Ooops.
As Philip Veerman reminded me (at least to me), arachnids are not
insects of course . So while I had thought that their eyes are compound
eyes, in fact they have multiple simple eyes or ocelli.
Andrew
On 18/11/2013 8:08 PM, Andrew Hobbs wrote:
Given the nature of insect eyes, is it possible that you are seeing
some sort of irridescent effect, similar to the irridescent colours
seen in some insects and bird feathers etc. (due to diffraction,
multiple reflections and interference etc.)
Andrew
On 18/11/2013 7:03 PM, Denise Goodfellow wrote:
And wolf spiders' eyes (for that's what you're seeing), appear to change
colour as you approach them!
Denise
Denise Lawungkurr Goodfellow
PO Box 71, Darwin River,
NT 0841
043 8650 835
On 18/11/13 6:24 PM, "Mona Loofs Samorzewski"
<>
wrote:
Thanks Andrew for that excellent explanation.
Along the same vein, I have walked around the bush at Howard Springs
near
Darwin at night with a head torch on, and seen many many reflections
coming
from the ground from spiders eyes. And they were blue! A friend with a
hand-held torch wasn't seeing them at all until he put my head torch
on. And
then we realised how many spiders we were walking on...
Mona
On 18/11/2013, at 4:54 PM, Andrew Hobbs wrote:
There are two types of 'eye shine'.
As Chris said it is partly simple physics. By the nature of the eye
and its
focus on the back of the eye when light is shone into it there is a
reflection from the back of the eye. As is the case with anything
being
illuminated, at least some of the light is reflected back along the
same path
and will appear to cause the eye to 'shine' (compared to the rather
non-reflective coat of hair, feathers etc).
But there is another factor. Some animals (including some birds
such as
owls) which are active at night, have a layer at the back of the
eye called a
'Tapetum Lucidum'. This layer contains mineral crystals which have
the
property of reflecting almost all the incident light back along the
same path
as the incident light. (Look up 'Corner reflector' on Wikipedia)
This layer
is behind the layer of light sensitive cells, doubling the
sensitivity of the
eye. (The cells have two chances of capturing photons; coming and
going).
When you shine a light into an animal or bird with these eyes they
really do
appear to shine very brightly. However the reflection is not
perfect and some
light is reflected back somewhat 'off axis'. In this case, because the
reflection is so strong, you can still see a reflection even if
your eyes are
well off the same axis as the torch beam.
In contrast, in animals (and birds) without a Tapetum Lucidum, the
light is
reflected of the Fundus at the back of the eye. This is not a good
reflector
but some does get reflected back along the axis. Because the
reflection is
so much weaker your eyes need to be much closer to the axis of the
torch beam
to see the effect.
The colour of the eye reflection from the Tapetum Lucidum depends
upon the
properties of the crystals in the layer. It can vary more or less
across the
full visible spectrum. However in animals without that layer the
reflection
is always red. The red colour is mainly due to the haemoglobin in
the blood
vessels at the back of the eye. (This is the basis for the red eye
effect in
people. We don't have a Tapetum Lucidum.)
Andrew
On 18/11/2013 12:18 PM, Chris Corben wrote:
Roger
It's very simple physics. Almost anything looks brightest when the
light
source is close to your eyes. But in addition, a bird's eye is
pretty much
retro-reflective. That is, the light reflected by it mostly goes
back along
the path it came from. If you think about it, you are looking at
the inside
of a sphere, so wherever the light comes from, it is reflected
back in the
reverse direction. The same principle is used to make road
markings shine
brightly at night. The paint on a road is filled with tiny glass
spheres, so
that from whatever direction the light arrives, some portion of it is
retro-reflected. Since the headlights of a car are not too far off
the line
of your sight, a lot of that light comes back to your eyes, and
the markings
look bright. If the paint was just plain gloss paint, it would be
much more
reflective, but nearly all the light from the headlights would be
reflected
away from the driver, and the paint would look essentially black.
As does
smooth ice,
and for the same reason.
Owlet Nightjars eyes are not nearly so bright as White-throated
Nightjar's
eyes. But even a White-throated can be seen at much greater
distance with a
light which is close to your eyes. It makes such a difference,
that you can
wear a headlamp and see the eyes of things like owls at reasonable
distance
even though the headlamp is not very bright. A side benefit of
this is that
the lower light levels will scare the bird a lot less, so you can
actually
gain by having a lower intensity light if it is close to your eyes. A
headlamp is perfect for that!
Incidentally, if you know to look for it, you can see eye-shine of
animals
in all sorts of unexpected ways. A classic case is to get to a
place where
there are frogs on the surface of the water. If you get the sun
straight
behind you (eg in early morning or late afternoon), you will be
able to see
their eye-shines surprisingly well, especially if you use binoculars.
Using binoculars with a headlamp is a great way to find all sorts of
creatures at night. Frogs, snakes, geckoes, spiders, small mammals
and even
bats in the right situations. Just use your binoculars to look at
the spot
of light from the headlamp.
Cheers, Chris.
On 11/17/2013 08:39 PM, Roger McNeill wrote:
A few weeks ago Gus McNab was over and we were discussing
spotlighting and
I mentioned how I have a good population of Owlet Nightjars on
our block
but I never see them at night because their eyes don't eyeshine,
despite
wandering the woods after hours.
He told me (politely) how wrong I was and the issue was that I
was not
holding the torch in the right place to see it. What I needed to
do was
walk around like a unicorn with the torch beam emenenting from
between my
eyes. I (polietly) said that is faseniting, thinking that this
was surely
some ploy to make me look like an idiot...not that help is
required...and
thinking how that could possibly be true?
Last night, I had a visiting bird-o who wanted to see Nightjars
and other
things so we decided to wander the tracks and see what we could
find.
First try was for White-throated Nightjars...two birds pearched
up for us,
brilliant eyeshine. A koala started calling back at the house so we
wandered back, yep bright eye shine...we then decided to walk
down to
"owlet-nightjar grove" and I had three birds respond and two come
in, one
quite close. I put the torch on the bird and as expected
no-eyeshine...but
then I tried Gus' recomendation and move the torch between my
eyes and wow,
its eyes shown back bright red like a Christmas tree. Amazing!
Gus you
were right, but I have no idea what the explanitaion was or why
this is the
case! Thanks for the tip...wanted to share publically.
Cheers,
Roger
Roger McNeill
Samford Valley, SEQ
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