Why do some butterflies have spotty eyes (and hairy ones too)?

Factual Questions
1

Lately I have been taking some close up shots of butterflies, and I have noticed some interesting things that I haven’t been able to find an explanation for. So I thought the Straight Dope might be able to shed some light on these questions.

Previously I have photographed some smaller butterflies from the Lycaeninae subfamily (coppers), and they appear to have regular compound eyes like most other insects I have looked closer at. However, a few days ago I happened upon some larger butterflies, such as the Vanessa cardui (Painted lady) and the Aglais Urticae (Small Tortoiseshell), and their eyes are different.

Each compound eye seems to have darker spots, and the spot that happens to be facing in my view direction appears particularly dark, like a pupil looking right at me. What are these spots, and what is their function? Do they have higher visual acuity there? Are they meant to imitate a pupil for some nefarious reason?

Secondly, I also noticed that the eyes seem to be covered in straight strands of hair, sticking out in all directions. What are they for? Are they supposed to stop particles from getting onto the eyes like our eyelashes do? I have also seen similar hairs on bees’ eyes, I believe.

Thirdly, I might as well get this question out of the way too. Why do they have so much fur? Most other insects that I’ve found aren’t furry in this manner, except bumblebees and bees. The question applies to them as well. I was under the impression that for most butterfly species, the adults only live during the summer and they only overwinter as eggs or pupae, so why do they need such a thick fur? Is it because of the large wing area causes a lot of heat loss?

Hmm, that’s all I wonder for now. I hope I got the species’ names correct, it’s quite difficult for some species when they look alike. :slight_smile:

Thanks in advance for any answers.

To get a better idea of what I’m talking about, here are some photos to illustrate:

Vanessa cardui keeping an eye on me
Another Vanessa cardui, different spot visible
Pieris napi uses different colors, here you can more easily see the rest of the spots
Aglais urticae looking evil

2

I don’t know about the purpose of the hair, but I’m pretty sure that the “spots” are just an optical illusion - they are the result of the spherical array of the compound eyes. When you are looking normal to the surface of the eye, the axis of that individual eye is aligned with your eye, and doesn’t reflect much light. Other, lighter spots appear at different angles, probably due to the number of reflections the light makes as it bounces down the axis of the eye.

3

That sounds plausible. Though why don’t other compound eyes, for example those of other butterfly species show the same effect? Is it due to them being black and thus appear in a uniform color? What makes this guy’s eyes different?

4

“His” eyes are very dark - it’s possible they exhibit the same effect, but it’s not very visible because the eye itself is so dark.

5

That’s what I meant to say in my previous post, though I incorrectly used the word color. I haven’t been able to determine the gender either :). I think it’s a Hesperia comma though. Hmm, wouldn’t the intensity just change in a circular pattern according to your theory? So you also think that the spots are entirely view dependent, without fixed positions?

There seem to be different kinds of compound eyes (apposition, superposition) but I am not sure which bugs use which kind, or whether this phenomenon depends on that.

Wikipedia has this image of a carpenter fly which shows a similar effect, but they don’t seem to show the same view dependent intensity difference as the butterflies’ eyes do. Here’s a photo I made of some unidentified kind of fly. It has reddish eyes but shows no kind of darkening patterns either.

I am not convinced yet.

Yikes. I did some extra googling on the subject but found my own thread as the second result. Google is fast.

6

I think you will find that the position of the spots changes with your viewpoint - try it and let us know.

7

I’ll give it a try tomorrow if the sun is out. Those bugs are picky regarding the weather, despite their nice fur. Evolve some body temperature regulation mechanisms already!

It will probably be difficult to see though due to their petite size, and my surely intimidating appearance (from their viewpoint) makes it a bit tricky to get close.

8

Wikipedia’s Eye article says:

Could they be something like this? The surface might not be “spherical” but have flatter spots which would explain why the whole spots seem to darken without a smooth transition (I will have to check if this is true) like you would expect if the eye had a more even curvature. Together with your observation that not much light is returned when the angle between the normal and your view direction gets smaller.

9

I don’t know from compound eyes, but your pictures are amazing! I didn’t know how furry some of those little dudes are.

10

Hairs on both bees and butterflies help to catch pollen and pollinate the other flowers. Since ensuring a new generation of flowers is important to any animal that feeds on nectar, it would make sense that they’re hairy where other insects aren’t.

I’m not sure if that’s the only reason, but if you compare flying insects, you see that the flower feeders are pretty much hairy (bees, moths, butterflies) and the others (wasps, dragonflies, flies) are not so much.

11

Thanks! I haven’t been very interested in or observant of bugs before, so I was also a bit surprised when I saw how hairy they are. There are a lot of interesting things you notice when taking close up shots, that would be hard to see with the naked eye. It takes some patience to get close enough and get some decent photos out of it though.

Another interesting thing I noticed is that many butterflies often appear to have only four legs while I had the impression that all insects should have six as long as they haven’t lost some to a predator. After digging around I found out that many butterfly species’ frontal legs have diminished in size and are mostly useless, but they’re still there. I think they are visible in this photo that I snapped recently, tucked up like arms along the body.

That seems like a good explanation for the fur. However, at the field where I have been looking for butterflies I haven’t seen any with more than a trace amount of pollen on them, unlike the bees that have shorter legs and rub much closer to the plants. The butterflies have long legs and don’t even have to lean in towards the flowers to reach the nectar due to their very long proboscises. Maybe we don’t have plants that are very suitable for butterfly pollination here. There are plenty of bumblebees sharing the same flowers though, so the pollination will get done that way too.

I suppose it’s still a reasonable explanation for the body hair, but I am guessing that the eye hair has another function.

12

I went back today to look for some butterflies to test the theory, but they were fewer in number today, and they didn’t let me come very close. After stalking them for a good while I managed to come close to one. I only got a brief glimpse of one butterfly through the viewfinder, and since it’s difficult to change view direction smoothly while keeping the butterfly’s eye sufficiently in focus, my observation could be incorrect.

Anyway, I tried to move my head and the camera sideways while observing the compound eye. The spot seemed to fade out as I moved, but remained stationary, while another spot that became more aligned to my camera got darker. So the spots do seem to have a physiological cause, and not only an optical effect. Of course, the darkening of them is an optical effect, but I think they appear due to some difference in the eyes’ construction at those places.

13

I just found this (scroll down to the next section):

Apparently the spots are called pseudopupils and are areas of higher visual acuity. A few pages down it is also mentioned that many insects have eyes that are too dark for the pseudopupils to be visible (like you said, beowulff). So it seems like the correct answer to my question was a combination of our ideas.

I am still looking for more information regarding my other questions.

14

That’s interesting.
BTW - your link doesn’t work.

15

It’s supposed to point to a book called Animal Eyes by Michael F. Land, Dan-Eric Nilsson on Google books, pages 139-142. It has some nice images and illustrations to show how they work.

Where does the link take you? I used their ‘Link’ feature and copied the link it gave me, and it works for me. Hmm.

16

That’s interesting - when I go to the page, it gives me an error like “you have reached a page that’s unavailable blah, blah, blah” but I happened to click on the message, and the photo of the compound eyes loaded - go figure.