There is an old saying, “Listen to the eyes, they have much to say.” This expression can be extended to butterflies. Their eyes are self-revealing: Through external observation, their eyes reveal their structure, from which we can determine their sensitivity, acuity, field of view, and more.
By and large, butterflies are visual creatures, and rely heavily on their vision to navigate, search for food, find potential mates, and avoid predators. Butterflies have slightly lower visual acuity compared to humans. However, they are better at detecting fast-moving objects, they have a wide range of color sensitivity extending into the ultraviolet range, and they can detect polarized light. Their large compound eyes also give them nearly a 360-degree field of view.
Two properties of compound eyes that illustrate how butterfly eyes reveal their functionality are pseudopupils and eyeshine.
Pseudopupils are not unique to butterflies. If you look closely at the eyes of a praying mantis, you’ll see a singular, tiny black dot that is the pseudopupil in each eye. Their eyes seem to follow you regardless of your viewing angle. As the name suggests, pseudopupils are not like our own pupils, but are visual effects of compound eyes. The compound eyes of insects typically contain thousands of long, tapered eye units called ommatidia, arranged radially. Each ommatidium contains light-absorbing pigments within their rhabdom (sort of an elongated equivalent of our retinas). When you view a pseudopupil, you are essentially looking “straight down the barrel” at the dark pigments of one or more ommatidia that are aligned with your viewing angle.
Compared to praying mantises, the pseudopupil patterns in butterflies can be richer and more complex. For example, if you look at the eye of a Blomfild’s beauty butterfly (Smyrna blomfildia, see photo on right), you’ll see a repetitive, hexagonal pattern of pseudopupils, roughly resembling a soccer ball. The darker spot in the center is the principal pseudopupil, where your line of sight is aligned with the axis of the ommatidia. The other, less-defined spots are the accessory pseudopupils. With accessory pseudopupils, you are viewing the dark pigments of ommatidia indirectly, through the facet lenses of adjacent ommatidia.
The ommatidia in butterfly eyes are typically packed in a hexagonal arrangement. So, the “soccer ball” pattern seen in the Blomfild’s beauty’s pseudopupils is no mere coincidence. We’re seeing an expanded approximation, or a sampling of their eye structure.
If you consider the “design” of compound eyes, there is a general tradeoff between light sensitivity and visual acuity. Each ommatidium produces its own functional pixel. If there are more ommatidia in each surface area, then there are more total functional pixels, and therefore higher resolution. However, this results in less light entering each (narrower) eye unit. Each species of butterfly evolves to find its own sweet spot in this tradeoff. By examining the structure of compound eyes non-invasively, entomologists can quickly and conveniently determine how the compound eyes of each species favor sensitivity versus acuity.
Another way that entomologists learn about butterfly vision is by observing eyeshine in butterfly eyes. If you’ve taken flash photos of dogs or cats, or if you’ve ever seen the sparkling eyes of wolf spiders at night, then you’re familiar with eyeshine. Eyeshine is the luminous or glowing effect that many animals, especially nocturnal ones, exhibit when a light is shined on their eyes in low light conditions. The eerie glow is caused by light being reflected by a special, reflective tissue behind their retinas called the tapetum lucidum (Latin for “shining layer”).
The reflected light from the tapetum lucidum allows animals to see much better at night. Essentially, it gives the photoreceptors in retinas two opportunities to absorb light: first with incident light, and a second time when any remaining unabsorbed light gets reflected by the tapetum lucidum. In cats, it is estimated that the tapetum lucidum increases light sensitivity by about 44%, allowing them to see things that we can’t in low light conditions.
Although butterflies are generally diurnal, they also possess a tapetum lucidum layer. It is believed that butterflies inherited this trait from the nocturnal moth ancestors from which they evolved approximately 100 million years ago. Not only does the tapetum lucidum improve butterfly night vision, but it is also believed to help improve contrast and reduce glare.
When butterfly eyeshine is photographed up close, the resulting image shows an array of brightly colored, iridescent dots. The variety of colors varies from species to species. Entomologists analyze these arrays to gain insight into, for example, how color detection varies for different regions of their eyes, or for which colors their ommatidia are tuned.
We’ll never know what “color” ultraviolet light looks like, and we may never fully experience how polarized light appears to a butterfly. But we can gain some insight and get a hint of how butterflies see by getting up close and “listening” to small details in their eyes that have much to tell us.
(For more photos, please visit my website at https://ajijic-up-close.com)
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- Listening to the Eyes of a Butterfly - August 30, 2025
