A miniature brain can achieve accurate performance in highly demanding sensorimotor tasks. ~ English neurobiologist Trevor Wardill et al
At 6 millimeters in length, a robber fly is 10 times smaller than a dragonfly. Compared to dragonflies, robber fly compound eyes have 1/3rd the total number of lenses, and 99% of those are almost 4 times smaller; yet robber fly vision is nearly as sharp as dragonflies. (Though robber fly sight is acute looking forward, their peripheral vision is no match for the wide-angle acuity of dragonflies.)
Robber flies give dragonflies a run for their money with regards to spatial resolution of the retina. ~ Paloma Gonzalez-Bellido
The lenses in a robber fly’s compound eyes range from 20 to 78 microns (μ) in diameter. 78 μ, which is about the width of a human hair, matches dragonfly lens size.
The largest lenses in robber fly eyes are clustered together in the center of each eye and pointed forward. There only ~20 of these foveal ommatidia. They occupy ~20% of the eye volume, while covering only ~0.1% of the eye’s visual space. These large lenses are paired with small light-receptor cells set farther back from the lens than in other flies. The result is astonishing binocular vision.
The effect of this is like zooming in on a camera lens. By extending the focal length, the sensor at the bottom samples a smaller region of visual space. ~ English entomologist Sam Fabian
Most saliently, the neural equipment for vision processing that robber flies have is much tinier, and many times more modest, than the meager resources that dragonflies manage their impressive aerial feats with.
A robber fly attentively sits, waiting for a tempting prey to fly past. A robber fly can detect a morsel on the wing smaller than 2 mm from up to 100 body lengths away: acuity so astonishing as to be physically implausible.
Once a meal is spotted, a robber fly takes off, using an interception trajectory known as constant bearing angle: keeping the target at a constant angle to ensure eventual intersection. This same strategy is also used by fish, bats, and sailors.
Once a robber fly has closed in to 29 cm, it changes tack to a direct intercept. The lock-on phase transition is driven by invariant image properties of the targeted prey, not by any specific subtended angular size or by distance estimation.
It has been considered that the lock-on trigger relates to the angular size of the object over its rate of expansion as the robber fly closes in. This ratio, called optical tau, yields an estimated duration to contact. But the rapid robber fly attack violates 2 conditions necessary for optical tau to be reliable: constant approach speed and symmetrical head-on approach.
Whatever triggers robber fly lock-on, it is an instantaneous calculation of an advanced geometrical algorithm; something well beyond human ability at any speed.
There’s a trade-off going on between having excellent vision – which requires bigger lenses – and the size of the insect. The only way a robber fly could have vision as excellent as a dragonfly would be to have an eye with many more and larger lenses ; but then the fly itself would need to be much larger to be able to carry it. ~ Paloma Gonzalez-Bellido