Mantis Eyes

I learned something interesting tonight. I was out pretty late for a concert in London, so by the time I got home and turned on the front room light, it had already been dark for several hours. When I get in, the first thing I like to do is to go over to the Critter Corner and check on the critters. When I did, I found Catchy in her tank, looking at me like this:

As you can see, her eyes have completely changed colour. Usually her eyes are the same vibrant green as the rest of her body (see below) but tonight, they were a tanned brown colour. Initially, I thought something was wrong - but wrong in the sense of "maybe she's preparing to molt" rather than "oh no, there's something wrong with her eyes". So I found myself doing a little bit of late night drunken research into Mantis vision.

One of the first things I discovered was that mantises have five eyes, rather than two. On the image below, (which I shamelessly stole from the internet and now I can't remember where I stole it from!) you can see the two large compound eyes which make Catchy look like a "bug-eyed" alien, but you can also see that there are three ocelli in the middle of the head, between the antennae. These ocelli are very simple eyes that pretty much only detect light. I haven't been able to locate them on Catchy, but her head is still relatively small, despite her size. The two big eyes are the interesting part though.

It is well known that insects have compound eyes - each single eye is made up of hundreds or thousands of tiny independent "photoreception units" called ommatidia, that each consist of a cornea, lens, and the photoreceptor cells which distinguish brightness and color. In mantises these ommatidia are heavily pigmented to aid in their camouflage, which is why Catchy's eyes appear green. However, when you are looking directly at ("down") a unit, the pigment appears to disappear and you can seea "pseudopupil" - The black dot on the surface of the eye that makes it look like Catchy is looking right at you.
Mantises have other adaptations to their vision, however. They have much more complex vision than many other insect species - They can see in three dimensions, for example, but this ability is based on movement. If they fixate on their prey and it stops moving, they can sway themselves back and forth to create movement, enabling them to attain a level of depth perception in order to reach out and capture their prey more efficiently.

Another adaption that mantises have is night vision. When a mantis detects lowering light levels, it is able to relocate the pigment in the surface of its eyes to other areas. This allows more light to enter the ommatidia rather than light reflecting back off improving the mantises ability to see in the dark, but also having the colour changing effect. If left for long enough, the mantises eyes will go completely black, making them all the more resemble a "grey" alien. This gradual transition can be seen below:

I am unsure whether Catchy was disturbed mid change so hadn't finished the transition, or if my turning the light on resulted in the lighter, more brown colour. One day I hope to photograph her with completely black eyes. But in the meantime, I thought it was a pretty interesting feature and it needed to be discussed here.