Eyeing Up The Opposition, or Is Compounding The Solution?

In yesterday’s post, the topic was our visual perception. Clearly, our eyes are very sophisticated sensors and one of our great assets is our ability to focus on objects at both long and short distances. Color is also of critical importance in the resolution of shapes. Some birds and mammals have sharper visual acuity than us, but we do pretty well.

The majority of creatures out there now, and in the past, don’t have our type of eye, though. Most have compound eyes made up of hexagonally packed units or ommatidia. Each unit has a lens and an arrangement of pigment cells to ensure that only light that comes full on gets focused onto the nerve cells, so there is no peripheral information coming into that unit.

That would be pretty limiting if there were only one unit, though. But with a large number of units packed together on a curved surface a nice pixelated picture is formed. The resolution depends on the number of pixels.  At the bottom of the pecking order is the grasshopper with only 5 compared to a dragonfly with 30,000.

The downside of the compound eye is the distance problem where far off objects are not resolved. For good resolution, they have to be pretty close. However, motion is well resolved as movement across the field of vision produces a flicker effect. Really nice for close up and personal hunting, which is one reason dragonflies have been successful for such a long time.

Eyeing today’s pixelated version of the Guardian, my attention was drawn to a report of fossilized eyes from the early Cambrian period, that is over 500M years ago (1, 2). The large sea-going insect was an Anomalocaris that had large compound eyes on stalks. The 16,000 ommatidia in each eye would have made it easy to spot lunch as it swam around close to the bottom looking for large trilobites to bite.

Having great sensors must have been of critical importance, even rivaling sex as the number one priority way back in the Cambrian and even the pre-Cambrian era – as it is today, of course.

  1. http://www.guardian.co.uk/science/2011/dec/07/predator-compound-eyes-stalks-cambrian
  2.  J.R. Paterson et al, Nature, 480, 237, (2011).

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