Tropical Orb-weaver (Eriophora ravilla)

Physical description and identification

The tropical orb-weaver is a big, solidly built araneid with a broad abdomen and thick, powerful legs. It is also one of the most variable spiders in the region: individuals may be grey, brown, olive, rust or almost white, plain or heavily mottled, which makes colour an unreliable guide on its own. Two features do the work instead — the underside of the abdomen and the shape of the shoulders.

Adults

• Size: females measure roughly 12–24 mm in body length and males 9–13 mm. A mature female with her legs spread easily covers a matchbox, which puts her among the largest orb-weavers in the southern United States.
• Colour: females usually have reddish-brown legs and carapace with a grey, brown or olive abdomen, often with pale flecks. Males are typically greyer with distinctly banded legs. In Florida many adults and juveniles carry conspicuous white spots on the upper abdomen.
• The belly: the most reliable field mark is on the underside. The centre of the abdomen carries a dark, almost black patch bordered by paler markings that point backwards. Turning a spider's web gently so the light catches the venter is a far better test than arguing about colour.
• Shoulders: many individuals show two low humps at the front of the abdomen — a genuine feature of the species rather than something restricted to one region, though it is more obvious in some spiders than others.
• Sexes: males are smaller with a narrower abdomen, longer legs and swollen pedipalp tips. They stop building webs once mature.

Eggs

The female wraps several hundred small eggs in a silk sac fixed to vegetation or hidden in a rolled leaf near the web site. In the warmer parts of the range more than one sac may be produced in a season.

Spiderlings

Young spiders hatch pale and disperse after their first moults, many on wind-borne silk. Florida juveniles often show the same white abdominal spots as the adults, so small spiders on a porch in autumn are frequently this species rather than something else.

The web

This is the part that makes the species remarkable. Soon after dark the female spins a large vertical orb — the capture spiral itself typically spans 30–60 cm, occasionally more — but she slings it from bridge and anchor lines that can run for several metres between trees, eaves, wires or fence posts. The widely quoted figure of a “19 ft (6 m) web” refers to that total span of supporting threads, not to the orb, and repeating it as the size of the wheel is one of the most common errors written about this spider. Before dawn she takes the whole structure down, eating most of the silk to recover the protein, and retreats to a curled leaf or a crevice for the day. Our spider web guide covers how an orb is built and why nightly recycling pays.

Where the tropical orb-weaver lives

Eriophora ravilla is a warm-climate American species. In the United States it is commonest in Florida and along the Gulf Coast through Alabama, Mississippi, Louisiana and southern Texas, with scattered records inland. From there the range runs south through Mexico, the Caribbean and Central America into northern South America, including Colombia and Venezuela. It does not survive hard winters, which is what limits it to the southern edge of the country.

Typical habitat is woodland edge, hammock, scrub, citrus groves and orchards, parkland and suburban gardens — anywhere with well-spaced trees or structures that give the spider the long anchor lines its web design depends on. Outdoor lights are a strong attractant, since a web strung near a lit doorway or a street lamp intercepts the night-flying insects the light pulls in.

Are tropical orb-weaver spiders venomous?

Yes, like virtually all spiders: the tropical orb-weaver injects venom to subdue the moths, beetles and other insects it catches. The venom is not medically significant to humans, pets or livestock, and there is no antivenom because none has ever been needed. Despite the spider's size and the impressive web, no serious envenomation has been attributed to Eriophora ravilla or to any other member of the orb-weaver family Araneidae in the Americas.

Can tropical orb-weaver spiders bite?

It can, and because it is a large spider with fangs strong enough to pierce skin, a bite from this species is more noticeable than that of a small orb-weaver. It is still uncommon. These spiders are not aggressive and their reaction to a person is to drop from the web and run, or to freeze in the leaf retreat.

When a bite does happen — usually after a face-first walk into a night web, or when a spider is caught inside clothing — it is generally described as similar to a bee sting: a sharp pain at the moment of the bite, local redness and mild swelling, and soreness that fades within a day. Wash the site with soap and water, use a cold pack for the pain, and keep it clean. Individual reactions vary, so seek medical advice if swelling spreads well beyond the bite or you have symptoms of an allergic reaction.

Ecological importance and behavior of the tropical orb-weaver

The tropical orb-weaver is strictly nocturnal, and its whole biology is organised around that. Building at dusk means the web is invisible to birds through the hours it is in use; dismantling it at dawn means the spider is not exposed on open silk in daylight and recovers the silk protein for the next night's construction. The spider waits head-down at the hub, senses prey by vibration, and bites and wraps it on the spot.

Diet: night-flying insects dominate — moths above all, plus beetles, craneflies, lacewings, katydids and the swarms of small insects that gather at lights. A large female takes prey that smaller orb-weavers cannot handle, and in citrus groves the species is a useful part of the natural predator community.

Natural predators: mud-dauber and spider wasps hunt orb-weavers for their nests, and bats, geckos, lizards, tree frogs, birds and larger spiders all take them. The spider's day retreat is as much protection from predators as from heat.

Prey and predator balance: because the web occupies a large aerial gap that few other spiders use, an Eriophora population removes a meaningful share of the insects moving through woodland edge at night without competing directly with the daytime webs of Neoscona or golden silk orb-weavers nearby.

Seasonal behaviour: adults are present most of the year in Florida and the tropics and are most obvious from late summer into autumn further north. Mature males abandon web-building and search for females at night, approaching on the frame lines and signalling with plucked silk.

Relationship with humans: harmless and worth keeping. A web across a path can simply be avoided for the night — it will be gone by breakfast.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• Rebuilding every night is affordable only because the silk is recycled: the spider eats the old web, digests the proteins and reuses them in the next one.
• The genus name Eriophora means “wool-bearing”, a reference to the woolly appearance of the silk and hairs rather than to any cribellate silk — these are ordinary sticky-silk orb-weavers.
• A relative, Eriophora fuliginea, flew on a NASA mission in the 1970s as part of an experiment on whether spiders could still build orbs in microgravity. They could, after a shaky start.
• The species was described in 1844 by Carl Ludwig Koch, one of the most prolific arachnologists of the nineteenth century.
• Because the web vanishes at dawn, homeowners often assume a large spider has “moved in overnight” when in fact it has been in the same garden for weeks.

In summary, the tropical orb-weaver is a large, harmless, strictly nocturnal spider that builds and destroys one of the biggest webs in the southern United States every single night. Check the underside for the pale-edged dark patch and you have your identification.