High-speed imaging reveals the ballista spider’s ballistic silk snare

Phantom T1340 captures a prey strike too fast for the human eye

Researchers studying Australia’s ballista spider have used a Phantom T1340 high-speed camera supplied by Adept Turnkey to capture its remarkable spring-loaded silk snare in action.

Led by Professor Ajay Narendra, School of Natural Sciences, Macquarie University NSW the study, Ballistic high-powered spider webs overcome dangerous prey defences, was recently published in Current Biology. The research reveals how the spider uses a highly tensioned silk structure to pull aggressive green tree ants from the surface of a leaf at extraordinary speed.

Capturing a previously unseen behaviour

The project began when Professor Greg Anderson a biomedical researcher and spider taxonomist who heads the Chronic Disorders Program at the QIMR Berghofer Medical Research Institute in Brisbane, discovered the spider and noticed it constructing an unusual trap. As this behaviour had not been documented before, he approached Professor Ajay Narendra to investigate it further.

The research team travelled from Sydney to Cape York by road to safely transport the camera equipment. On their first night in the field, they observed a spider spend around four hours building a supporting scaffold and a tiny conical silk snare measuring just 3–4 mm.

Once complete, the spider returned to the centre of its web and waited. Within a short time, an ant approached and appeared to disappear instantly. The speed at which this event occurred caught the researchers by surprise and so both the trigger and capture rate were set far too slow. They setup at much higher rates with the T1340 and waited again.

After waiting another three hours for the spider to rebuild the trap, the researchers captured the next strike at between 5,000 and 7,000 frames per second. What was discovered was remarkable. The upturned silk cone was attached to both the leaf and a high branch with a thin silk cord. The ant was attracted to the silk with pheromones deposited by the spider causing the ant to attack the cone. As soon as the ant's pincers were entangled with the silk cone web, the cone detached from the leaf flinging the ant upwards at an acceleration 13x times greater than gravity and to achieve extremely high speeds in a short time, giving the ant no chance to recover or disentangle.

The act of surprise is essential for the spider to achieve its goal, with it's diet being solely green ants, a very aggressive prey that's well equipped with chemical defences and the ability to call on an army at short notice.

Why higher frame rates were essential

Previous attempts to record the spiders at between 250 and 1,000 frames per second had shown little more than a blur.

Using the Phantom T1340 at much higher frame rates allowed the researchers to freeze individual stages of the movement and observe the ant being rapidly pulled from the leaf. The footage could also be reviewed immediately in the field, confirming that the event had been captured successfully.

Recording the behaviour presented several technical challenges. The relatively high magnification required, amplified the need for high capture rates, as well as making it more difficult to achieve a bright image with enough depth of field.

Selecting the right high-speed camera

Adept Turnkey worked with the researchers and helped them to assess a range of camera options and identify the most suitable system for their work. Apart from the optical and speed characteristics required, portability was a very important aspect for the selection because much of the team’s research takes place in the field.

The Phantom T1340 proved to be the most suitable option as it provided the speed, pixel size and sensitivity to allow for lens settings to achieve the required depth of field and image quality to analyse the rapid prey-capture sequence.

Lighting was also an important consideration. In order not to interfere with either spider or ant behaviour infrared lighting was chosen to illuminate the scene and this in turn influenced the camera selection. The T1340 demonstrates sensitivity to around 1000nm and so allowed the selection of an infrared light that was completely invisible to both spider and ant.

The next stage of the research

Although the study has revealed an extraordinary hunting strategy, several questions remain. The researchers still want to understand how the spider generates such extreme acceleration and if the trapped ant survives the forces involved. 

The team plans to return to Cape York early next year, with further high-speed filming expected to play an essential role in answering these questions. 

The study demonstrates how high-speed imaging can reveal biological movements that are invisible to the human eye and turn fleeting events into measurable scientific evidence.