Bolas Spider

It replaces the web with a single line and a chemical trick. Shortly after dusk the female spins a horizontal "trapeze" thread between twigs, hangs beneath it, and spins a short vertical thread tipped with one or two big globules of a specially runny, viscous glue — the bolas. Holding the line with a front leg, she then begins releasing into the air a scent that mimics the sex pheromone a female moth uses to attract a mate. Male moths of the target species, flying upwind in search of females, home in on the source and blunder close to the spider. As a moth flutters within reach, the spider swings, flicks, or lengthens the bolas to slap the sticky droplet against the moth; the glue holds, the spider hauls the line in, wraps and bites the moth, and feeds. Missed swings are common.

Yes, and it is one of the most sophisticated known cases of chemical mimicry (aggressive mimicry) by a predator. Each bolas spider species targets one or a few moth species, and chemical analysis has shown that the spider's emitted blend closely matches the actual pheromone components those moths use, in roughly the right proportions. Some species go further — the moths a spider can catch change through the night as different species take flight, and those spiders appear to alter the composition of their lure over the course of the night to track which moths are currently on the wing, catching one species early and another later. Young spiderlings, which are too small to handle moths, instead lure and catch tiny male midges or flies, again apparently using a chemical mimic.

During the day the spider does not hunt; it sits still on a leaf or twig, and it is extremely vulnerable to birds, wasps, and other predators while exposed. Its body has evolved to be lumpy, glossy in patches, mottled white, grey, brown, and black, and roughly the size and shape of a fresh splash of bird excrement, and it often rests in a hunched posture with its legs tucked in to complete the effect. Bird-dropping mimicry is a common defensive strategy in spiders and caterpillars, because almost nothing wants to eat a bird dropping, and it lets the animal hide in plain sight on the upper surface of a leaf rather than needing a crevice.

They are completely harmless to humans — small, non-aggressive, with venom suited only to subduing moths, and no medically significant bites are known. They are also genuinely hard to encounter. Bolas spiders are never abundant, they hunt only for a few hours on suitable nights, the hunting female is a small dark shape on a nearly invisible thread, and by day the bird-dropping disguise makes them easy to overlook. Many naturalists go a lifetime without seeing one in the act. They occur across warm and temperate parts of the Americas, Africa, Asia, and Australia, typically at the edges of woods, in scrub, and in gardens, wherever their specific target moths fly.