very often across a trail at between waist and head height.
Two habits set this species apart. First, it is diurnal and rebuilds in the morning, whereas most orb weavers take the web down at dusk and spin at night. Second, it does not stay put: the spider abandons a site and relocates roughly once a week, which is why a web that blocked a path on Monday is gone by the weekend. For the mechanics of orb construction, see our guide to spider webs.
Where the spined micrathena lives
Micrathena gracilis is an eastern North American spider. Records run from southern Ontario and New England south to Florida, and west to eastern Texas, Nebraska and the eastern Dakotas, with populations continuing south through Mexico into Central America. Older summaries that add Argentina to the list are almost certainly confusing this species with one of the many South American members of the genus; the range of M. gracilis itself is not known to reach there.
The habitat is deciduous and mixed woodland, especially oak and hickory forest, along with wooded edges, ravines and shaded gardens backing onto trees. It favours humid, sheltered spots and is most abundant near streams, ponds and damp hollows, where flying insects concentrate. Within a wood the webs are built in the understory, between saplings and low branches, and along the open corridor of a trail — which is exactly why hikers collect so many of them on the face.
Are spined micrathena spiders venomous?
Yes, in the way that nearly every spider is venomous: the spined micrathena injects venom to subdue the small flies and other insects it catches. That venom has no medical significance for humans or pets. There are no documented cases of serious reactions, no antivenom exists because none is needed, and the species is not related to the widow spiders or to recluses.
In practical terms the spider's size is as much of a limit as its chemistry. Its fangs are short and fine and frequently cannot penetrate human skin at all.
Can spined micrathena spiders bite?
They can, but it is genuinely rare. A spined micrathena that is disturbed drops out of the web on a dragline or shakes the orb rather than confronting whatever caused the vibration, and a spider brushed off a shirt will run for cover. Bites are recorded only when the animal is pinned against skin — squeezed in a hand, or caught between clothing and body.
The result is a momentary pinprick, sometimes with a small pink spot that fades within an hour or two. No swelling, no lasting pain, and no aftercare beyond soap and water. If any bite site instead spreads, blisters or worsens over several days, that indicates a secondary infection or a different animal and is worth a doctor's opinion.
Not to be confused with: the other familiar spiny orb weaver of the eastern United States, Gasteracantha cancriformis, is wider than long with six long lateral horns and a shell-like white abdomen, and it hangs its web in shrubs and citrus rather than across forest trails. Both are harmless.
Ecological importance and behavior of the spined micrathena
This is a daytime hunter in a family of night workers. The spider spends the day at the hub, legs on the radii, and responds to a struggling insect within a fraction of a second — running down, biting, and wrapping the prey in silk. The fine mesh of the orb suits the catch: small flies, gnats, mosquitoes, leafhoppers, parasitic wasps, small moths and other weak-flying insects. The frequently repeated claim that micrathenas eat birds is simply wrong; a spider of this size could not handle one, and no orb weaver of 10 mm takes vertebrate prey.
Natural predators: mud dauber wasps stock their mud cells with paralysed orb weavers and take micrathenas readily; spider-hunting wasps, birds, praying mantises and jumping spiders take the rest. The abdominal spines and the hard cuticle are thought to make the spider awkward to swallow, and experiments on spiny orb weavers generally support the idea that armour discourages small birds.
Prey–predator dynamics: because it hunts by day in the woodland understory, the spined micrathena occupies a layer and a shift that most local orb weavers leave empty, overlapping little with nocturnal relatives such as spotted orb weavers.
Seasonal behaviour: the life cycle is annual. Spiderlings appear in spring, mature from July, and adults are most obvious from mid-August to early October. Males abandon web-building on maturing and wander in search of females, approaching the edge of an orb and signalling by plucking the silk before crossing.
Relationship with humans: entirely benign, if occasionally irritating on a trail. Walking with a stick held up in front of you clears webs without harming the spider, and any web taken down is rebuilt elsewhere within a day.
