There’s more than one way a spider can spin its web. Some construct large vertical orb webs, while others build horizontal sheet webs or tangled cobwebs that ensnare crawling insects.
There’s also more than one way a spider can catch its dinner. Net-casting spiders throw small silk nets over unsuspecting prey, while slingshot spiders use their conical webs like catapults, launching both themselves and the web toward nearby prey.
In the tropical rainforests of the Cape York Peninsula in Queensland, we discovered a spider with a new hunting strategy: a small nocturnal animal of the genus Propostira which we call the ballista spider.
As we describe in a paper published today in Current Biology, it constructs a unique spring-loaded snare triggered only by a single kind of prey: the highly territorial and aggressive green tree ant (Oecophylla smaragdina).
How the ballista spider catches ants
Ajay Narendra, Pranav Joshi, Daniele Liprandi, Gregory J Anderson, Jonas Wolff, CC BY
During the day, this spider rests within a silken retreat on the underside of leaves. As the night unfolds, the spider slowly descends on a silk line until it finds a suitable structure on which to lay an anchor point.
Next it returns to the core web, leaving a “tension line” behind it, and repeats the process with incredible precision to build a fan-shaped web of silk tension lines. This gradually leads to the formation of a small conical scaffold which the spider then wraps in a thinner kind of silk before retreating to the core web.
The thinner silk seems to attract ants and provoke an attack response, possibly by means of pheromones. After the thinner silk is laid, worker green tree ants appear in moments. The ants react aggressively, biting the cone.
The bite detaches the cone from the surface, and the ant is pulled up and propelled into the core web in a fraction of a second. The ant is hauled off at accelerations of up to 1,367 metres per second squared – that’s roughly 140 times the acceleration due to gravity, or 15 times the most extreme g-forces experienced by jet pilots.
The spider waits for the ant to be fully entangled in its web. When it’s safe to approach, the spider wraps the ant in silk until it is ready to eat.
A superior system
The ballista spider’s snare exhibits superior energy performance compared with other silk-based catapult systems.
Gram for gram, the webs store more energy and exert more power than any known biological catapult. A kilogram of the web would store 78.17 kilojoules of kinetic energy and very briefly exert 11.73 megawatts of power.
The exceptionally high power of the ballista spider’s snare has likely evolved to rapidly yank ants away from the vicinity of their nests and trail, where fellow ants might come to their defence.
Ajay Narendra, Pranav Joshi, Daniele Liprandi, Gregory J Anderson, Jonas Wolff, CC BY
Extreme specialisation
There are two more unusual things about the ballista spider.
First is its extreme specialisation to a single prey species. This suggests the spiders may add specific pheromones to the thin wrapping silk on their snares to attract green tree ants.
Second is that the snare is triggered by the prey itself, rather than the more common situation in which the predator senses the prey and triggers the snare.
The ballista spider demonstrates how extreme prey specialisation can drive the evolution of exceptional biomechanical performance.
