Bug-eyed: a thread. After putting up mantis pics, I've had questions about their eyes, and whether the black dots in them are actually pupils. They're not - they're called "pseudo-pupils," and are an optical artifact of compound eyes' structure. A compound eye is-- 
--composed of many individual optical "modules" called ommatidia, tube-like structures for gathering and sensing light, all bundled together, like a bunch of... eye pipes? I'll call them eye pipes. Each one produces a small piece of a larger image mosaic (so no, insects do NOT--
--see hundreds of the same image, despite what "The Fly" taught you). When light comes into ommatidia at an angle, it is reflected back out to us, but any ommatidia that are directly in line with our eyes (when we're looking straight down the eye pipe, if you will) absorb--
--all the light and we only see black. The pseudopupil therefore appears to move as you or the insect moves, so that the dot is always "looking" at you, like a bug version of that creepy painting in your grandma's house.
By the way--
By the way--
--it's not just mantis eyes that show pseudopupils. You can see them in many insects (as well as crabs & shrimp). A few examples include stick insects...
...cicadas, caddisflies, dragonflies...
...katydids and grasshoppers. And--
--not all compound eyes show pseudopupils, sometimes simply because the eye is either too dark or is heavily patterned so that you can't see the effect, as in this mantis.
Finally--
Finally--
--here's a 15-second animation demonstrating the effect (the site has other content about compound eyes if you want to learn more): biology.anu.edu.au/research/proje…
1 more pic: This mantis had just molted the night before, and something must have bumped one eye while the exoskeleton was still soft. The dent in its eye shows the pseudopupil as almost a pinpoint, because in the center of the crease, literally only 2 ommatidia now face my lens.
