Spiny Orb-Weaver (Gasteracantha)

Physical description and identification

Gasteracantha is one of the few spider genera an untrained eye can name with confidence. The abdomen of an adult female is not the soft, rounded bag carried by most orb weavers but a broad, hardened plate, glossy as enamel and rimmed with pointed spines, with the small dark head and legs tucked underneath.

Adults

• Size: females of most species measure 5–10 mm from front to spinnerets and 10–15 mm across the widest pair of spines, so the spider really is wider than it is long. Males are far smaller, roughly 2–4 mm, and narrow-bodied.
• Color: the shield is usually white, cream, yellow or orange, often bordered in red or black; a minority of species are deep red or nearly all black. Scattered over it are small sunken pits called sigilla, which mark where internal muscles anchor to the cuticle. The spines are commonly red, orange or black.
• Spines: six is the usual number, three on each side, but the genus is variable. Several species carry four, a few only two, and the shape ranges from blunt cones to the long curved horns of G. arcuata. Spine number and curvature, with the pattern of sigilla, are what specialists use to separate species.
• Eyes: eight small eyes in two rows on a low head, the outer pairs on slight bumps. Vision is poor; the spider reads its world through vibration in the web, which is why it sits still while a camera comes within centimetres.
• Males: dull brown or grey-black, with the spines reduced to five or six low humps on a soft abdomen. They look nothing like the female and spend their short adult lives on the outer threads of her web.

Eggs and egg sacs

The female leaves the web to lay. She fixes an elongated sac, in many species 20–25 mm long, under a leaf or twig nearby and covers it with pale silk crossed by dark green or yellow threads — a pattern distinctive enough to identify on its own. A sac holds roughly 100 to 260 eggs, which hatch about eleven to thirteen days after laying.

Spiderlings

Hatchlings stay inside the sac for a further two to five weeks, moulting once in that shelter before chewing their way out. They emerge pale and unspined, darken over the following moults, and disperse by ballooning on strands of silk. A half-grown spiny orb-weaver already has the hardened shield but only the beginnings of the horns.

The web

The web is a classic vertical or slightly tilted orb, usually 20–40 cm across, strung between shrubs or low branches at about head height. Two details set it apart from other spider webs. Most species decorate the frame lines with small tufts of white woolly silk at regular intervals, widely read as a visual signal that keeps birds from flying through and wrecking the orb. And the orb is short-lived: the spider often eats and replaces the sticky spiral every day or two.

Where spiny orb-weavers live

Gasteracantha is a pantropical genus. Its centre of diversity is Southeast Asia and the Indo-Pacific — India, Sri Lanka, Thailand, Malaysia, Indonesia, the Philippines, southern China and southern Japan — with a second concentration in sub-Saharan Africa and Madagascar, where the spiders are usually called kite spiders. Several species reach Australia and the Pacific islands.

One point is worth correcting, because general accounts blur it. Only one member of the genus lives in the Americas: Gasteracantha cancriformis, which ranges from Florida and the Gulf states west to southern California, through the Caribbean and Central America into northern South America. Every spiny orb-weaver photographed in the United States belongs to that species.

Habitat is consistent across the range: sunlit edges rather than deep shade. Webs go up in shrubby gardens, hedgerows, plant nurseries, citrus groves, woodland margins and roadside scrub, usually in open air between shrubs at walking height — which is why the first encounter is so often a face-first collision with a web across a path.

Are spiny orb-weaver spiders venomous?

Yes, in the ordinary sense that applies to almost all spiders: Gasteracantha uses venom to immobilise the small flying insects it catches. That venom has no medical significance for humans or pets. No serious envenomation has been documented from any species in the genus, and its chemistry is nothing like the neurotoxins of widow spiders or the tissue-damaging enzymes of the recluses.

The spines cause more worry than the venom, and they should not. They are inert armour — hardened outgrowths of the abdominal wall with no gland, duct or opening. The spider cannot sting with them, inject anything through them, or flick them.

Can spiny orb-weaver spiders bite?

In principle yes, but bites are close to unheard of. The fangs are short and built for insect cuticle, and the spider's answer to a large animal is to drop on a dragline or shake the orb rather than advance. The realistic scenario is someone closing a hand around a female, or a spider pressed against skin after a walk through a web.

Where a bite does happen it feels like a pinprick, may leave a small red mark for an hour or two, and needs nothing more than soap and water and a cool compress. Spreading redness, fever or swelling that grows over days points to a secondary skin infection rather than venom, and that is worth a doctor's attention.

Ecological importance and behavior of spiny orb-weavers

Spiny orb-weavers are day-active, which is the opposite of what many popular accounts claim. The female hangs head-down at the hub through daylight hours, in plain sight, relying on armour and warning colour rather than on hiding. She stays at the hub at night too, and does not build the separate retreat that many nocturnal orb weavers use.

Diet: the orb intercepts small, weak fliers — whiteflies, fruit flies, mosquitoes, moths, leafhoppers, small beetles and flying ants. In citrus groves and nurseries dense populations remove large numbers of pest flies, which is why growers leave them alone.

Natural predators: mud-dauber wasps in the genus Sceliphron and spider-hunting wasps paralyse adults and stock their mud cells with them; birds and lizards take spiders that stray from the hub; parasitoid wasps and flies attack the egg sacs, where most of the genus's mortality occurs.

Seasonal pattern: in seasonal parts of the range adults mature late in the warm season, mate, and the females lay through autumn and then die. Winter is passed as spiderlings inside the sac, which is why sacs found under leaves in winter look full but inactive. In the wet tropics generations overlap and adults occur year round.

Mating: a mature male sits on the outer frame of a female's web and plucks the threads in a recognisable rhythm to announce that he is not prey. Males die soon after mating.

Relationship with humans: entirely benign. The webs are a nuisance across paths and nothing more. Moving the anchor lines with a stick usually persuades the spider to rebuild a metre away overnight; there is no reason to kill one.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The Swedish zoologist Carl Jakob Sundevall erected Gasteracantha in 1833. The name is Greek for “belly thorn”, from gaster (belly) and akantha (spine) — a scientific name that doubles as a field description.
• Around seventy species are recognised. Because colour varies so much within a species, the group has a long history of the same spider being described more than once and the names later merged.
• The hardened abdomen is unusual among araneids, most of which have a soft, expandable body. With the bright warning colour it looks like an anti-predator package aimed at birds.
• The woolly tufts outlast the orb itself, so an abandoned site can be recognised days later from the tufts alone.
• Despite the nickname “crab spider” and the species name cancriformis — crab-shaped — the genus is unrelated to the true crab spiders, which belong to a different family and spin no snare.

In summary, the spiny orb-weaver is a small, ornamental, harmless spider whose armour is aimed at birds, not at you. Learn the wide enamelled shield, the ring of spines and the tufted frame lines and you can name the genus from across a garden.