Similar spiders and how to tell them apart
| Spider | How it differs from the spined micrathena |
|---|---|
| Arrow-shaped micrathena (Micrathena sagittata) | Same genus and often the same wood. Bright yellow, triangular abdomen with a red-brown head and legs, and only three pairs of spines, the rear pair long and flaring. Never black and white. |
| Other Micrathena species | Around 120 species, overwhelmingly tropical. M. mitrata, the third species in the eastern United States, is smaller, glossy white and has only two small spines. |
| Spinybacked orb weaver (Gasteracantha cancriformis) | Much broader than long, with six sharp lateral horns on a flat, shiny white or yellow shell spotted with black. Builds in shrubs and around houses, not across trails. |
| Other spiny orb weavers | A broad group across several genera; almost all are wider-bodied and more brightly coloured, and few are woodland-trail spiders. |
| Verrucosa | Similar size and also a woodland orb weaver, but it sits head-up in the web and carries a smooth triangular abdomen with a white or yellow patch and no spines. |
Quick facts
| Scientific name | Micrathena gracilis (Walckenaer, 1805), family Araneidae. |
|---|---|
| Other names | Castleback orbweaver, spined orb weaver. |
| Distribution | Eastern North America from southern Ontario and New England to Florida and eastern Texas, south through Mexico into Central America. |
| Habitat | Deciduous and mixed woodland, particularly oak and hickory; wooded edges and shaded ravines, most abundant near water. |
| Web | Small, tightly woven vertical orb, 10–20 cm across, rebuilt in the morning and relocated about once a week. |
| Diet | Small flying insects: gnats, flies, mosquitoes, leafhoppers, small moths and wasps. |
| Lifespan | About one year. Adults appear in mid-summer and die after autumn egg-laying. |
| Predators | Mud dauber and spider-hunting wasps, birds, mantises and jumping spiders. |
| IUCN status | Not evaluated. Common and widespread throughout its range. |
Did you know?
- The species name gracilis means slender — a poor fit for the stout female, and a better one for the long, flattened male that early naturalists described alongside her.
- It is one of the very few orb weavers that rebuilds in the morning. Most take their web down at dusk and spin again in darkness.
- The alternative name castleback comes from the row of spines and blocky rear edge, which in silhouette resemble the battlements of a castle wall.
- The habit of moving house weekly is unusual among orb weavers, many of which hold one site all season.
- Female micrathenas often hang so still in the hub that they are taken for a seed or a fragment of bark until the web moves.
In summary, the spined micrathena is a harmless, hard-shelled little orb weaver that works the day shift in eastern woodlands. Learn the black-and-white armoured abdomen and the ten short spines and you will recognise it instantly — usually about a second after you walk into its web.
Frequently asked questions about the spined micrathena
What does a spined micrathena look like?
The female has a hard, boxy abdomen wider than it is long, patterned in black and white and ringed with ten short black spines in five pairs. The carapace is amber to brown and the legs are glossy black. Males are much smaller, flatter, plainer and almost spineless.
How big does a spined micrathena get?
Adult females reach about 8 to 11 millimetres in body length, roughly the size of a shirt button, and males only 4 to 5 millimetres. Older sources quote a range of 4.2 to 10.8 millimetres, which combines both sexes rather than describing either one on its own.
Is the spined micrathena dangerous to humans?
No. It uses venom on insects, but that venom has no medical significance for people or pets, and no serious reaction has ever been documented. The spider is small, its fangs are short, and it frequently cannot pierce human skin at all. It is not related to widows or recluses.
Why does the spined micrathena rebuild its web?
Sticky capture silk dries out and tears, so the orb loses its grip within a day. This species rebuilds in the morning rather than at night, and unusually it also relocates entirely about once a week, abandoning a site whose insect traffic has dropped in favour of a fresh gap in the understory.
How can you tell a male spined micrathena from a female?
Size and shape settle it. Females are two to three times larger, with a bulky armoured abdomen and five pairs of prominent spines. Males have a small, flattened, elongated abdomen with the spines reduced to low bumps or absent, and they leave the web entirely once mature.
When are spined micrathenas most often seen?
From July to early October, with the peak in late August and September when the females are fully grown. Because the species is diurnal it sits in the open at midday, which is when almost every encounter happens, usually on a shaded woodland trail.
Do spined micrathenas bite people?
Very rarely, and only if pressed against skin. Their first response to disturbance is to drop out of the web on a dragline or shake the orb. A bite feels like a pinprick and leaves at most a small pink spot for an hour or two; soap and water is all it needs.
What do spined micrathena spiders eat?
Small, weak-flying insects caught in the fine mesh of the orb: gnats, flies, mosquitoes, leafhoppers, small moths and parasitic wasps. The old claim that micrathenas eat birds is incorrect. A spider of ten millimetres cannot handle vertebrate prey of any kind.
Where does a spined micrathena build its web?
In the understory of deciduous woodland, strung vertically across a gap between saplings or low branches, often at waist to head height. Trails, ravines and the edges of streams and ponds are favourite spots, because insects funnel along those open corridors.
What are the spines on a spined micrathena for?
They are defensive. The combination of hard cuticle and projecting cones makes the spider awkward for a small bird or wasp to grip and swallow, and the same armour is found across the spiny orb weavers. The spines have no role in web building and are barely developed in males.
References
- World Spider Catalog (2026). Micrathena gracilis (Walckenaer, 1805). Natural History Museum Bern. wsc.nmbe.ch/spec-data/11357/taxon
