Many have heard of the fight-or-flight choices most creatures face in the wild, but new research has shown that for the fruit fly the choice is a bit more complex.

Studies on the neural network of the fruit fly have shown that it has two tiers of ‘flight’ response.

When a fruit fly detects an approaching predator, it takes just a fraction of a second to launch itself into the air and soar gracefully to safety - but when threats demand a quicker getaway, things get a little bit clumsy.

Scientists in the US have revealed how a quick-escape circuit in the fly's brain overrides the fly's slower, more controlled behaviour when a threat becomes urgent.

Researchers were looking at escape behaviours in the fruit fly to find out about the circuits and processes that underlie decision-making, and how the brain integrates information to respond to a changing environment.

It turns out that two neural circuits mediate fruit flies’ slow-and-stable or quick-but-clumsy escape behaviours.

The choice is made, researchers found, during a spike in activity for a key neuron in the quick-escape circuit, which can override the slower escape and prompt the fly to spring to safety.

A pair of neurons - called ‘giant fibres’ - in the fruit fly brain had long been suspected to trigger escape.

Researchers are able provoke this behaviour by artificially activating the giant fibre neurons, but no one had actually demonstrated that those neurons responded to visual cues associated with an approaching predator.

To do this, researchers at the Howard Hughes Medical Institute's Janelia Research Campus created an engaging and realistic cinema experience for their subjects.

“It's really like a domed IMAX for the fly,” research leader Gwyneth Card explained.

Genetic tools enabled the team to switch the giant fibre neurons on or off, and then observe how flies responded to a predator-like stimulus.

They conducted their experiments in an apparatus that captures videos of individual flies as they are exposed to a looming dark circle. The image is projected onto a hemispheric surface and expands rapidly to fill the fly's visual field, simulating the approach of a predator.

To ensure their experiments were relevant to fruit flies' real-world experiences the team recorded and analysed the trajectories and acceleration of the damselfly – natural predators of the fruit fly – as they attacked. They designed their looming stimulus to mimic these features.

By analysing more than 4,000 flies, Card and her colleagues discovered two distinct responses to the simulated predator: long and short escapes.

To prepare for a steady take-off, flies took the time to raise their wings fully. Quicker escapes, in contrast, eliminated this step, shaving time off the take-off but often causing the fly to tumble through the air.

Card and her colleagues wanted to understand how flies decide when to sacrifice stability in favour of a quicker response. They set up experiments in which she could directly monitor activity in the giant fibre neurons and surprisingly, discovered that the giant fibres were not only active in short-mode escape, but also during some of the long-mode escapes.

Based on their data, the research team proposed that a looming stimulus first activates a circuit in the brain that initiates a slow escape, beginning with a controlled lift of the wings. When the object looms closer, filling more of the fly's field of view, the giant fibre activates, prompting a more urgent escape.

“What determines whether a fly does a long-mode or a short-mode escape is how soon after the wings go up the fly kicks its legs and it starts to take off,” Card says.

“The giant fibre can fire at any point during that sequence. It might not fire at all – in which case you get this nice long, beautifully choreographed takeoff.

“It might fire right away, in which case you get an abbreviated escape,” she said,

The more quickly an object approaches, the sooner the giant fibre is likely to fire, increasing the probability of a short escape.