The depths of your eyes

Yeah, that title’s fairly similar to a post from about a year ago, but the difference is significant. That one was about a fly with a maze-like pattern in its eyes (thus, “lost,” get it?) while this one really does involve depth. I spend hours on these titles…

Anyone who’s had a close enough encounter with a praying mantis knows about the false pupil, even if they haven’t discovered that it’s false, believing instead that it indicates where the mantis is actually looking, as our own eyes do. Mantids, though, have compound eyes like most arthropods, lots of simple optical mechanisms bundled together into a knobby group that provides a wide field of view. Even with this field of view, mantids have an optimum angle of sight, and so will still turn their heads to face potential prey or danger; when this happens, the false pupil may become minimized or disappear altogether, enhancing the illusion, but the bare truth is, the false pupil (when visible) always faces the viewer. The mantis may or may not be focusing its primary attention on us, but those little black spots give us the impression that it’s looking right at us.

two images showing false pupil depthThere is a particular trait that I’ve noticed before under high magnification, and managed to capture in images the other day: the false pupils are not on the surface of the eye, but actually down beneath. This makes sense when you know why they occur, but seeing it firsthand is pretty cool.

Notice how the false pupil isn’t visible in the top image where the mantis’ face is sharp, but is pretty distinct in the bottom image where the face is out of focus; also note the comparative focus on the shoulder. Working in natural light with the macro lens at its widest aperture of f4, the depth of sharp focus is incredibly short. The false pupils are actually there in the top image, but blurred into indistinction by being out of focus. A slight twitch closer in the bottom image brought them into focus. Using even a slightly smaller aperture would have increased the depth of field enough to have face and false pupil in focus simultaneously (especially for a subject this small.)

You see, the false pupil is an optical effect. The compound eyes of most arthropods aren’t little clusters of spheres, but something more like a globular flower blossom, originating deeper within the owner’s head. Each eye is a tapered tube, with a simple lens on top and an optic nerve at the bottom; this gives each eye a very specific direction that it sees. Most times, it is the walls of these tubes that give the eyes their collective colors, since we are seeing nearly all of them obliquely, at an angle. It is only when we can see directly down the tubes that the color vanishes, and we get darkness instead, perhaps even seeing the optic nerve at the bottom. So yes, it really is farther away than the surface of the compound eyes, with the possibility that the effect is enhanced by the lenses themselves.

It is believed that this is an evolved protective trait, much like the coloration resembling eyes that several different species possess. Something that is staring right at you is aware of your presence, perhaps ready to defend itself vigorously. This not only runs against the hunting instincts of many species that want to capture their prey unawares, even we feel it; mantids are routinely described as having an “evil stare.” They have no more stare than a housefly, but just saying that isn’t enough to dispel the feeling, is it?

Another interesting trait about mantis eyes can be seen in the last image in this post; at night, the camouflage coloration fades to black. Presumably, this provides some benefit to their night-vision capabilities, but as yet I cannot tell you how or why. It’s also a trait that has to develop. The Chinese mantises, at least, are born with darker eyes but they turn to much the same color as the body within hours (see also this post,) and for the next several weeks, the eyes remain that way day or night. At a certain age, perhaps following a molt, their eyes can become dark at night, as I found out the other evening, when the same model we see above posed for a tight closeup well after sundown.

tight portrait of juvenile Chinese mantis
I wasn’t around when these hatched, so I only have a guesstimate of how old they are – we’ll use the known date of the hatching I witnessed and consider these nine weeks old. What I do have is a measurement of the eyes, since this guy held still for a close pass of the calipers: it’s 4.5mm across the outside of the eyes.

I have no information or trivia to pass along regarding this next image – I’m just including it for variety, and because I obtained it during the same photo session. It appears we also have a resident grey treefrog (Hyla versicolor,) though it might actually be a Copes grey treefrog (Hyla chrysoscelis,) a rarer species – I have to record their mating call to be sure, and so far I haven’t heard a sound from this one. Since they are largely identical, call it either one for the sake of the image.

grey treefrog
The little bit of cottony fluff near the toe, by the way, is some form of leafhopper nymph, partially demonstrating the same trait as the mantis, only for this species, the eyes turn red at night; by day, they are very pale blue-white. I’ll come back later on with more detail pics of the species.