Eyes Of 30,000 Honeycombs

Lower Peirce
North, Singapore
July 2013

Merlion Wayfarer spotted a Brachydiplax Chalybea (Blue Dasher) dragonfly perched on a twig. A few shots were taken with this very accommodating subject. On zooming, she realized that those few shots had managed to capture a very important facet of a dragonfly's anatomy - its compound eyes.

Thousands Of Honeycombs

Most insects have multi-faceted "compound eyes". For example, houseflies have about 6,000 eye facets that give them a panoramic view of their surroundings. These eyes look like honeycombs when viewed up close. 

With 30,000 individual facets, dragonflies have the most number of facets among insects. Each facet, or ommatidia, creates its own image, and the dragonfly brain has eight pairs of descending visual neurons to compile those thousands of images into one picture.

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. These eyes are common in arthropods, and are also present in annelids and some bivalved molluscs. In arthropods, compound eyes, grow at their margins by the addition of new ommatidia.

What's The Big Deal About Compound Eyes?

The compound eye consists of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). 

Each ommatidium consists of:
  • A lens (the front surface of which makes up a single facet)
  • A transparent crystalline cone
  • Light-sensitive visual cells arranged in a radial pattern like the sections of an orange
  • Pigment cells which separate the ommatidium from its neighbors.

The pigment cells ensure that only light entering the ommatidium parallel (or almost so) to its long axis reaches the visual cells and triggers nerve impulses. Thus each ommatidium is pointed at just a single area in space and contributes information about only one small area in the field of view.

The dragonfly's compound eyes consist of a series of bigger ommatidia (the red portion) occupying about 40% of each eye with finer ommatidia making up the rest of the lower portion...

The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, and pointing in slightly different directions. The composite of all their responses is a mosaic image - a pattern of light and dark dots rather like the halftone illustrations in a newspaper or magazine. And just as in those media, the finer the pattern of dots, the better the quality of the image.

(Source : Elise Fallson)

Grasshopper eyes, with relatively few ommatidia must produce a coarse, grainy image. The honeybee and dragonfly have many more ommatidia and a corresponding improvement in their ability to distinguish detail. Even so, the resolving ability of the honeybee eye is poor in comparison with that of most vertebrate eyes and only 1/60 as good as that of the human eye; that is, two objects that we could distinguish between at 60 feet (18 m) could only be discriminated by the bee at a distance of one foot (0.3 m).

Compared with simple eyes, due to their sheer number of ommatidium and shape, compound eyes encompass a very large view angle, can detect fast movement, and in some cases, the polarisation of light. As an object moves across the visual field, ommatidia are progressively turned on and off. Because of the resulting "flicker effect", insects respond far better to moving objects than stationary ones. Honeybees, for example, will visit wind-blown flowers more readily than still ones.

With prey snagged in its jaws...

Compound eyes are especially adept at "locking on" to anything that is moving quickly. Like a heat-seeking missile, when a honey bee sees a fast-moving object, its attention is "locked" and an alarm goes off in the honey bees' colony protective instincts. This explains why experienced bee keepers hark on the importance of slow and careful movements in the apiary.

(Source : Laser Focus World)

Imagine a tennis player having to pick out a small ball from the crowd when the ball is traveling at almost 200kmh - selective attention is needed to hit that ball back into play. Likewise, the dragonfly hunts for insects which may exist in swarms - a collection of fast-moving tiny objects. Yet once it has selected a target, its neuron activity filters out all other potential prey. It then swoops in on its prey and gets it right 97% of the time.

Any Limitations?

Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require ridiculously large compound eyes, around 11 m in radius.

The spherical field of vision means that dragonflies are still watching you after they have flown by. However, the backward-looking part of the eye has rather low resolution. This means that dragonflies and bees are less responsive to predators which attack from behind.

In dim light, an optical system needs to collect more light in order to see clearer. In general insects especially those diurnal ones are limited by the small apertures of each ommatidium in the compound eye, hence detect weak contrast especially in bright daylight or dim light. And in order to survive the insect eyes collect lights for about 0.1 second to form a given image. However for dragonflies, they may have apposition eyes with wider facets and they may collect light over a longer period (up to 0.5 seconds) before integrating the signal to produce the final image.

Dusk-active dragonflies have sacrificed most of their color vision in favor of increased light-collecting capacity by having fewer, larger facets in their eyes. They also lack all color sensitive opsins except green, which provides the broadest range of light sensitivity for any opsin. As a result, these dragonfly species probably also have a corresponding decrease in overall color perception.

More Eyes?!

But wait... A dragonfly has more than 2 eyes!

Dorsal ocelli are light-sensitive organs found on the dorsal (top-most) surface or frontal surface of the head of many insects. They coexist with the compound eyes, thus most insects possess two anatomically separate and functionally different visual pathways.

The number, forms, and functions of the dorsal ocelli vary markedly throughout insect orders. They tend to be larger and more strongly expressed in flying insects (particularly bees, wasps, dragonflies and locusts), where they are typically found as a triplet. Two lateral ocelli are directed to the left and right of the head, respectively, while a central (median) ocellus is directed frontally. In some terrestrial insects (e.g. some ants and cockroaches), only two lateral ocelli are present: the median ocellus is absent. The unfortunately labelled "lateral ocelli" here refers to the sideways-facing position of the ocelli, which are of the dorsal type. They should not be confused with the lateral ocelli of some insect larvae.

One common theory of ocellar function in flying insects holds that they are used to assist in maintaining flight stability. Given their underfocused nature, wide fields of view, and high light-collecting ability, the ocelli are superbly adapted for measuring changes in the perceived brightness of the external world as an insect rolls or pitches around its body axis during flight. Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs, to polarization sensors and circadian entrainers.

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