Mapping the Butterfly Eye

Like most insects, the butterfly’s vision system is based on a compound eye. Instead of having one lens and one retina per eye, the butterfly has many long thin photoreceptors called ommatidia. Each ommatid has its own lens and a collection of sensory cells.

The ommatidia are packed into a sphere. Each eye of an Empress Leilia butterfly, for example, has about 6,000 ommatidia. This gives the eyes their disco ball appearance.

Unlike some popular portrayals, however, an insect’s compound eye does not view the world as through a kaleidoscope. The view provided is much like human vision—a pixilated view. The pixels of butterfly vision are just much larger.

To measure the acuity of different regions of a butterfly’s eye, ASU biologist Ron Rutowski and his team use microscopes to measure the angles between the ommatidia. They then compare these measurements with the angle at which light is accepted into the lens.

The ASU scientists measure ommatidial angles at least two different ways. The first method is to take a slice of the actual butterfly eye and measure the angle between the axes of adjacent ommatidia.

The second technique uses the pseudopupil—a false pupil produced by light reflecting in the butterfly’s eye. The location of the pseudopupil will change as the viewer’s position changes. Researchers can count the number of facets between the original and this new location. Divide this number into 10 degrees and the result is the average interommatidial angle in that part of the butterfly’s eye.

Understanding the structure of the butterfly eye has its benefits. The knowledge allows Rutowski to pursue questions about how butterflies actually perceive one another in the wild.

Rutowski has found that butterflies often use cues such as ultraviolet light (UV) and UV reflecting scales to identify one another. That finding leads to other questions.

“We found that the reflectance is highly directional among the butterflies that can reflect ultraviolet light,” he explains. “Males have special scales on the dorsal surface of the forewing. These scales selectively reflect ultraviolet light at certain angles. As the wings move, the direction of reflectance would shift.”

Because butterflies are typically flying around when they encounter one another, the way one butterfly sees another is from the side as the wings are flapped. Rutowski’s questions: What exactly does this UV reflectance look like in flight? For how much of a wing stroke is the UV reflectance visible? In other words, does one butterfly see another as a flashing beacon on the horizon?

According to Rutowski’s evidence, they do, but probably only when viewed from above.

“As expected, we found that for typical sun positions, the UV reflectance is only visible when the viewer is positioned above a flying animal,” he says. “The reflectance appears to be brightest when the viewer is almost directly overhead.”

Rutowski’s team has designed a series of experiments and observations. They want to see if male butterflies approaching females will first move above the females to check on gender before they move below to display an ultraviolet flash.

To date, the scientists have collected some evidence to suggest that individual males vary in terms of the intensity and flash duration of their UV signals. Rutowski wants to develop methods to quantify these differences.—Matthew Shindell