Robber flies: the mini assassins with binoculars and two navigation systems

By Clarie Lo

Holcocephala fusca, also known as robber fly, is one of the smallest aerial predators in nature. With a total body length of no longer than 7 mm,1 comparable to a grain of rice, its sensory capability and brain capacity are expected to be inferior to other species of the Insecta class. Much to scientists’ amazement, robber fly demonstrates an outstanding ability in detecting, tracking, and capturing its prey, even in comparison to vertebrates. It can identify a target that is half a metre away, which after scaling up in proportion to its size, is equivalent to a human precisely spotting a football across the entire pitch.

The secret weapon owned by the Top Gun of the fly world is its highly sophisticated eyes. Like most arthropods, robber fly has compound rather than single-aperture eyes. The evolution of such a curved array of ommatidia containing individual lenses and photoreceptors provides a 360-degree panoramic view and the power to perceive high-speed movement. However, the robber fly has a clear physiological limitation: the surface area of its visual organ is very restricted. Hence, the balance between the number and size of lenses has to be carefully designed. The former determines sight width, while the latter dictates resolution,3 which is crucial for targeting from afar. 

To overcome this challenge, the eyes of the robber fly have been adapted to house lenses with sizes ranging from 20 microns to 78 microns. The larger lenses, which are as wide as a human hair, are concentrated at the centre of the eye and occupy only one thousandth of the matrix. The closer the lenses are located to the periphery, the tinier they get. This distribution pattern allows robber fly to accommodate lenses that would normally only fit in the eyes of dragonflies despite being 10 times smaller in size, without bearing excessive weight or metabolic burden. Mini sensors are planted at the core of the eyes to complement the virtually parallel optical axes extending from the target.4 Although a localised high-resolution region within the eye is not unheard of in other species of flies, the fovea is unquestionably the most exaggerated structure out of the family.

Being born with a pair of permanent binoculars, the remaining task for the robber fly is to process all the visual inputs and generate a flight path to its prey. This may not be difficult for a mammalian brain like that of humans, which has up to 86 billion neurons to perform all sorts of complex calculations. In contrast, a typical fly brain is 109 times lighter with around 100 thousand neurons to work with.5 The innate lack of computation power forces the robber fly to capture only the necessary information from its environment to feed into a combination of two simple systems. 

The first system is analogous to the “pure-proportional navigation” guidance law that drives most homing missiles. The detailed physics behind will not be discussed, but it revolves around the concept of “line of sight”, which is an imaginary line connecting the eyes of the robber fly to its air target. As the latter is constantly moving, the angle of the line of sight will shift accordingly. The robber fly is programmed to correct their trajectory in terms of speed and direction in response to that change. This rule, coined as constant bearing strategy (CBA) ensures the hunter and the prey are on a collision course at any given time point, regardless of the path of the target itself.6 CBA is a robust mechanism that is independent of the brain’s processing capacity, such that robber fly can react promptly to reduce the time-lag between detection and execution. 

No matter how accurate the robber fly can plan its actions in advance, the chase will be cut short by an accidental crash mid-way into an obstacle. Therefore, a second system is introduced through evolution. It is a simple avoidance algorithm that behaves like a feedback loop. If an obstacle is situated along the intended trajectory of the fly, its perceived image should increase in size as the fly approaches it. This visual expansion triggers the fly to turn immediately away from the original vertical track. The larger the obstacle is in its field of view, the greater the magnitude of adjustment. Each redirection response will only be switched off when the blockage diminishes and eventually disappears from its sight.7

These two competing navigation systems produce a single output, which maps a flight path for the robber fly to steer swiftly around different obstacles, and ultimately makes an interception on its target. Once the gap between the fly and its prey is below 29 cm, a “lock-on” process automatically becomes the dominant system. Instead of tackling the target from the side, this mechanism helps the fly pick up speed and attacks from the front. This is because an interception is much more likely to be successful if both creatures are heading along the same axis, instead of meeting at ninety degrees, similar to how a baton is passed during a relay race.4

The robber fly has become an interesting creature to study not only because of its masterfully crafted hunting mechanism, but also its potential to act as a prototype for the design of drones. One of the major problems with drones is its massive demand for battery supply to process all the visual data they receive, which undoubtedly is very resource-draining. Perhaps we can learn from the robber flies created by nature, and construct robots that can operate with just as little sensing as what these miniature assassins do. 


1.         Bromley SW. Asilid notes (Diptera), with descriptions of thirty-two new species. American Museum novitates. 1951.

2.         University of Cambridge. Detect. Lock on. Intercept. The remarkable hunting ability of the robber fly. 2017.

3.         R Volkel, Eisner M, Weible K. Miniaturized imaging systems. Microelectron Eng. 2003.

4.         Wardill TJ, Fabian ST, Pettigrew AC, Stavenga DG, Nordström K, Gonzalez-Bellido PT. A Novel Interception Strategy in a Miniature Robber Fly with Extreme Visual Acuity. Curr Biol. 2017 Mar;27(6):854–9. 

5.         Scheffer LK. A connectome and analysis of the adult Drosophila central brain. Elife. 2020.

6.         Yanushevsky R. Modern missile guidance. Boca Raton: CRC Press; 2008. 226 p. 

7.         Fabian ST, Sumner ME, Wardill TJ, Gonzalez-Bellido PT. Avoiding obstacles while intercepting a moving target: a miniature fly’s solution. J Exp Biol. 2022 Feb 15;225(4):jeb243568. 

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