What’s that on your face? You may have barged into a spiny-backed orb-weaver’s house

As spiders go, spiny-backed orb-weavers (Gasteracantha cancriformis) aren’t very intimidating. Fully-grown females are barely half an inch across, and males are even tinier.

The spiders are distinctive, though, because of their compact, crab-shaped bodies and spine-like projections. Female coloration varies geographically – their black-spotted abdomens may be glossy white or yellow, and the spines may be red or black. Males have gray abdomens with white spots, plus smaller spines.

This mainly tropical species is found in Central and South America, Bermuda, Jamaica, Cuba, and other warm areas, including the southern U.S.

As is typical of orb-weavers (family Araneidae), females are the web builders. But unlike banana spiders (Nephila clavipes), their huge Lowcountry relatives, spiny-backed orb-weavers seem dwarfed by their own spreading, circular snares. Look out for their webs at eye-level or higher above the ground, attached to buildings, trees, and shrubs.

A distinctive feature of spiny-backed orb webs is the presence of small tufts of silk here and there, particularly on the foundation lines. These tufts may make the web more conspicuous, reducing the chance of inadvertent damage by birds or other large animals (including humans). Perhaps the silken tufts attract prey, or warn off lizards or other potential predators that might find the spiny spiders an unpleasant mouthful. At any rate, the function of the tufts is still unclear.

Females spend much of their days repairing and rebuilding their webs, staying in more or less the same location unless disturbed. The rest of the time, they employ the typical sit-and-wait strategy of a web-building predator. They feed on a wide variety of captured insects, biting and paralyzing them first, then sucking up their bodily fluids.

Meanwhile one or several diminutive males may hang out (literally) in a female’s web, each dangling from a single silken thread, waiting opportunistically for a chance to mate.

Males typically die within a week or so after mating. And after producing an egg mass wrapped in silk and attached to a leaf, the female dies, as well.

Are spiny-backed orb-weavers dangerous? It’s unlikely. Like other spiders, they may bite if handled or molested, and their tiny spines could puncture human skin. However, they’re not considered a major threat to humans.

This story was originally published October 16, 2017 at 8:56 AM with the headline "What’s that on your face? You may have barged into a spiny-backed orb-weaver’s house."