Ballista Spider: The Prey Pulls Its Own Trigger — A 4-Hour Silk Catapult and an Ant's 1300 m/s² Flight

Forum topic · 小凯 · 2026-08-29

Summary

In the rainforests near Cooktown, Queensland, Australia, an undescribed spider of the genus Propostira — informally dubbed the 'ballista spider' — was documented by researchers from Macquarie University, including Professor Ajay Narendra and graduate student Pranav Joshi, alongside biomechanics expert Jonas Wolff of the University of Greifswald. Published in Current Biology, their study reveals a never-before-recorded hunting mechanism: each night the spider spends roughly four hours anchoring a cone of 15–60 tensioned silk threads to the ground, then hiding in a web about 30 cm above. The trap targets the green weaver ant (Oecophylla smaragdina), a fiercely territorial species. When an ant bites the cone's fragile outer 'pin' thread, the structure detaches and all stored elastic energy releases at once, catapulting the ant upward at over 1300 m/s² — roughly 133 g, far beyond human tolerance — directly into the waiting web. Uniquely among known spider traps, the prey's own aggressive behavior triggers the mechanism, possibly aided by a suspected pheromone lure. The silk cone reportedly exceeds all known silk-based biological catapults in instantaneous power density. The spider appears strictly monophagous, feeding only on this one ant species — an extreme evolutionary specialization with no fallback plan.

Ballista Spider: The Prey Pulls Its Own Trigger

Key points

A newly discovered spider of the genus *Propostira* (species not yet formally described), informally called the "ballista spider", builds a nightly silk-powered catapult trap in the rainforest near Cooktown, Cape York Peninsula, Queensland, Australia.
Research by Ajay Narendra and Pranav Joshi (Macquarie University), with silk biomechanics expert Jonas Wolff (University of Greifswald), was published in *Current Biology*; it followed ten days and nights of field observation and high-speed/infrared filming after Greg Anderson first photographed the spider in 2024.

How the trap works

1. After dark, the spider (less than 1 cm long) descends and anchors a cone-shaped structure (~2–3 cm tall) to the ground or low branches, spending about four hours spinning it. 2. The cone is built from 15–60 tensioned silk threads, each storing elastic potential energy like parallel springs. 3. An outer layer of fine, easily broken silk acts as a release pin. 4. The spider retreats to a main web positioned roughly 30 cm above the cone. 5. Its target is the green weaver ant (*Oecophylla smaragdina*), a highly aggressive, territorial, canopy-dwelling species that attacks anything suspicious with mandibles and formic acid. 6. When an ant bites the fragile outer silk, the pin breaks, the cone detaches from its anchor, and all tensioned threads release simultaneously — launching the ant upward at over 1300 m/s² (≈133 g), into the web above. For comparison: fighter ejection seats deliver ~12–20 g; the human tolerance limit for brief g-forces is roughly 46 g. 7. The ant is likely kept attached by its own adhesive pads sticking to the silk, and possibly also by a suspected (unconfirmed) pheromone lure that mimics alarm or territorial signals and provokes the bite.

Why the finding matters

This is the only known spider web trap triggered by the prey's own action rather than by prey blundering into it passively — the spider sets up the mechanism; the ant pulls the trigger by biting.
Jonas Wolff's SEM analysis concluded the silk cone achieves an "incredible instantaneous power density", reportedly exceeding all other known silk-based biological catapults. Comparison points from prior research:
Jumping spider legs: ~500 W/kg
Asilid fly takeoff: ~1000 W/kg
Slingshot spider (theridiosomatid, Alexander et al. 2020): elastic energy density ~3.92 kJ/kg, peak power density ~5.53 kW/kg
The mechanism exploits the ant's defensive system against itself: the ant's attack response is precisely what fires the trap — an evolutionary arms-race endgame against a prey that no other predator specializes on.
The spider is apparently strictly monophagous (eats only green weaver ants), an extreme specialization with no fallback if the ant species declines.

Sources

Macquarie University / ScienceDaily, "This newly discovered ballista spider catapults ants into a deadly trap," 2026-06-26
Narendra, Joshi, Wolff et al., *Current Biology*, 2026
Mongabay, "Game of tiny thrones: Parasitic ants grab power by turning workers against their queen," 2025-11-20 (background on weaver ant behavior)
Alexander et al., 2020 (slingshot spider energy density comparison data)