Bolas spiders hunt moths by mimicking female mating pheromones
Instead of building large webs, bolas spiders produce a single strand of silk tipped with a sticky, scented droplet. The spider emits chemical compounds that perfectly match the sex pheromones of specific female moth species. When male moths fly in looking for a mate, the spider swings the weighted silk line like a lasso, snagging the prey out of the air before reeling it in for a meal.
Abandoning the Traditional Web
Most orb-weaving spiders invest significant energy into spinning expansive geometric webs designed to catch passing insects by chance. Bolas spiders, which belong to the orb-weaver family Araneidae, have evolved away from this passive strategy. Instead of maintaining a large, fragile net of sticky threads, an adult female bolas spider hunts using a single suspended strand of silk terminated with one or more dense, sticky droplets. This specialized tool functions like the throwing weapon known as a bolas, giving the spider its name.
The spider perches quietly on a twig or leaf margin, producing a silk line that it grips with one of its forward legs. Rather than waiting for prey to blunder into an obstacle, the spider actively deploys this single weighted line to strike approaching flying insects out of the air. This structural reduction saves immense amounts of silk and maintenance time, but it demands an entirely different set of sensory and mechanical adaptations to ensure that prey actually approaches within striking range.
Aggressive Chemical Mimicry
To lure prey directly toward its perch, the bolas spider uses aggressive chemical mimicry. Female moths typically release species-specific sex pheromones into the air to attract mates across long distances. Bolas spiders synthesize and emit volatile chemical compounds that closely match the exact chemical blends produced by female moths of particular target species, such as owlet moths in the family Noctuidae.
Male moths detect these chemical plumes with their sensitive, feathery antennae and fly upwind toward the source, expecting to locate a receptive mate. Because the airborne signals match their target mate signals with high fidelity, the moths fly directly toward the awaiting spider. The spider requires no visual lure or floral decoy; the invisible chemical trail is sufficient to draw the prey straight into striking distance in complete darkness.
The Mechanics of the Strike
When an incoming male moth nears the spider, the vibration of its wingbeats alerts the predator to its exact approach. The spider raises its foreleg, holding the suspended silk droplet steady until the prey enters range. It then swings the weighted thread in a rapid, fluid motion toward the moth, effectively casting the adhesive droplet into the path of the flying insect.
Moths are famously difficult for ordinary orb webs to retain because their bodies and wings are covered in loose, detachable scales that slough off upon contact with standard web glue. The adhesive droplet on a bolas, however, is composed of a viscous, elastic glue specifically suited to penetrate through the loose scales and bond firmly to the underlying cuticle of the wing or body. Once the droplet connects, the moth is securely snagged, and the spider reels in the line, paralyzes the prey with venom, and wraps it for consumption.
Temporal Tuning Across the Night
The flight schedules of different moth species vary depending on the hour of the night, creating a challenge for a predator reliant on a single chemical recipe. Observations of certain North American species, such as Mastophora hutchinsoni, reveal that bolas spiders can adapt to these shifting activity windows by changing their chemical output over the course of an evening.
Early in the night, the spider may emit a pheromone blend tailored to attract one moth species that is actively flying at dusk. As the night progresses and that species becomes inactive, the spider alters its chemical emissions to match a second moth species that flies primarily in the late hours. This temporal adjustment ensures that the spider maintains a steady influx of prospective prey throughout its active hunting period without needing to change locations.
Dimorphism and Juvenile Strategies
Bolas spiders exhibit extreme sexual size dimorphism. Mature females are substantial spiders capable of overpowering relatively large moths, whereas adult males and early-stage spiderlings are minuscule by comparison, often only a fraction of the female's size. Because of these drastic size differences, male and juvenile bolas spiders do not construct or swing a bolas at all.
Instead, young spiderlings and adult males hunt by ambushing tiny prey with their front legs. Rather than targeting large moths, they capture much smaller insects, such as moth flies (psychodids), which they attract or encounter without using weighted silk droplets. As female spiderlings grow through successive molts, they eventually transition to constructing the silk bolas and producing the chemical lures necessary to capture larger prey.
Daytime Camouflage and Survival
Hunting by active chemical mimicry is an exclusively nocturnal endeavor, leaving the spider exposed during daylight hours. To protect themselves from birds and other visual predators while resting on exposed foliage, bolas spiders rely on elaborate morphological crypsis. Many species have lumpy, irregularly shaped abdomens patterned in shades of white, cream, brown, and black, making them closely resemble bird droppings deposited on a leaf.
During the day, the spider tucks its legs tightly beneath its body and remains motionless on a leaf surface or twig. This mimicry is so convincing that predators overlook the spider entirely. When dusk arrives, the spider emerges from its cryptic posture, climbs to a hunting perch, synthesizes its chemical lure, and prepares a new bolas for the night ahead.
Key takeaways
•Bolas spiders abandon traditional orb webs in favor of a single strand of silk tipped with an adhesive, weighted droplet.
•They emit chemical blends that mimic the sex pheromones of female moths, luring male moths directly to their perch.
•The specialized glue droplet penetrates loose moth scales to bond directly to the insect's body or wing upon contact.
•Extreme sexual dimorphism means only larger adult females use the bolas, while tiny males and juveniles hunt smaller prey without silk lines.