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  • Writer's pictureDave Black

A Five-Eyes Partnership

Did you know that honey bees’ eyes are essential to their ability to fly? Science writer Dave Black takes a deep look into the eyes of the bee and explains their varied and important purposes, which are so naturally ingenious that NASA has harnessed their features to aid exploration of far-away planets.

Honey bees have five eyes, working together to form some essential functions. The compound eyes on the side of the head, and three ocelli (aka ‘simple’ eyes) on the top of the head.

Honey bee have two, relatively large, compound eyes. They also have three other ‘eyes’ called ocelli (one would be an ‘ocellus’), and these are a little more enigmatic. All the bee families follow much the same pattern and have two compound eyes and three ocelli, and it’s a blue-print followed by all flying insects, even if the number of ocelli and their structure or shape vary a bit. A few insects that don’t fly (like ants) can also have them, but flying = ocelli is a pretty good rule-of-thumb and tells us something about what they are for.

Putting aside silly jokes about bumble bees, just by looking at an insect you’d never imagine it was constructed for flying, they just don’t look air-worthy. Other things that fly and seem to defy physics include a lot of modern aircraft. By prioritising ‘stealth’ and manoeuvrability we have managed to deliberately build some extraordinarily unstable aeroplanes, apparently only kept up there by the lightning-fast reactions of enormous engines, smart sensors and cunning computers. That’s not a tactic insects can use; it’s largely ocelli that make that possible.

In honey bees their three ocelli are arranged like a dainty crown on a slight bump on top of their head in a small equilateral triangle above the compound eyes[i]. In drones they get shifted forward a bit because of the much larger eyes. The middle ocellus is positioned in front of the other two at the tip of the triangle in a more forward-looking position. The left and right ocelli collect a view mostly from light to the side, and above, the middle one detects light from the front, and from the top and both sides. There is some overlap, controlled to a greater or lesser extent by sitting on a mildly curved surface and being shaded by surrounding body hairs, or bigger eyes, but the orientation of the head determines their view point. They are completely immobile.

NASA has developed autonomous drones for far away exploration missions – such as to Mars – which use navigation systems constructed, in part, from studying insect’s eyes.

In principle the structure of ocelli is not too different from the two compound eyes, there is a fixed lens (much of which has a faintly granular surface), pigment cells analogous to an iris, a light conducting tubes called rhabdoms, around which light sensitive retinula cells make up a ‘retina’ layer. In honey bees there is a dimple in the outer surface of each corneal lens which divides the lens into two regions, which corresponds with the retina that also has two regions, a lower one with long retinula cells and the other upper region with short retinula cells. The layers of straight rhabdoms make the ocelli very sensitive to the orientation of light waves (their ‘polarisation’), and the pigments screen and select particular wavelengths (the pigment in the short cells screen out red for instance).

Because these ‘eyes’ are fixed and the light they receive is determined by the position of the head (and body), you can imagine that turning a horizontal body to the left or right, tilting up or down, or rotating it, will change the field of view for each or the three ocelli. Aviators talk about this as ‘yaw’, ‘pitch’, and ‘roll’. Usually the light coming from above will be bright and polarised, the light coming from below will be scattered, darker, and unpolarised, and there will be a ‘boundary’ that we call a ‘horizon’. The different regions in an ocellus are constructed to be particularly sensitive to light from distinct sources. They are not built to form sharp, focused images and have very limited spatial resolution[ii]. It’s worthwhile trying to imagine how honey bee ocelli will react in a netted enclosure, or when it snows.

The honey bee can fly with its three ocelli blinded, studies have shown, but they fly ‘differently’ as the three eyes on the top of their head help them react very quickly to changes in light.

A honey bee has all sort of sensory mechanisms that provide feedback about its position, not just ocelli. Honey bees with ocelli that have been ‘blinded’ can still fly, they still have two big eyes for one thing, but they fly ‘differently’. Part of the reason for a multiplicity of sensors is specialisation, another reason is ‘redundancy’, that is, they have a backup in case. On a more fundamental level, all insects function by comparing all the data they get from relevant sensors as a form of reality check and on-going system calibration. Being sensitive to the same signal in various wavelengths and orientations eliminates some of the ambiguity in information that occurs in the natural world.

Insects don’t ‘measure’ an absolute status with their sensors, they are built to detect a change of state[iii]. The absolute light level from one ocellus is not important, but an increase in one matched with a decrease in another is, and the bee acts to correct it to the default condition. This use of ‘comparators’ in nervous systems is very fast and can be executed very simply, you don’t need a large complex brain to remember or calculate sensor data. Signals from the ocelli reach their destination within 6ms, twice as fast as those from the compound eye that follow more circuitous, multi-staged processing route. If you want to be as manoeuvrable as a ‘bee, that speed is essential.

The honey bee’s two compound eyes detect colours and patterns, and are much more noticeable to the human eye than their three small eyes on the top of the head.

These features of ocelli have been used to construct autonomous drones, most notably (in 2003) a prototype ‘Mars flyer’ for NASA inspired by dragonfly ocelli[iv]. The ‘biomorphic ocellus’ was small and used very little power or computing, and didn’t rely on gravity or magnetic fields (not much of either on Mars!). The honey bee’s simple ocelli are mainly (but not only) rapid, very sensitive horizon detectors, and the triangular layout of a bee’s three ocelli detects the apparent horizon tilt produced when it’s flying. They add to the sensory range of the compound eyes, and reduce the ‘processing overhead’ enabling the brain to concentrate on something else.

Just like combat aircraft, honey bees rapidly and continually correct their flight trajectory, adapting to agitation in the air, collisions, and the strange aerodynamics of tiny, fragile, flexible wings.

References

[i] Willi Ribi, Eric Warrant, Jochen Zeil (2011), The organization of honeybee ocelli: Regional specializations and rhabdom arrangements. Arthropod Structure & Development 40 (2011) 509e520, doi:10.1016/j.asd.2011.06.004

[ii] Yu-ShanHung and Michael R.Ibbotson (2014), Ocellar structure and neural innervation in the honeybee. Frontiers in Neuroanatomy, Vol.8, Article 6 doi: 10.3389/fnana.2014.00006

[iii] Graham K. Taylor and Holger G. Krapp (2008), Sensory Systems and Flight Stability: What do Insects Measure and Why? Advances in Insect Physiology, Vol.34 ISBN 978-0-12-373714-4, DOI: 10.1016/S0065-2806(07)34005-8

[iv] Chahl et al. Bioinspired Engineering of Exploration Systems: A Horizon Sensor/Attitude Reference System Based on the Dragonfly Ocelli for Mars Exploration Applications, Journal of Robotic Systems 20(1), 35–42 (2003) DOI: 10.002/rob.10068. (The current ‘Dragonfly’, proposed for exploring the Saturn moon Titan, uses gyrocompassing and accelerometers for flight stability.)


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