A mantis can see you almost irrespective of your position relative
to its eyes. If you look at its eyes you will see a little dark dot in each
eye. And both seem to follow you no matter where you look from. Those dots are
the bottoms of the roughly prismoidal component eyes (ommatidia) of the
compound eyes. Only the ommatidia directed at you can “see” any part of you.
However, this characteristic of the eyes means that by positioning itself
so that the prey is visible by the correct patch of both eyes at once, the
mantis can estimate the range and snatch it with great speed and reliability.
No doubt because of this form of hunting, the mantids have developed
unusually mobile necks, and if anything takes their attention, they turn their
heads and look more or less directly at it, much as you or eye have reasonable peripheral
vision, but turn eyes and heads to look directly at tigers, pretty girls and a
plate of dessert (not necessarily in that order). There is a difference between
an animal with binocular vision “seeing” something, and “looking” at it.
In contrast, a chameleon seldom looks directly at anything that it
is not aiming at. It largely reserves binocular vision for prey.
Mantispids are not related to mantids (Neuroptera, rather than Mantodea)
but look startlingly like them. However, they are much rarer and much shyer, so
I have never been able to compare their behaviour.
As for the eyes going darker in the dark, I find this puzzling.
Insects with low-light adaptation tend to retract the light-absorbing protective
pigment in their eyes in the dark. If you are sure of the details, I can only
imagine that the species of mantids that you are dealing with have
light-coloured, reflective, protective pigments that expose the dark retinal
pigments when withdrawn. Please supply more details. For example, are you sure
that the eyes really were darker in the dark and the pigment did not just flood
back in defence against the light when you illuminated them to see? Sorry to sound niggly, but I have never seen
that effect and I don’t know even what kind of mantid you were observing.
Jon
