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