Pop Quiz: Are the creatures in this photo butterflies or moths? Which is the male and which is the female?
Sarpy County, Nebraska
Answer:
These are moths. While they have enough color that many people might call them butterflies, the antennae distinguish them as moths. Butterflies have long straight antennae with little knobs at the tip. Moths, on the other hand, have fuzzy antennae, and males (such as the one on the right in this photo) have much fuzzier antennae than females. The males use their antennae to find females by following their pheromone trails.
Now, maybe one of you can tell me why at least some moths appear to have a darker spot in the center of their big compound eyes – making it look like they have a large pupil in each eye. I’ve noticed the trait on other moths I’ve photographed, but don’t know whether or not the dark spots have a particular function. I’m guessing that among the readers of this blog someone will know the answer. Thanks in advance!
(Oh, and if you’d like to identify the moth species for me, that’d be great too!)
The Prairie Ecologist 
Hi Chris,
Cool photo! These are Crocus Geometer Moths – there are two species that look alike, that can’t be distinguished from photos. Xanthotype urticaria and X. sospeta. I love seeing them – they have such bright colors. Here’s my page about them, with a few photos: http://www.aprairiehaven.com/?page_id=8183
Thanks Marcie! Great site.
Hi Chris,
I’ve noticed, with dragonflies, that the location of the dark spot depends on where you stand. It seems to be the facets which can see you that are darkest. Do they change the angle of their retina(s) to focus on you like a spider can, or is it just differences in light reflection? I don’t know.
Cyndi
Cyndi is right. The dark spot occurs when you look directly down one of the hexagonal cylinder constructions making up the compound eye. If you are not seeing the “dark” spot you otherwise see the reflection off one of the sides of the hexagonal cylinders. The different angles of reflection available off the sides of these constructions can also make it appear like the eye is changing color. Somebody correct me if I’m wrong.
I find it intersting that Chinese mantis eyes have just a small “pupil” in daylight, and this expands to a fully black compound eye at night. I’ve never noticed if moth eyes do this.
Since compound eyes are fixed structures, my guess (and it’s only this) is that those facets of the eye of which the outer lens is perpendicular to the light coming from you are the ones the inner base of which you can see, and that functionally, their reception of more light from the object in straight line view (or not in neighboring facets) helps the insect perceive position and form of the object. Having written this, I see it may raise more questions than it provides answers.
Cyndi, Tom, and James – very interesting, thank you. It makes sense that I’d just be seeing the dark “bottom” of those cylinders, and that I can’t see the bottom of all the cylinders because they’re set at different angles. I think that fits what all three of you are saying, right?
James, the changing size of the “pupil” is really interesting. Not sure I can come up with a reason for that happening based on our initial hypothesis?
Chris,
John Rawlins at the Carnegie Museum of Natural History had this answer for you:
These are moths….family Geometridae, subfamily Ennominae.
A pair of mating Xanthotype moths, either Xanthotype sospeta or Xanthotype urticaria (probably the former, but despite pattern differences, a determination without genitalic examination is guesswork….i.e.., some urticaria are very similar to some sospeta based on wing patterns only.
Male is skinny guy to right in image with big antennae.
Dark spot in compound eye is not a pupil in the normal sense, rather it is evidence of a heavily pigmented basal region of the individual ommatidia that make up the compound eye….the dark pigment moves outward and the “pupil” appears larger, or pigment moves downward and the “pupil” is smaller. Movement of pigment is a response to light levels and changes visual acuity (smaller “pupil”, sharper vision at higher light levels).
Nice image.
John E. Rawlins
Carnegie Museum Bugrooms
4400 Forbes Avenue
Pittsburgh PA, 15213-4080
Interesting posting,
Thanks, Paul
Hello Chris,
John Rawilns of the Carnegie Museum in Pittsburgh, PA informs me:
These are moths….family Geometridae, subfamily Ennominae.
A pair of mating Xanthotype moths, either Xanthotype sospeta or Xanthotype urticaria (probably the former, but despite pattern differences, a determination without genitalic examination is guesswork….i.e.., some urticaria are very similar to some sospeta based on wing patterns only.
Male is skinny guy to right in image with big antennae.
Dark spot in compound eye is not a pupil in the normal sense, rather it is evidence of a heavily pigmented basal region of the individual ommatidia that make up the compound eye….the dark pigment moves outward and the “pupil” appears larger, or pigment moves downward and the “pupil” is smaller.
Movement of pigment is a response to light levels and changes visual acuity (smaller “pupil”, sharper vision at higher light levels).
Nice image.
John E. Rawlins
Carnegie Museum Bugrooms
4400 Forbes Avenue
Pittsburgh PA, 15213-4080
Wow – excellent information! Big thank you to John and Paul both!
Chris
I think it would be fun to do a moth survey within the Platte River Prairies.
You’re on. When can you start?
: )
Oh, golly. Now that I am a graduate a lot of people expect me to get a job in my field but with this wedding planning jazz (Andy and I are getting married in July, I’m not sure if I told you) the time frame is just off, which is a bummer. But as long as the leps are around, I will be too.