Acanthepeira

Physical description and identification

Acanthepeira is a small genus of orb-weavers in the family Araneidae, confined to the Americas and best known in North America as the starbellied orb-weavers. Every member shares one unmistakable feature: the abdomen is not smooth but carries a ring of firm, blunt, conical tubercles, so that the spider's outline is spiky and star-like rather than rounded. No other North American orb-weaver genus combines that outline with plain brown colouring.

Adults

• Size: females run about 8–16 mm in body length across the genus, males 6–10 mm. Females are markedly bulkier as well as longer.
• Colour: brown in almost all cases, from pale tan through russet to dark chocolate, usually mottled with cream and often with a paler central band down the abdomen. The legs are banded and carry stout spines. The overall effect is dry and woody, not glossy.
• The tubercles: the number, size and arrangement of the abdominal points vary between species and are part of how they are told apart. In the common starbellied orb-weaver there are usually ten to twelve, with the rear pair the largest.
• Head region: the front of the carapace is raised and produced forward, lifting the anterior median eyes onto a distinct projection. This is often described loosely as “two eyes on stalks”; what is actually elevated is the head itself. See our guide to spider eyes for the standard araneid pattern.
• Identifying to species: reliable separation needs an adult and, in most cases, a microscope. Field identification beyond A. stellata should be treated cautiously.

Eggs

Females of the genus build rounded silk sacs containing a few hundred small eggs, fastened to vegetation or hidden in leaf litter rather than hung in the web. Most are produced in late summer, with the young appearing from the following spring.

Spiderlings

Hatchlings stay together briefly, then disperse — many on wind-caught silk — and start building small orbs within days. The abdominal tubercles are present from early instars and become proportionally more pronounced with each moult.

The web

All Acanthepeira build classic vertical orb webs, typically modest in size and slung low among grasses, forbs and low shrubs. Construction and repair are usually carried out around dusk and dawn rather than in the middle of the day, and the spider often abandons the hub to wait in a retreat of curled leaves or bunched stems connected by a signal line. Our spider web guide explains how the radii and the sticky spiral are laid down.

Where Acanthepeira spiders live

The genus is restricted to the New World. Its centre of diversity is North America, where Acanthepeira stellata ranges from southern Canada through the eastern and central United States to Florida and eastern Texas and on into Mexico. Further species are recorded from the south-eastern states, the Caribbean — Hispaniola among them — and parts of South America, including Brazil.

Habitat across the genus is consistently open and herbaceous: meadows, old fields, prairie remnants, roadside verges, marsh margins, woodland clearings and rough grassland of all kinds. Woodland interiors and buildings are largely avoided, which is one reason these spiders are seldom reported despite being common. The requirement is simply a stand of grass or tall herbs firm enough to anchor a small orb a short distance above the ground.

Are Acanthepeira spiders venomous?

Yes, in the sense that applies to almost every spider: Acanthepeira species use venom to immobilise the insects caught in their webs. The venom is insect-specific and carries no medical significance for humans, pets or livestock. No antivenom exists for any species in the genus because none has ever been needed, and no case of serious envenomation has been recorded. Araneidae as a whole contains no medically important spiders in the Americas.

Can Acanthepeira spiders bite?

They can, but it is a rare event that follows deliberate handling or a spider being pressed against skin. The normal response to disturbance is to drop out of the web on a dragline, pull the legs in and lie still — a posture that turns the spider into a convincing piece of plant debris.

A bite produces a brief sting, a small red mark and sometimes a few hours of itching. Wash the area with soap and water and apply a cold pack if it is uncomfortable; nothing further is required. Medical advice is warranted only if redness and swelling spread well beyond the site, which would indicate an allergic reaction rather than a toxic effect of the venom.

Ecological importance and behavior of Acanthepeira spiders

Acanthepeira spiders are web-bound ambush predators of open country. The spider keeps a foot on a signal line, reads the vibration when something strikes the silk, and rushes out to bite and wrap the prey. Because the webs sit low in the herb layer, the genus intercepts insects that the higher webs of larger orb-weavers never meet, which is part of how several araneid species coexist in one meadow.

Diet: small beetles, moths, flies, leafhoppers, mosquitoes and small wasps. Across a field margin or a hayfield edge this amounts to a steady, unpaid contribution to insect control, and a healthy population is a reasonable sign that the grassland food web is intact.

Camouflage and defence: the knobbly outline, the mottled brown colour and complete stillness together are the genus's main protection. The armoured abdomen is an example of convergence — unrelated orb-weavers such as the tropical spiny orb-weavers and the woodland micrathenas evolved spined abdomens independently.

Natural predators: birds, mud-dauber and spider wasps, ants, lizards, robber flies and larger spiders. Parasitoid wasps take a steady share of adults, and egg sacs are attacked by parasitic wasps and flies.

Seasonal behaviour: adults appear from late spring, peak in midsummer and persist into early autumn. Mature males stop building webs and wander in search of females, approaching along the frame threads. In most of the North American range adults die before winter and the population carries over as eggs and juveniles.

Relationship with humans: entirely benign. Leaving field edges and grassy corners unmown keeps them, and the insects they feed on, in the landscape.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1883 by George Marx, a German-born physician who settled in Washington, worked for the US Department of Agriculture and became one of the first serious students of American spiders.
• The name combines the Greek akanthos, a thorn or spine, with Epeira, the old genus name once used for most orb-weavers — literally “thorny Epeira”.
• A sweep net through a July meadow usually turns up starbellied orb-weavers in numbers that surprise people, because the genus is far more abundant than its visibility suggests.
• The armoured abdomen does not make these spiders slow: an Acanthepeira can drop from the hub and vanish into the litter in under a second.
• Most of the modern North American taxonomy rests on Herbert W. Levi's 1976 revision at Harvard's Museum of Comparative Zoology, which sorted the genus alongside several other spined orb-weaver groups.

In summary, Acanthepeira is a small, harmless American genus of grassland orb-weavers defined by one feature — a ring of blunt points around a dull brown abdomen. Learn that outline and you can name the genus at a glance, even if the species usually needs a specialist.